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Treatment during engineered wood product manufacture

Some engineered wood panel products, such as plywood and laminated veneer lumber (LVL) are able to be treated after manufacture with preservative solutions, whereas thin strand based products (OSB, OSL) and small particulate and fibre-based panels (particleboard, MDF) are not. The preservatives must be added to the wood elements before they are bonded together, either as a spray on, mist or powder.

Products such as OSB are manufactured from small, thin strands of wood. Powdered preservatives can be mixed in with the strands and resins during the blending process just prior to mat forming and pressing. Zinc borate is commonly used in this application. By adding preservatives to the manufacturing process it’s possible to obtain uniform treatment throughout the thickness of the product.

In North America, plywood is normally protected against decay and termites by pressure treatment processes. However, in other parts of the world insecticides are often formulated with adhesives to protect plywood against termites. 

Buildings

Wall Types for Water Control

Building envelope experts generally speak of three or four different approaches to design of a wall for moisture control. Face seal walls are designed to achieve water tightness and air tightness at the face of the cladding. An example would be stucco applied directly to sheathing or masonry without a moisture barrier membrane such as building paper. Joints in the cladding and interfaces with other wall components are sealed to provide continuity. The exterior face of the cladding is the primary – and only – drainage path. There is no moisture control redundancy, i.e., there is no back-up system. A face seal system must be constructed and maintained in perfect condition to effectively control rain water intrusion. In general, these walls are only recommended in low risk situations, such as wall areas under deep overhangs or in dry climates. Concealed barrier walls are designed with an acceptance that some water may pass beyond the surface of the cladding. These walls incorporate a drainage plane within the wall assembly, as a second line of defense against rain water.

The face of the cladding remains the primary drainage path, but secondary drainage is accomplished within the wall. This drainage plane consists of a membrane such as building paper, which carries water down and out of the wall assembly. An example is siding or stucco applied over building paper. Concealed barrier walls are appropriate in areas of low to moderate exposure to rain and wind. Rainscreen walls take water management one step further by incorporating a cavity between the back of the cladding and the building paper. This airspace ventilates the back of the cladding, helping it to dry out. The cavity also acts as a capillary break between cladding and building paper, thereby keeping most water from making contact with the building paper. An example of a rainscreen wall is stucco or siding applied to vertical strapping over the building paper. Rainscreen walls are appropriate in high rain and wind exposures. An advancement of the rainscreen technology is the pressure-equalized rainscreen. These walls use vents to equalize the pressure between the exterior and the cavity air, thereby removing one of the driving forces for water penetration (when it is pushed through cracks due to high pressure on the face of the wall and low pressure in the cavity). These walls are for very high risk exposures.

Importance of an Overhang

In a rainy climate, an overhang is one of the simplest and most effective ways to reduce the risk of water intrusion. An overhang is an umbrella for the wall, and the deeper the better. A survey of leaky buildings in British Columbia commissioned by Canada Mortgage and Housing Corporation in 1996 showed a strong inverse correlation between depth of overhang and percent of walls with problems. However, even a small overhang can help protect the wall, largely due to its effect on driving rain. One important benefit of overhangs and peaked roofs often not appreciated is the effect of these elements on wind pressure. Wind-driven rain is typically the largest source of moisture for walls. An overhang and/or sloped roof will help direct the wind up and over the building, which reduces the pressure on the wall and thereby reduces the force of the driving rain striking the wall. This means water is less likely to be pushed by wind through cracks in the wall.

Minimize the Holes

Most rainwater problems are due to water leaking into the wall through holes. If care isn’t taken to protect discontinuities in the envelope, water can leak around window framing and dryer vents, at intersections like balconies and parapets, and at building paper joints, for example. Good design detailing and careful construction is critical! So is maintenance of short-life sealants like caulk around window frames. BC Housing-Homeowner Protection Office has updated the “Best Practice Guide for Wood-Frame Envelopes in the Coastal Climate of British Columbia” originally developed by Canada Mortgage and Housing Corporation and published “Building Enclosure Design Guide for Wood-Frame Multi-Unit Residential Buildings” with extensive information on design and construction detailing.

Use our Effective R calculator to determine not only the thermal resistance of walls, but also a durability assessment of the wall based on representative climate conditions across Canada.

Related Publications
For on-line design and construction tips, try the following:The Build a Better Home program, operated by APA-The Engineered Wood Association, runs training courses, operates a demonstration houses, and offers publications. The web site offers construction information and provides links to all relevant APA publications.

Building Enclosure Design Guide: Wood-Frame Multi-Unit Residential Buildings.

 

Bâtiments

Types de murs permettant de contrôler l’eau

En règle générale, les experts en enveloppes de bâtiment considèrent qu’il existe trois ou quatre approches différentes pour la conception de murs au profit du contrôle de l’humidité. Les murs avec barrière d’étanchéité en surface sont conçus de façon à obtenir une étanchéité à l’eau et à l’air à la surface du parement. Un exemple de ceci serait le stuc appliqué directement sur le revêtement ou la maçonnerie, sans membrane d’étanchéité comme le papier de construction. Les joints entre le parement et les interfaces, et les autres composants, sont scellés afin d’assurer la continuité. La face extérieure du parement est la principale et unique voie d’évacuation de l’eau. Il n’y a pas de renfort pour le contrôle de l’humidité, c.-à-d. qu’il n’y a pas de système complémentaire. Un système d’étanchéisation en surface doit être construit et maintenu en parfaite condition afin de contrôler efficacement l’infiltration de l’eau de pluie. En général, ces murs sont recommandés uniquement dans les situations où les risques sont faibles, comme les zones murales situées sous de larges avant-toits ou là où le climat est sec. Les murs dotés d’une membrane dissimulée sont conçus dans la perspective où il est possible qu’un peu d’eau s’infiltre au-delà de la surface du parement. L’intérieur de ces murs comporte un dispositif d’évacuation de l’eau, en guise de deuxième ligne de défense contre l’eau de pluie.

