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Searching for: pressure treated wood

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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. 

The Importance of Proper Specification: Agricultural, Commercial, and Industrial Applications for Pressure Treated Wood

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.

Environmental Issues Environmental Issues

 

 

 

 

 

 

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

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Assessing and Restoration of Decay

Sometimes it happens – wood in service suffers from decay. How can you identify decayed wood and what are the recommended actions to take? First, be sure you actually have decay. The wood may only be harmlessly discoloured, for any number of reasons. See the publication in the side bar for help if your wood is stained but you’re not sure why.

If wood is badly decayed, this will be quite obvious. The wood will be softer than normal and perhaps even be breakable by hand. Decayed wood often has a colour change, either darker or lighter than normal, although this could be due to weathering or could just be a stain. The wood may display an unexpected cracking pattern, or may look stringy- this is a sign of fairly advanced decay. If fungal growth is visible on the surface, the wood has quite likely already suffered strength loss even if this isn’t visibly obvious. However, do not rely on visual cues alone.

Wood can appear stained and yet be sound, or can appear normal yet have already suffered significant strength loss due to decay. Some researchers or engineers use the pick test to determine if the wood is sound. They insert the point of a knife at a shallow angle to the surface and attempt to lever up a thin splinter. If the wood splinters with longer fragments, it is likely sound. If instead it breaks or crumbles in small pieces over the blade, it could be decayed. Decayed wood breaks somewhat like a carrot snapping in half, at one section, versus the splintering along the length of sound wood. See our Biodeterioration page to learn more about the science of decay.

Assessing and Restoration of Decay

If you are still unsure whether or not you have decayed wood, you are advised to seek help from a wood restoration specialist.

How urgent is a decay problem? By the time you notice decay, the wood typically has lost substantial strength already. In cases where the decayed wood is supporting load you are strongly advised to contact a structural engineer or other appropriate expert to more thoroughly assess the problem and proceed with a repair.

A small, localized and non-critical case of decay may be a do-it-yourself project under some conditions. All decayed wood should be removed. If you are unable to remove the entire affected piece, remove the decayed portion plus an additional portion of adjacent wood beyond the visible decay. A rule of thumb is to remove an additional two feet (60 cm) of adjacent wood from each side, although this will of course depend on the extent of the decay. The removal of adjacent wood is because the fungus may have extended deep into the wood beyond the area of decay and may be ready to cause more damage in adjacent sound wood.

Then apply a field treatment to the remaining adjacent wood, such as a borate solution in roll-on, rod or paste form, before replacing the removed pieces. Use treated or naturally durable wood to replace the removed pieces. If damaged wood must be left in place, a penetrating epoxy can sometimes be applied as a stabilizer. In those cases and for best results in all wood repair projects we recommend you consult with a wood restoration expert.

Indoors, it is extremely important that you find the source(s) of the moisture that allowed wood decay fungi to grow. If you had wood decay in a location that is supposed to be dry, then you have a leak or a condensation problem that needs fixing to prevent any future problems. Look for primary and secondary sources of moisture. A short term leak may have allowed decay to start, for example, and condensation may be sustaining the decay. If the location of the decayed wood was outdoors or in a wet location, you need to use treated or naturally durable wood.

If you have building moisture problems on a large scale, you need to hire some experts and be prepared for a potentially substantial remediation project. Seek out a qualified consultant, who will begin by using a variety of techniques and tools to determine the extent of the damage. This will include a visual examination for staining, bulging, cracking, presence of water, and warping. Subsurface moisture penetration will be tested with probes and/or thermography.

In a building with wood structural members, the consultant will probably use a moisture meter to sample wetness of structural wood components in several locations. Based on the results of this investigation, the consultant will recommend a course of action for repair and future prevention. Canada Mortgage and Housing Corporation has developed a guide for building envelope rehabilitation, in two volumes: one for owners, one for consultants.

More Information
Click Here for a fact sheet Discolourations on wood products: Causes and Implications for help if your wood is stained and you’re not sure why.
Click here for more information on biodeterioration and the science of decay.
Click here for more information on remedial treatments.
Click here for links on decay assessment and other durability topics

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.

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.

Durability Solutions

Wood has been a valuable and effective structural material since the earliest days of human civilisation. With normal good practice, wood can deliver many years of reliable service. But, like other building materials, wood can suffer as a result of mistakes made in storage, design, construction, and maintenance practices.

How can you ensure long life of a wood building? The best approach is always to remember that wood meant for dry application must stay dry. Start out by buying dry wood, store it carefully to keep it dry, design the building to protect the wood elements, keep wood dry during construction, and practice good maintenance of the building. This approach is called durability by design.

If wood won’t stay dry, you have two choices in approach. Because wet wood is at risk of decay, you must select a product with decay resistance. One choice is to choose a naturally durable species like Western red cedar. This approach is called durability by nature.

Most of our construction lumber is not naturally durable, but we can make it decay resistant by treating it with a preservative. Preservative-treated lumber is more reliably resistant to decay than naturally durable lumber. This approach is called durability by treated wood.

The level of attention you give to durability issues during the course of design depends on your decay hazard. In other words, the more that your circumstances put wood at risk, the more care you must take in protecting against  decay. In outdoor applications, for example, any wood in contact with the ground is at high risk of decay and should be pressure-treated with a preservative. For wood that is exposed to the weather but not in direct ground contact, the degree of hazard correlates with climate. The fungi that harm wood generally grow best in moist environments with warm temperatures. Researchers have developed hazard zones in North America using mean monthly temperature and number of rainy days. This map in particular shows the rainfall hazard and applies to exposed uses of wood such as decks, shingles and fence boards. A high degree of hazard would indicate a need to carefully choose a wood species or preservative treatment for maximum service life. In the future, building codes may provide more specific directives as a function of decay hazard. For wood not exposed to weather, such as framing lumber, this map is only moderately useful. This is because the environmental conditions in the wall may be substantially different than those outdoors.

Supplemental Treatment

Supplementary treatment may be added wherever on-site cutting or drilling of wood is unavoidable, or where it is suspected the original protection measures may be inadequate. This is most commonly done in applications such as wood foundations, agricultural buildings, or non-residential long-life applications such as utility poles and bridge timbers.

For wood foundations and agricultural buildings, it is normal to expect some end cutting and boring for bolts, pipes or electrical wiring. Typically copper naphthenate is brushed on the cut ends or holes in the treated wood to protect the exposed surfaces. Experience has shown that this is adequate for the limited exposure resulting from such cases.

For cases such as poles or bridge timbers, the original preservative protection can be lost over time due to degradation or depletion of the active ingredients. A need for supplementary treatment may be indicated by damage to similar structures in the same area. Or there may be evidence that the risk of damage has increased, for example, if new termites move into the area.

In cases like utility poles, where these are part of the physical infrastructure of an organization, inspection, maintenance and remediation are regularly practiced to ensure continued safety in use and to schedule replacement. Often the cost of supplementary treatment is relatively small compared to the cost of inspection, and is a very small fraction of the cost of premature failure. Supplementary treatment may also be prudent in terms of due diligence (reducing legal liability). During inspection of these structures, drills or increment borers may be used to determine the condition of the interior of the wood members. It is advised to treat these holes, to avoid infection from non-sterilized drills and borers. In addition, as holes are typically drilled where decay is suspected or anticipated, treating the holes is wise to supplement protection at that site.

Solids

Borate, copper/borate and fluoride rods have seen increasingly widespread use as supplementary treatments for internal decay due to their convenience in handling and very low toxicity. Copper moves more slowly in the wood than borate, providing protection to the zone around the rod if the borate is removed over time through mass flow of water. This is mainly of concern for utility poles in wet climates, where moisture moves into the pole from the soil, wicks up the pole and evaporates above ground, moving the borate up the pole with it – this leaves the borate in a part of the pole not especially at risk for decay. The rate of water flow may be relatively slow in Douglas fir (an impermeable wood species) treated with an oil-borne preservative having some water repellency. It may be more rapid in southern pine (a very permeable wood species) treated with a waterborne preservative.