La face du parement reste la voie d’évacuation principale, mais une évacuation secondaire est exécutée à l’intérieur du mur. Le dispositif de drainage se compose d’une membrane comme du papier de construction, qui achemine l’eau jusqu’en bas et à l’extérieur du mur. Un bardage ou du stuc appliqué sur du papier de construction constitue un exemple d’un tel dispositif. Les murs comptant une membrane dissimulée sont appropriés aux endroits modérément exposés à la pluie et au vent. Les murs à écran pare-pluie vont un pas plus loin dans le contrôle de l’eau, en incorporant une cavité entre le dos du parement et le papier de construction. Le vide d’air ventile le dos du parement et l’aide à s’assécher. De plus, la cavité fait office de coupure capillaire entre le parement et le papier de construction, empêchant ainsi la majeure partie de l’eau d’entrer en contact avec le papier. Un mur avec stuc ou parement appliqué sur la fourrure verticale par-dessus le papier de construction constitue un bon exemple de mur à écran pare-pluie. De tels murs conviennent à des bâtiments fortement exposés à la pluie et au vent. L’écran pare-pluie à pression équilibrée constitue l’un des progrès de la technologie des écrans pare-pluie. Ces murs font appel à des orifices pour équilibrer la pression entre l’air extérieur et celui de la cavité, éliminant ainsi l’une des forces favorisant la pénétration de l’eau (lorsque celle-ci est poussée au travers des fissures en raison de la pression élevée à la surface du mur et de la pression basse dans la cavité). Ces murs sont réservés aux endroits où les risques d’exposition sont très élevés.

Importance des avant-toits

Lorsque le climat est pluvieux, un avant-toit constitue l’un des moyens les plus simples et efficaces de réduire le risque d’infiltration de l’eau. Un avant-toit peut être comparé à un parapluie pour les murs, et plus il est large, mieux c’est. Une étude sur les bâtiments aux prises avec des problèmes de fuites en Colombie-Britannique, demandée par la Société canadienne d’hypothèques et de logement en 1996, a démontré la forte corrélation inverse entre la largeur d’un avant-toit et le pourcentage de murs problématiques. Par contre, même un avant-toit étroit peut aider à protéger le mur, en grande partie en raison de son effet sur la pluie battante. L’un des avantages importants mésestimés des avant-toits et des toits à double pente est leur effet sur la pression du vent. En règle générale, la pluie poussée par le vent est la plus grande source d’humidité dans les murs. Un avant-toit ou une toiture inclinée aidera à rediriger le vent vers le haut et par-dessus le bâtiment, réduisant ainsi la pression sur le mur et, par conséquent, force de la pluie battante qui martèle le mur. L’eau sera donc moins susceptible d’être poussée par le vent dans les fissures du mur.

Minimiser les orifices

La grande majorité des problèmes causés par l’eau pluviale est attribuable à l’eau qui s’infiltre par les trous des murs. Si aucune mesure n’est prise pour remédier aux irrégularités de l’enveloppe, l’eau pourra s’infiltrer par exemple autour des cadrages de fenêtres et du conduit d’évacuation de la sécheuse, aux intersections comme les balcons et les parapets, et aux joints du papier de construction. Une conception détaillée et une construction soigneuse sont donc essentielles! Tout comme l’est l’entretien des éléments d’étanchéité de courte durée, comme le mastic de calfeutrage autour des cadrages de fenêtres. Le BC Housing-Homeowner Protection Office a mis à jour le « Best Practice Guide for Wood-Frame Envelopes in the Coastal Climate of British Columbia », initialement conçu par la Société canadienne d’hypothèques et de logement, et a publié le « Building Enclosure Design Guide for Wood-Frame Multi-Unit Residential Buildings », qui comprend des renseignements exhaustifs sur la conception et la construction.

Utilisez notre calculatrice de résistance effective non seulement pour établir la résistance thermique des murs, mais également pour procéder à une évaluation de leur durabilité en fonction des conditions climatiques représentatives à l’échelle du Canada.

Publications associées
Pour obtenir des conseils en ligne sur la conception et la construction, consultez ce qui suit : Le programme « Build a Better Home », dirigé par l’APA – The Engineered Wood Association, anime des cours de formation, présente des maisons témoins et offre des publications. Le site Web fournit des renseignements sur la construction, de même que des liens vers toutes les publications pertinentes de l’APA.
Building Enclosure Design Guide Guide: Wood-Frame Multi-Unit Residential Buildings.

Environmental Issues

Safe Handling

Using common sense and standard safety equipment (personal protection and wood-working machinery) applies when working with any building products. Gloves, dust masks and goggles are appropriate for use with all woodworking. Here are a few key points specific to treated wood:

  • Pressure-treated wood is not a pesticide, and it is not a hazardous product. In most municipalities, you may dispose of treated wood by ordinary garbage collection. However, you should check with your local regulations.
  • Never burn treated wood because toxic chemicals may be produced as part of the smoke and ashes.
  • If preservatives or sawdust accumulate on clothes, launder before reuse. Wash your work clothes separately from other household clothing.
  • Treated wood used for patios, decks and walkways should be free of surface preservative residues.
  • Treated wood should not be used for compost heaps where free organic acids produced early in the composting process can remove the fixed chemicals. It is, however, safe to use for growing vegetables in raised soil beds. If, after reading this, you are still concerned, place a layer of plastic sheet between the soil and the treated wood wall.
  • Treated wood should not be cleaned with harsh reducing agents since these can also remove the fixed chemicals.

Environmental Concerns

All wood preservatives used in the U.S. and Canada are registered and regularly re-examined for safety by the U.S. Environmental Protection Agency and Health Canada’s Pest Management and Regulatory Agency, respectively. 

Wood preservation is not an exact science, due to the biological – and therefore variable and unpredictable – nature of both wood and the organisms that destroy it. Wood scientists are trying to understand more about how wood decays to ensure that durability is achieved through smart design and construction choices where possible, so that as a society we can be selective in our use of preservatives.

Comparing treated wood to alternative products

A series of life cycle assessments has been completed comparing preservative treated wood to alternative products. In most cases, the treated wood products had lower environmental impacts.

 

 

 

 

 

 

Click for consumer safety information on handling treated wood (Canada).

Read More

About Treated Wood

When you want to use wood that is not naturally decay resistant in a wet application (outdoors, for example) or where it may be at risk for insect attack, you need to specify preservative-treated wood. This is lumber that has been chemically treated to make it unattractive to fungi and other pests. In the same way that you would specify galvanized steel where it would be at risk of rusting, you specify treated wood where it will be used in a setting conducive to decay. 

Wood does not deteriorate just because it gets wet. When wood breaks down, it is because an organism is eating it as food. Preservatives work by making the food source inedible to these organisms.

Properly preservative-treated wood can have 5 to 10 times the service life of untreated wood. This extension of life saves the equivalent of 12.5% of Canada’s annual log harvest.

Preserved wood is used most often for railroad ties, utility poles, marine piles, decks, fences and other outdoor applications. Various treatment methods and types of chemicals are available, depending on the attributes required in the particular application and the level of protection needed.

Environmental product declarations (EPDs)

Stakeholders within the building design and construction community are increasingly being asked to include information in their decision-making processes that take into consideration potential environmental impacts. These stakeholders and interested parties expect unbiased product information that is consistent with current best practices and based on objective scientific analysis. In the future, building product purchasing decisions will likely require the type of environmental information provided by environmental product declarations (EPDs). In addition, green building rating systems, including LEED®, Green Globes™ and BREEAM®, recognize the value of EPDs for the assessment of potential environmental impacts of building products.

EPDs are concise, standardized, and third-party verified reports that describe the environmental performance of a product or a service. EPDs are able to identify and quantify the potential environmental impacts of a product or service throughout the various stages of its life cycle (resource extraction or harvest, processing, manufacturing, transportation, use, and end-of-life). EPDs, also known as Type III environmental product declarations, provide quantified environmental data using predetermined parameters that are based on internationally standardized approaches. EPDs for building products can help architects, designers, specifiers, and other purchasers better understand a product’s potential environmental impacts and sustainability attributes.

An EPD is a disclosure by a company or industry to make public the environmental data related to one or more of its products. EPDs are intended to help purchasers better understand a product’s environmental attributes in order for specifiers to make more informed decisions selecting products. The function of EPDs are somewhat analogous to nutrition labels on food packaging; their purpose is to clearly communicate, to the user, environmental data about products in a standardized format.

EPDs are information carriers that are intended to be a simple and user-friendly mechanism to disclose potential environmental impact information about a product within the marketplace. EPDs do not rank products or compare products to baselines or benchmarks. An EPD does not indicate whether or not certain environmental performance criteria have been met and does not address social and economic impacts of construction products.

Data reported in an EPD is collected using life cycle assessment (LCA), an internationally standardized scientific methodology. LCAs involve compiling an inventory of relevant energy and material inputs and environmental releases, and evaluating their potential impacts. It is also possible for EPDs to convey additional environmental information about a product that is outside the scope of LCA.

EPDs are primarily intended for business-to-business communication, although they can also be used for business-to-consumer communication. EPDs are developed based on the results of a life cycle assessment (LCA) study and must be compliant with the relevant product category rules (PCR), which are developed by a registered program operator. The PCR establishes the specific rules, requirements and guidelines for conducting an LCA and developing an EPD for one or more product categories.

The North American wood products industry has developed several industry wide EPDs, applicable to all the wood product manufacturers located across North America. These industry wide EPDs have obtained third-party verification from the Underwriters Laboratories Environment (ULE), an independent certification body. North American wood product EPDs provide industry average data for the following environmental metrics:

  • Global warming potential;
  • Acidification potential;
  • Eutrophication potential;
  • Ozone depletion potential;
  • Smog potential;
  • Primary energy consumption;
  • Material resources consumption; and
  • Non-hazardous waste generation.

Industry wide EPDs for wood products are business-to-business EPDs, covering a cradle-to-gate scope; from raw material harvest until the finished product is ready to leave the manufacturing facility. Due to the multitude of uses for wood products, the potential environmental impacts related to the delivery of the product to the customer, the use of the product, and the eventual end-of-life processes are excluded from the analysis.