Liquids, Pastes and Gels

Spray and foam application of liquids and gels are increasingly used for supplementary treatment of wood frame buildings against termites and wood boring beetles. Holes are drilled into each stud space and the liquids or gels are pumped in under pressure. Coverage cannot be expected to be as effective as that achieved by spray treatment during construction. Liquids can be poured or pumped into drilled holes to treat internal decay in utility poles or timbers. Typically the loading of preservative that can be achieved is limited in the first case by the size and location of the holes and the solubility of the chemical, and in the second case by the permeability of the wood. Another approach is to leave a pressurized device attached to the pole below ground, which pushes a larger amount of liquid into the pole over a longer time period. Care must be taken to ensure that drilled holes do not intersect voids or checks leading to the surface of the wood; otherwise, the liquids can flow out. Pastes can be packed into drilled holes to treat internal decay. Alternatively, they can be brushed or trowelled on or applied on bandages to treat external decay.

Fumigants

Fumigant treatments have been used successfully for decades on utility poles and timber structures. The gas moves rapidly through the wood, adsorbing to the lignocellulose and providing several years of residual protection.

Fasteners

Fasteners, Connectors and Flashing for Wood Treated With Copper-Based Preservatives

The presence of moisture is a precondition for corrosion of metals. Treated wood is typically used in applications where it may be exposed to moisture for considerable periods so any fasteners and connectors used with treated wood must also be resistant to these conditions.  In addition, most wood preservatives designed for exterior use contain copper that may react with the metals used to fabricate fasteners and connectors therefore, it is important to use the right type of fastener and/or connectors. Where treated wood is used in dry environments to prevent damage by wood-destroying insects, including termites, corrosion is of less concern.

Users and specifiers should also be aware that corrosive industrial, or salt air, environments may also require the use of appropriate corrosion resistant metals.

Types of Wood Preserving Treatments

Most copper-based preservatives are corrosive to unprotected fasteners and connectors. More recent systems such as MCA where the copper isn’t introduced in an ionic salt form, are designed to reduce the corrosion of metals, and the preserved wood is approved for use in contact with aluminum (e.g. brackets or outdoor furniture legs). Borate treatments do not increase the risk of corrosion.

Recommendations on Connectors for Treated Wood

Connectors used for wood treated with a copper-based preservative must be manufactured from steel either hot–dipped galvanized in accordance with ASTM A653 or hot dipped galvanized after manufacture in accordance with ASTM A123.  Galvanizing nails and screws is actually a sacrificial coating to protect the structural integrity of the fastener, and the presence of some white corrosion product on the surface is normal. Red rust appearing is an indicator of coating failure. The service life of these components can be extended by using a barrier membrane between the connector and the treated wood surface. Stainless steel connectors (type 304 or 316) should be used for maximum service life, for high preservative retentions (i.e. ground contact products) or severe applications such as salt spray environments.  For borate-treated wood used inside buildings, the same connectors can be used as for untreated wood.

Recommendations on Fasteners for Treated Wood

Fasteners for use in treated wood that will be exposed to the weather should be selected to withstand weathering as long as the treated wood itself. 

As a minimum, nails for wood treated with a copper-based preservative must be hot-dipped galvanized in accordance with ASTM A153. Hot-dipped galvanized nails should not be fastened using a high pressure nail gun due to the risk of damage to the coating during firing. The protective coating on electroplated galvanized fasteners is too thin and will perform poorly, and common nails will corrode rapidly after fastening most copper-based treated wood.  Stainless steel should be used for maximum service life, for high preservative retentions or severe applications such as salt spray environments. Where appropriate, copper fasteners may also be used. Fasteners used in combination with metal connectors must be the same type of metal to avoid galvanic corrosion caused by dissimilar metals.  For example stainless steel fasteners should not be used in combination with galvanized connectors.

Screws intended for use on wood treated with a copper-based preservative must be hot dipped galvanized in accordance with ASTM A153 or, if recommended by the manufacturer and the preservative supplier, high-quality polymer coated. Stainless steel should be used for maximum service life, for high preservative retentions or severe applications such as salt spray environments.

For borate treated wood used inside buildings, the same fasteners can be used as for untreated wood.