For further information, refer to the following resources:

Choosing and Applying Exterior Wood Coatings

Choosing a coating depends on what appearance is desired and what level of maintenance would be tolerable.  For many people, the basic choice is paint versus stain. The trade-off is often between maintenance frequency and appearance.

For many people, additional criteria include VOC emissions, ease of clean up, and cost.  See our Links page for web sites and books with detailed information on choosing and applying wood finishes.  Read our About exterior wood coatings page for an understanding of the differences between paints and stains, pigmented versus clear coatings, and so forth.

Because exterior wood shrinks and swells with moisture changes, the coating needs to be flexible. Flexibility varies by product – some products may be clearly identified as suitably flexible for wood’s dimensional changes.  Water-borne coatings are generally more flexible than alkyds. Coatings containing urethanes tend to be more flexible than coatings containing acrylics.

For factory finishing with transparent coatings, with special considerations for UV and mildew control, please see our fact sheet Factory Finishing with Transparent Coatings: Requirements for Maximizing Longevity.

Special Considerations

If a coating is desired for a wear surface such as a deck or stairs, consult carefully with the coating manufacturer to choose the right product for this demanding application.  All coatings will be challenged by foot traffic and increased exposure to weather in a horizontal application.  High traffic routes will show wear faster than other areas. Paints and other thick film-formers may fail quickly in this situation, and a time-consuming refinishing process will be necessary each time the coating fails.  Hence many people will find a stain the more convenient choice for decks and stairs.

Knots may require a bit of extra care as some wood extractives or resin may leach out or bleed. Extractive bleeding can cause discolouration, but this can usually be prevented by applying special stain-blocking primers. In some species, especially the pines and Douglas-fir, knots and pitch pockets contain resin. The resin can bleed and may discolour the finish, leave hard beads of resin on the surface, or may otherwise interfere with the coating bond. The best way to prevent this is to purchase kiln-dried wood where the resin should be set (hardened and fixed in place). If painting is desired, choose higher grades of lumber as these will have fewer knots, and choose kiln-dried lumber if using a resinous species.

If siding or sidewall shingles are to be painted, the US Forest Products Laboratory (USFPL) recommends they be backprimed.  This application of a coating to the back side will plug the wood pores, preventing extractive bleed without blocking water vapour transmission and also preventing liquid water uptake.

If possible, round out any sharp corners for best coating adhesion on these edges – for example, a square-edged stair tread will show coating degradation quickly, but bullnosed stair tread edges will retain a coating much longer.  This is because a coating applied to a corner tends to pull away from the corner, leaving a much thinner layer there than elsewhere.

Surface Preparation

Durability of any finish is highly dependent on proper application, which includes good preparation of the surface to be coated.  Specific details on surface preparation depend on what condition the wood is to begin with – read on for tips that apply to various scenarios.

Surface Preparation for Fresh Wood

While fresh, clean wood can be coated without surface preparation, a light sanding with 100 grit sandpaper (and dust removal) can double the service life of some water-based coatings. For best results apply a coating to a fresh wood surface as soon as possible after planing or sanding.  If exposed to rain and sun for more than two weeks, adhesion of coatings will not be as good. The surface must also be free of anything that will interfere with coating adhesion, such as dirt, damaged wood fibres and moisture. Grade stamps on wood should also be removed before applying a semitransparent stain, preferably by sanding.

Cleaning

If there are discolourations caused by dirt, iron stains or other discolourations on the wood surface, cleaning may be desired. It is always preferable to achieve cleaning with sanding when possible.  Another safe way to clean wood without damaging the surface is to simply use a garden hose, with or without a pressure nozzle.  Use pressure-washing only with extreme care as it can damage wood, especially low-density species such as western red cedar.  The pressure should be kept at a minimum, and never hold the nozzle in one place for a long time.  If necessary, use a little bit of dish detergent, and lightly scrub (not with steel wool, as this will leave iron stains) in the direction of the grain for any stubborn discolourations.  For discolourations that resist soap-and-water cleaning, chemical cleaners will be effective.  The chemicals in commercial wood cleaners can be caustic soda (sodium hydroxide), sodium metasilicate, oxalic acid, citric acid, phosphoric acid, borax or some mixture. Wood cleaners containing caustic soda at a 1% –  2% solution will remove nearly all discolourations with the least damage to wood. Some acid cleaners are especially effective for removing extractive stains and iron stain.  Bleach is commonly used for cleaning wood, but we do not recommend this, since a poor wood substrate will usually be left behind for subsequent coating.  Resin (pine pitch) can be generally removed with mineral spirits. Please note that all acidic or alkaline chemicals need to be thoroughly rinsed off before coating. Chemicals can be toxic, corrosive and harmful, so handle all these chemicals with care and follow all manufacturer’s instructions.

Surface Preparation for Aged Wood

Wood coatings need a fresh surface or the coating simply won’t last. The longer wood has been allowed to weather, the poorer the coating adhesion. If a fresh surface is allowed to weather or age outdoors for more than two weeks, coating adhesion will deteriorate. This is mainly due to wood damage from sunlight. Weathered wood surfaces usually have a higher acidity, higher contact angle, and lower surface energy.

Restoring an aged wood surface is necessary before applying a coating.  The damaged (aged/weathered) wood fibres must be removed, exposing fresh wood.  Also, any discolourations will typically be removed along with the damaged fibres, so the process of restoration is simultaneously a cleaning process.  Wood restoration can be achieved with sanding or with chemicals, but sanding is always preferable when possible.  Sanding can be done by hand or machine until the true wood colour shows. Then brush off the sawdust and apply the coating immediately.  For many jobs, a chemical method will be far easier.  Read the label of each product to identify the active components.  In general, caustic soda (sodium hydroxide) is the best chemical choice for both cleaning and restoration.  It effectively removes weathered wood fibres from the surface and leaves the surface at a suitable pH for coating.  Oxalic acid is also commonly identified as a wood restorer, however, it is only effective at discolouration removal and does not remove the damaged wood fibres from the surface – in other words, it is not restoring the wood to be an appropriate substrate for a coating.  However, oxalic acid can be used to return the original wood colour after the use of sodium hydroxide.  Sodium hydroxide will slightly darken the wood, and, if this is undesirable, simply rinse the wood with oxalic acid after restoration with sodium hydroxide.  Please note that all these chemicals must be handled with care and all manufacturer’s instructions should be followed, as the chemicals can be toxic, corrosive and harmful. Where the wood is close to plants, wet down the leaves with a garden hose prior to and after chemical use. Wood surfaces should also be thoroughly rinsed with water before coating.

Maintenance

Maintaining a coating means giving it a wash occasionally, watching for signs that the coating is losing integrity, and applying a fresh coat before full failure sets in.  If a coating is reapplied before the last coat has failed, the stripping process may not be necessary. It’s time to apply another coat when paint has worn down to the primer, or if the coating colour has undesirably faded, or if the surface of water-repellent treated wood no longer beads water.  Then wash or brush off dirt and apply a new coat.  Any areas showing failure (the coating has lifted from the surface or cracked, or bare wood is showing) can be spot-treated.  Remove any loose pieces of paint and use sandpaper to feather the edges of adjacent sound paint so the transition won’t be evident through the new paint layer.  Also sand away any weathered wood.  For large scale failure, refinishing will be necessary. For all coating systems, there is a limit to the number of coats a surface can support. When the coating gets too thick, refinishing will also become necessary.

Refinishing

Refinishing a coating means stripping off the old coating and starting over.  This is necessary when large areas of the coating have failed, or the coating is getting too thick for refinishing, or if a decision is made to change the type of coating.  A coating has failed when it no longer adheres to the wood surface.  If the coating has bubbled, cracked, or peeled, it must be removed.  If the coating has simply faded but otherwise appears to still be well-bonded, it may not need to be removed.  When a change of coating type is desired, the new coating may be incompatible with the old coating – to ensure a good bond for the new coating, strip off the old one.  Remove coatings by sanding or with a chemical product.  Sanding has advantages over chemical stripping in restoring the fresh wood surface, but even if sanding is done by machine, it is still very labour-intensive for large painted areas typical of outdoor projects.  Sandblasting is not recommended except for large timbers and logs, as it will pit the wood and is hard to keep away from elements like window frames.  Powerwashing will only remove loose paint, leaving behind paint that is still adhered.  So, a chemical approach is generally regarded as the most effective and least labour-intensive way to strip a coating.  Sodium hydroxide at a 6% –  8% dilution is the recommended chemical for stripping – and offers the additional benefits of cleaning discolourations and restoring the wood surface at the same time.  Products containing sodium hydroxide are corrosive and should be prevented from touching skin. Follow manufacturers’ instructions.  There are also other chemical products for stripping coatings in the market.  After stripping with chemicals, always give the wood a final rinse with water.  Many projects will still require some light sanding around stubborn stains or heavily damaged wood.

Factory Finishing

  • Select heartwood where possible to minimize nutrient content of wood surfaces and prevent nutrients migrating through the coating to support fungal growth on the surface.
  • Round all corners to minimum 5 mm radius to eliminate sharp edges where coating can thin out.
  • Prepare surface by sanding with 100 grit sandpaper to physically and chemically activate the surface.  Pretreatment and coating should be applied immediately after sanding. Research shows sanding can double coating life.
  • Pretreat with an aqueous formulation containing a UV absorber designed to absorb the visible light that must penetrate transparent coatings to permit the wood to be visible. If the subsequent coating is not completely opaque to UV light, a hindered amine light stabilizer should be added to the visible light protection system. Not only does a visible light protection system prevent degradation of the wood-coating interface, it also prevents release of lignin breakdown products that can be used as a food source by black-stain fungi and prevents light induced breakdown of the biocide components. This pre-treatment must also contain three low-dose carbon-based biocides with differing chemistries to provide cross protection against detoxification and with complementary spectra of activity providing resistance to the full range of black-stain fungi. It should ideally have water repellent properties and must maintain wood surface pH close to neutral or slightly alkaline.
  • Apply a transparent water-based catalyzed urethane coating, containing organic and inorganic UV absorbers with absorbance that extends from UVB through to the high-energy part of the visible spectrum (violet light). The coating must virtually eliminate UV from penetrating to the wood, preventing breakdown of wood, biocides and water repellents. This coating will be formulated to be damp-wood friendly to allow application soon after pre-treatment. It will contain no nutrients for fungal growth. It must have an optimum combination of moisture excluding efficiency and vapour permeability to minimize moisture uptake and allow drying after rain. The first coat to be designed to penetrate and bond to the wood, subsequent coats to be designed to ensure maximum intercoat adhesion without sanding between coats. Sufficient coats to be applied to give a film thickness no less than 60 microns to minimize the ability of black-stain fungi to penetrate the film with their infection pegs. The surface layer to have sheeting rather than beading properties to ensure rapid drying after rain or dew, reducing the time available for spore germination.