As a general rule aluminum fasteners should not be used with treated wood, except new generation products (MCA treated) specifically tested, approved and labelled as suitable for contact with Aluminum. 

Recommendations on Flashing for Treated Wood

Flashing used in contact with treated wood must be compatible with the treated wood and be last long enough to be suitable for the intended application.  Flashing must also be of the same type of metal as any fasteners that penetrate through them to avoid galvanic corrosion. Copper and stainless steel are the most durable metals for flashing.  Galvanized steel, in accordance with ASTM A653, G185 designation, is also suitable for use as flashing.

Other Fasteners, Connectors or Hardware as Recommended by the Manufacturer

There may be additional products such as polymer or ceramic coatings for fasteners, or vinyl or plastic flashings that are suitable for use with treated wood products.  Consult the individual fastener, connector or flashing manufacturer for recommendations for use of their products with treated wood.

Current Recommendations for Drying and Conditioning of Treated Wood Prior to Construction.

Wood treated with copper-based preservatives should be at the least surface dried at the treating plant, in the store or at the job site before attachment of fasteners, connectors, flashing or other hardware. A moisture meter with a calibration for preservative treated wood should be used to verify that the wood is within a similar moisture content range to untreated construction lumber (i.e. about 12 to 18%) otherwise the treated wood can undergo similar shrinkage related cracking and deformation as incorrectly conditioned untreated lumber.

Canadian Preservation Industry

Canada has had a wood preservation industry for more than 100 years.  Canada is tied with the UK as the world’s second largest producer of treated wood (the USA is first, by a large margin).  In 1999, the most recent year for which we have data, Canada produced 3.5 million cubic metres of treated wood.  There are about 60 treating plants in Canada.

As with most other industrialized countries, Canada developed a wood preservation industry using creosote, initially to service railroads (the ties holding the rails) and then utilities (power poles).  Creosote production began declining by the 1950s, and by the 1970s was being somewhat replaced for these traditional uses by pentachlorophenol.  Today, these oil-borne preservatives only constitute 17% of Canadian treated wood production.

The remaining 83% of production uses water-borne preservatives such as CCA, ACQ, CA and MCA.  The industry began its substantial shift to the water borne products in the 1970s, as consumer interest in decks and other residential outdoor structures dramatically increased.  For many years, CCA was by far the dominant preservative for both residential and industrial applications.

In 2004, CCA regulations were changed such that CCA is no longer available for many residential applications.  Subsequently, Canadian treaters have shifted about 80% of their previous CCA production to ACQ, CA or MCA.

Most of Canada’s treated wood is used domestically; Canada exports only 10% of its production. Canada has its own wood preservation standards, supports several technical and marketing organizations, and maintains a lead position in certain areas of wood preservation research.  A major focus of the industry has been in response to increasing levels of health and environmental protection regulations.

More Information

For information on fasteners:

MiTek www.mitek.ca

Simpson Strong Tie

International Staple, Nail, And Tool Association

http://www.isanta.org/

Preservative supplier links

https://woodpreservation.ca

http://www.goodfellowinc.com/

http://www.uspconnectors.com/  

http://www.strongtie.com/ 

http://www.isanta.org/ 

 

Surface Pre-treatment

Liquid application: Dip diffusion treatment of green (wet) lumber

Dip-diffusion treatment involves immersion of freshly cut lumber, still wet from the tree, in a concentrated solution of preservative. The preservative may be thickened to increase the amount of solution retained on the surface. The lumber is stacked, covered and stored for periods of weeks to allow the preservative to diffuse deep into the wood. In New Zealand, framing lumber has been treated with borates using this process since the 1950s. Dip-diffusion works well with wood species that are mostly sapwood or have wet heartwood. The ratio of the surface area to the volume, the amount of solution retained on the surface, and the solubility of the preservative limit the amount of chemical that can be delivered deep into the wood using this process. For example, a boric acid loading of 0.5% by weight of the wood, sufficient to prevent decay and beetle attack, can be applied to nominal 2 inch lumber using this process. However, a boric acid loading of 2.0% by weight, sufficient to prevent attack by Formosan termites, cannot be achieved without multiple dips and months of storage.