Additional detailed information on coating wood surfaces has been assembled by the Joint Coatings and Forest Products Committee (http://www.fpl.fs.fed.us/documnts/pdf2004/fpl_2004_bonura001.pdf, 2004).

Performance Factors

How long will an exterior wood coating last?  Anywhere from a few months to 20 years or more, depending on the choice of product, how it was applied, and how severe the environment.

Paints tend to last the longest, assuming they are applied properly (see Choosing and applying exterior wood coatings page).  But the range of lifespan for a paint coating is very large.  A low quality product badly applied to a weathered wood surface may barely last two years.  If everything is done right, the coating might last 20 years.  High quality paints and stains generally last longest, and coatings that are in locations protected from sunlight and water tend to last longer.

Stains and water repellents have much shorter lives than paints, but are easier to maintain.  This is one of the reasons they are a popular choice for stairs and decks.  Depending on the degree of exposure to sun, water, foot traffic, and the pigment amount in the stain, expect a life of 1 to 2 years for a stain applied to deck boards and 2 to 5 for a stain applied to products that are not subject to wear.  Water repellents generally last 6 to 12 months.

Results from numerous tests on exterior wood finishes by many experts in this field, particularly by the US Forest Products Lab (USFPL), are summarized below.  See the USFPL link for more information.

Effect of wood anatomy

  • Coatings, particularly solid colour stains and paints tend to last longer on dimensionally stable species such as western red cedar, eastern white cedar and Alaska yellow cedar, as these will shrink and swell less than other species and will therefore put less stress on the coating bond.  However deck stains will not last as long on low density species such as western red cedar due to wear.
  • Coatings last longer on wood with narrow latewood bands (the dark part of the annual ring) due to density differences between the earlywood (the light part of the ring) and the denser latewood.  The southern pines are characterized by their wide bands of latewood, and therefore these species are considered to be somewhat poor for painting.
  • The amount of extractives or resin in wood also affects coating performance. Special primers can be used to block water-soluble extractives, and kiln drying is most effective for fixing resin in wood.  Nutrients in wood can migrate through the coating to support fungal growth on the surface, and heartwood can be chosen to minimize the nutrient content in wood.

Effect of grain

  • Finishes last longer on vertical (also called edge grain) versus flat grain, as these surfaces will shrink and swell less and therefore put less stress on the coating bond.  However, it can be difficult to specify type of grain when ordering a product.  Western red cedar and redwood may be available in a premium grade, which will likely be all heartwood, vertical grain.
  • If using flat grain, place it bark side out or up if possible, because the grain is less likely to raise on that side, particularly in species with dense latewood bands such as the southern pines, and raised grain is a problem for coating adhesion. This is not an issue when using vertical grain products. Placing bark side out also minimizes checking.

Effect of surface roughness

  • Rough-sawn (saw-textured) or roughened wood creates a better coating bond and thicker coating buildup than smooth wood.  The life of a coating can be substantially extended if the wood is roughened.

Effect of sanding

  • Sanding (100 grit) can double the life of a coating, for both weathered and freshly planed wood.  This is because sanding removes any damaged surface fibres and also changes the surface chemistry to improve bonding of the coating.

Effect of wood preservatives

  • Semitransparent stains last longer when applied to CCA-treated wood – treated wood purchased prior to 2004 was probably treated with CCA.  Research is under way on finishing for wood treated with new preservatives. Protection measures regarding use of treated wood apply when coating preservative-treated wood.

Effect of bluestain

  • Bluestain is caused by fungi, and bluestained wood is more permeable than unstained wood, therefore it may absorb more coating.  Make sure to apply sufficient coating.

Effect of weathering

  • Sunlight quickly degrades the ability of a wood surface to bond with a coating.  Research has shown a tremendous difference in paint performance on weathered versus unweathered wood.  Paint on boards with no exposure to weather prior to painting lasted at least 20 years.  Boards that had weathered for 16 weeks prior to painting began showing cracks in just 3 years.  For maximum coating life, sand the surface if the wood has been exposed to any sunlight at all, particularly if for more than two weeks.

Effect of product manufacturing

  • Plywood:  Coatings on plywood are challenged by the small cracks (face checks) on the surface that are caused by the lathe when the veneer is cut from the log during manufacturing.  As the plywood goes through moisture cycling outdoors, these cracks tend to get larger and stress the coating bond.  Plywood surface, edges and joints in outdoor applications should be protected, and coatings and other products for helping plywood resist cracking can be applied to prevent moisture ingress.  Generally a good stain can effectively protect plywood. Since checking in stained plywood usually occurs during the first six months of outdoor exposure, best coating results can be obtained by applying a first coat and allowing any checking to occur, then six months or so later applying a second coat.  Paints can fail quickly on plywood, unless efforts are made to reduce moisture uptake and also to use flexible products to accommodate dimensional changes of the wood. Roughening the surface is also important. For plywood protection and other issues with plywood, see the recommendations from the Canadian Plywood Association (http://www.canply.org/pdf/main/plywood_handbookcanada.pdf).
  • Finger-jointed products: Coatings may perform differently on different parts of these products, as they are not likely to be uniform in grain orientation, in heartwood versus sapwood content, or even in species.  Roughen the surface to extend the life of the coating and minimize these differences. Apply primer and paint all sides if possible to minimize moisture absorption.