Liquid application: Spray treatment of framing

Since this type of treatment is typically done during the construction phase, it can be applied to the whole structure or to selected parts of the structure that are anticipated to be at risk from fungal decay or insect attack. Solids and fumigants are not appropriate for these applications, and the only widely used formulations are based on borates. Because the wood is dry at this stage, and because borates require moisture for diffusion, it helps if such treatments are formulated to improve penetration in dry wood. This is usually achieved by adding glycols. Nevertheless, the initial preservative penetration cannot be expected to be as good as that provided by a pressure treatment process. Spray applications of borate are becoming popular in certain regions of the USA as part of termite management systems. Typically, whole house superficial treatments are used to protect against drywood termites and wood boring beetles. This replaces regular fumigation. For subterranean termite protection, concentrated glycol borates may be applied to the bottom two feet of all wood in contact with the slab or, for crawl space construction, two feet up and inwards from the foundation. This replaces a soil barrier.

Brush Application

Brush applications for surface pre-treatment are basically limited to field-cut preservatives for pressure treated wood and homeowner treatment of structures, presumably with limited life expectancy. Copper naphthenate works well above ground or in ground contact, but its dark green colour (fading to brown after a year or so) is not very appealing. Zinc naphthenate is colourless and can be tinted to suit, but does not work as well in ground contact. Borates are typically used for field cuts on interior sill plates. In addition, borate/glycol mixtures are available for domestic use.

  • Fire Ratings, Fire Resistance
fire-retardant-treated wood

Fire-Retardant-Treated Wood

“Fire-retardant treated wood” (FRTW), as defined by the National Building Code of Canada (NBC), is ‘…wood or a wood product that has had its surface-burning characteristics, such as flame spread, rate of fuel contribution and density of smoke developed, reduced by impregnation with fire-retardant chemicals.’ FRTW must be pressure impregnated with fire-retardant chemicals in accordance with the CAN/CSA-O80 Series of Standards, Wood Preservation and when fire-tested for its surface flammability, must have a flame spread rating not more than 25.

Fire-retardant chemical treatments applied to FRTW retard the spread of flame and limit smoke production from wood in fire situations. FRTW products are harder to ignite than untreated wood products and preservative treated wood products.

Fire-retardant treatments applied to FRTW enhances the fire performance of the products by reducing the amount of heat released during the initial stages of fire. The treatments also reduce the amount of flammable volatiles released during fire exposure. This results in a reduction in the rate of flame spread over the surface. When the flame source is removed, FRTW ceases to char.

FRTW contains different chemicals than preservative treated wood. However, the same manufacturing process is used to apply the chemicals. FRTW must be kiln-dried after treatment to a moisture content of 19% for lumber and 15% for plywood.

The fire-retardant treatments used in FRTW do not generally interfere with the adhesion of surface paints and coatings unless the FRTW has an increased moisture content. The finishing characteristics of specific products should be discussed with the manufacturer.

Typical interior applications of FRTW include architectural millwork, paneling, roof assemblies/trusses, beams, interior load bearing and non-load bearing partitions. Exterior-type fire retardants use different chemical formulations from those used for interior applications, since they must pass an accelerated weathering test (ASTM D2898), which exposes FRTW to regular wetting and drying cycles to represent actual long-term outdoor conditions. Generally, exterior-type fire retardants are applied to shingles and shakes.

FRTW can be crosscut to length (not ripped) and drilled for holes following treatment without reducing its effectiveness. End cuts in the field, whether exposed or butt jointed, do not require treatment, since any untreated areas are relatively small compared to the total surface area and the flame spread rating remains unaffected. Plywood can be both crosscut and ripped without concern, since the chemical treatment has penetrated throughout the individual layers/plys.

FRTW is not excessively corrosive to metal fasteners and other hardware, even in areas of high relative humidity. In fact, testing has demonstrated that FRTW is no more corrosive than untreated wood.