Effect of priming

  • Field tests have shown that coatings last much longer when a primer coat is used.
  • Field tests have shown that siding or shingles last much longer if they are back-primed.

Effect of design and installation

  • Use good design and installation practices to protect wood from sunlight and water, and prevent moisture accumulation in wood structures.
  • By providing adequate clearance to grade, adequate roof overhang, rainscreen wall and back-priming, the coating life on siding can be effectively extended.
  • If using flat grain, place the bark side out if possible to avoid raised grain.
  • Use corrosion-resistant fasteners.

Finishing Quick Tips

For new wood, remember:

  • The wood must be dry.  Drying time depends on a few factors.
    • Ideally the wood should be kiln-dried (stamped “S-DRY”, “KD” or “KDAT”, see glossary of “dry lumber”). If the wood is surface wet from rain or washing, let dry 1 to 2 days.
    • If the wood is wet through (green lumber, pressure-treated lumber not stamped “KDAT”), 2 days of drying is acceptable if using a “damp-friendly” coating.  Otherwise:
    • The wood must be allowed to thoroughly dry to a stable outdoor moisture content; about 15% in most climates. The characteristics of the wood and the climatic characteristics of its environment are so variable that drying time is hard to predict.  The common way to determine wood moisture content is with a moisture meter. (Note: specific correction factors should be applied if a moisture meter is used on preservative-treated wood.)
  • Weather conditions during coating application can affect the coating’s drying, appearance and performance. Follow the coating manufacturer’s recommendation.
  • Coat as soon as possible after the wood has been planed or sanded.  Apply finishes within two weeks of exposure, or sooner if possible (Surface Preparation for Fresh Wood).  Otherwise, follow the instructions for aged (weathered) wood below.
  • If the wood is very smooth, lightly sand it to roughen the surface with 100-120 grit sand paper.  This greatly improves the coating bond.  Brush free of dirt and sawdust.
  • If painting the wood, apply a primer coat. Use an extractive-blocking primer, if needed (for example, with western red cedar or redwood) over the entire piece, or a knot sealing primer if needed (Special Considerations).  When dry, apply two coats of top quality paint. For stains and water repellents, follow the  instructions on the can regarding number of coats.
  • Carefully follow the instructions on the can regarding best environmental conditions for coating, application recommendations, safety precautions and clean-up.

For aged (weathered) wood, remember:

  • For wood that has been previously coated, please read about refinishing.
  • Clean the wood and remove discolourations such as iron stain, if desired.  Expose fresh wood because coatings perform best when applied to freshly exposed wood surfaces.  Allow to dry. See Surface Preparation for Aged Wood.
  • Brush free of dirt and sawdust, and proceed with application of the coating.

When maintaining or refinishing, remember:

  • Avoid the need to refinish by keeping an eye on the coating and adding a fresh coat before the previous coat wears away, cracks or peels.  This may be as frequent as every six months with water repellents, every year or two with stains, and every few years with paint (See Maintenance).
  • Spot-treat worn areas to extend the period between full applications of a fresh coat.  Sand away any failed coating and any weathered wood, and re-apply the coating (See Maintenance).
  • If the coating has failed on a large scale, or the coating is getting too thick for refinishing, or if a change in type of coating is desired, completely strip away the old coating – please read about refinishing.

Glossary

Acrylic

A type of water-borne coating product containing acrylic polymers.

Alkyd

A type of polyester resin. Term often used to signify solvent-borne coatings, e.g., oil paints.

Backpriming

The application of a finish coat to the back side of wood such as shingles or siding.

Binder

The non-volatile film-forming solid portion in a coating, which binds the pigment particles together after the film is dry and creates the bond with the substrate.  Typical binders include alkyd resins, acrylic resins and polyurethane resins.

Bleeding

When the colour of a discolouration or other material works up through a coating to the surface.  Commonly used to describe leaching of tannins in extractive species like western red cedar and redwood (typically happens for the first year or so if not stain blocked).

Blistering

When a coating forms bubbles due to air, water vapour or solvent under the film.

Dry lumber

Lumber which has been dried to a moisture content of 19% or less. Any 4” and thinner boards or dimension lumber surfaced at a moisture content (MC) of 19% or less may be stamped “S-DRY” and stamped “KD” if kiln-dried to a maximum moisture content of 19%.  Lumber in the USA may be stamped “KDAT” if kiln-dried after pressure treatment with preservatives.

Enamel

Generic term for an alkyd-based pigmented coating that dries to a smooth, hard, glossy finish.  The term is often more broadly used for a coating which gives a hard, stain-resistant film.

Extractives

Soluble chemicals particularly present in the heartwood of some species which provide the wood with resistance to decay and insects.

Fungicide

A substance which inhibits the growth of fungus.  Often added to coatings to protect the coatings themselves from fungal growth.

Latex

Term used to signify water-borne paints.

Lacquer

Coating material characterized by rapid evaporation of the solvent to produce a thin, hard film.

Linseed oil

Obtained by crushing flax seeds, this natural oil can be used as a vehicle in paints, as a softening agent for the resins in varnishes, or can be used alone as a wood finish material.  Raw linseed oil is a food source for fungi and must be boiled to destroy these nutrients. Most “boiled” linseed oil is not boiled but contains metallic dryers and biocides.