 

Exterior use of FRTW
Fire retardant coatings
Fire-retardant-treated wood roof systems
Flame-spread rating

 

For more information on FRTW, visit the manufacture’s websites:

Arch Wood Protection, Lonza: www.wolmanizedwood.com

Viance LLC: www.treatedwood.com

  • dimension lumber, lumber

Preservative Treated Wood

Preservative-treated wood is surface coated or pressure impregnated with chemicals that improve the resistance to damage that can result from biological deterioration (decay) due to the action of fungi, insects, and microorganisms. Preservative treatment offers a means for improving the resistance and extending the service life of those wood species which do not have sufficient natural resistance under certain in-use conditions. It is possible to extend the service life of untreated wood products by up to ten times through the use of preservative treatment.

Preservative-treated wood can be used for exterior structures that require resistance to fungal decay and termites, such as: bridges, utility poles, railway ties, docks, marinas, fences, gazebos, pergolas, playground equipment, and landscaping.

Four factors are necessary to sustain life for wood destroying fungi; a suitable food supply (wood fibre), a sustained minimum wood moisture content of about 20 percent (common for exterior use conditions), exposure to air, and a favourable temperature for growth (cold temperatures inhibit, but do not eliminate fungi growth). Preservative treatment is effective because it removes the food supply by making it poisonous to the fungi and wood destroying insects such as termites.

An effective wood preservative must have the ability to penetrate the wood, neutralize the food supply of fungi and insects, and be present in sufficient quantities in a non-leachable form. Effective preservatives will also kill existing fungi and insects that might already exist in the wood.

There are two basic methods of treating wood; with and without pressure. Non-pressure methods include the application of preservative by brushing, spraying or dipping the piece of wood. These superficial treatments do not result in deep penetration or large absorption of preservative and are typically restricted to field treatment during construction. Deeper and more thorough penetration is achieved by driving the preservative into the wood cells with pressure. Various combinations of pressure and vacuum are used to force adequate levels of chemical into the wood.

For a wood preservative to function effectively it must be applied under controlled conditions, to specifications known to ensure that the preservative-treated wood will perform under specific in-use conditions. The manufacture and application of wood preservatives are governed by the CSA O80 series of standards. CSA O80 provides information on the wood species that may be treated, the types of preservatives and the retention and penetration of preservative in the wood that must be achieved for the use category or application. To ensure that the specified degree of protection will be provided, a preservative-treated wood product may bear a stamp indicating the suitability for a specific use category.

Wood preservatives in Canada are governed by the Pest Control Products Act and must be registered with the Pest Management Regulatory Agency (PMRA) of Health Canada. Common types of wood preservatives that are used in Canada include chromated copper arsenate (CCA), alkaline copper quaternary (ACQ), copper azole (CA), micronized copper azole (MCA), borates, creosote, pentachlorophenol, copper naphthenate and zinc naphthenate.

 

Acid salts can lessen the strength of wood if they are present in large concentrations. The concentrations used in preservative-treated wood are sufficiently small so that they do not affect the strength properties under normal use conditions. In some cases, the specified strength and stiffness of wood is reduced due to incising of the wood during the pressure impregnation process (refer to CSA O86 for further information on structural design reduction factors).

Hot dipped galvanized or stainless steel fasteners and connection hardware are usually required to be used in conjunction with preservative-treated wood. There may be additional materials, such as polymer or ceramic coatings, or vinyl or plastic flashings that are suitable for use with preservative-treated wood products. The manufacturer should be consulted prior to specification of fasteners and connection hardware.

 

For further information, refer to the following resources:

www.durable-wood.com

Wood Preservation Canada

Canadian Wood Preservation Association

CSA O80 Series Wood preservation

CSA O86 Engineering design in wood

Pest Management Regulatory Agency of Health Canada

American Wood Protection Association

Treatment during engineered wood product manufacture
The Importance of Proper Specification: Agricultural, Commercial, and Industrial Applications for Pressure Treated Wood
Environmental Issues
Assessing and Restoration of Decay
Choosing and Applying Exterior Wood Coatings
Finishing Quick Tips
Durability Solutions
Supplemental Treatment
Fasteners
Surface Pre-treatment
fire-retardant-treated wood
Fire-Retardant-Treated Wood
Preservative Treated Wood
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