Oil-based paints

Paints using natural oils such as linseed or tung oil as the binder, with turpentine as the usual solvent.  The term is now usually used to refer to paints with both alkyds and oil as the binders, and with a carrier of mineral spirits or other solvents.

Paint

An opaque coating generally made with a binder, liquids, additives and pigments. Applied in liquid form, it dries to form a continuous film that protects and improves the appearance of the substrate.

Pigment

Finely ground solids that impart colour, hiding power (opacity) and ultraviolet protection.

Pitch

Also called resin, this sticky substance is a mixture of rosin and turpentine and is found in most softwoods but particularly the pines, spruces and Douglas-fir.  Can ooze from the pitch pockets and sometimes the knots for a year or two if not set by kiln-drying.  Resin can bleed through finishes and will harden into beads, but this can be cleaned up with mineral spirits and will stop eventually.

Primer

The first complete coat of paint applied in a painting system. Many primers are designed to enhance adhesion between the surface and subsequent topcoats. Most primers contain some pigment, some lend uniformity to the topcoat, some inhibit corrosion of the substrate, and some stop the discolouration of the topcoat.

Resin

For tree resin, see Pitch. In coatings, see Binder.

Sealer

A liquid that seals wood pores so they will not absorb subsequent coats.  Sealers may be transparent, and can act as primers. Some sealers are designed to be left uncoated.

Semi-transparent stain

Stain that alters the natural colour of the wood, yet allows the grain and texture to show through. The term is generally applied to exterior products, but technically applies also to interior wiping stains used for trim, furniture and floors.

Shellac

Alcohol-soluble, clear to orange-coloured resin derived from lac, a substance secreted by insects.  Previously used as a sealer and clear finish for floors, for sealing knots, and in “alcohol-borne” primers; rarely in use anymore. Thinner is denatured alcohol. It is an environmentally friendly product and usually available from finish suppliers.

Solid-colour stain

Exterior stain that obscures the natural colour and grain of wood, but still allows the texture to show through – essentially, a thin paint.

Stain

A coating product which can either be opaque such as a solid colour stain or partly transparent such as a semi-transparent stain. Also refers to wood discolourations such as discolourations caused by tannins in wood extractives, or stain caused by fungi such as bluestain.

Solvent

In generic coatings terminology, refers to the volatile liquid used to improve the working properties of a coating, typically water or hydrocarbons.  In “solvent-borne” coatings, refers specifically to a coating based on hydrocarbons.

Tung oil

Obtained from the nut of the Asian tung tree. Hardly ever used in the raw state as it dries to a non-lustrous finish.  Used in varnishes.

Varnish

Generic term for clear film-forming finish. Transparent or translucent liquids applied as a thin film, which harden.  Can be solvent or water-borne.

VOC

Volatile organic compound.  VOCs are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere where they may participate in photochemical reactions. They are often associated with solvents, typically considered to be pollutants, and are the subject of regulations in many jurisdictions.

Durability Research and Development

FPInnovations has been field testing the performance of treated wood products for years. Click one of these categories for performance data from our field tests.

Borate-treated Wood vs. Termites

Round Wood Posts
Sawn Wood Posts

 

 

 

 

 

Lumber vs. termites
Shakes

 

 

 

 

 

Marine Pilings
Field Cuts

 

 

 

 

 

Naturally Durable Species

The heartwood of species reported to have some natural durability was evaluated in ground contact (stakes) and above-ground (decking) tests. 

Commodity: 2×4 and 2×6 lumber from naturally durable species: Western redcedar, yellow cypress, eastern white cedar, larch, tamarack, Douglas-fir

Control species: Ponderosa pine sapwood

Test method: Stake test (AWPA E7) and Decking test (AWPA E25)

Test sites: FPInnovations – Maple Ridge, BC; Petawawa, ON

Michigan Technological University – Gainesville, Florida; Kipuka, Hawaii 

Date of installation: 2004-2005

 

Estimated service life: In the ground-contact stake test, after 5 years moderate to high levels of decay were found in all species at all sites. Yellow cypress and western redcedar were the most durable at all site. Eastern white cedar had similar durability at the Canadian and Florida sites, but was less durable in Hawaii. There were no major performance differences observed between old-growth and second-growth materials used in this study. Untreated naturally durable heartwood is not recommended for long-term performance in ground contact.

In the above ground decking test, at the Canadian test sites after 10 years only small amounts of decay were observed in any of the naturally durable heartwoods tested. In contrast, the ponderosa pine controls had moderate to advanced decay. Decay was more rapid at the Florida and Hawaii test sites, with moderate to advanced decay present in all material types after 7 years. Untreated naturally durable heartwood is not recommended for long-term performance in exposed above ground applications in high decay hazard areas such as Florida and Hawaii. However, in temperate climates these naturally durable heartwoods can provide service lives greater than 10 years.

References:

Morris, P. I., Ingram, J., Larkin, G., & Laks, P. (2011). Field tests of naturally durable species. Forest Products Journal61(5), 344-351.

Morris, P. I., Laks, P., Larkin, G., Ingram, J. K., & Stirling, R. (2016). Aboveground decay resistance of selected Canadian softwoods at four test sites after 10 years of exposure. Forest products journal66(5), 268-273.

Treatment during engineered wood product manufacture
Buildings
Bâtiments
Environmental Issues
About Treated Wood
Environmental product declarations (EPDs)
Choosing and Applying Exterior Wood Coatings
Factory Finishing
Performance Factors
Finishing Quick Tips
Glossary
Durability Research and Development

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