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As prefabrication and hybrid timber systems become more widely adopted, tolerance coordination has emerged as a critical factor in project success. While components may meet material standards and fabrication targets, misalignment between design intent, manufacturing capability, and site conditions can still lead to fit-up issues, delays, and rework. Understanding Tolerances in Prefabricated Timber Construction introduces a practical framework to help multidisciplinary project teams better define, communicate, and manage tolerances across all stages of a project—from design and fabrication to installation and in-service performance. The publication outlines four core tolerance classes—Material Specification Limits, Standard Manufacturing Capabilities, Framing/System Deviations, and Installation Allowances—and explains how these interact in real-world construction. It also introduces key concepts such as Clearance Fits, designed fitment gaps, Critical to Fit (CTF) features, and Critical Dimensions (CDs), providing a structured approach to improving constructability at critical interface zones. A step-by-step workflow is included to help teams translate broad standards into clearly defined fitment strategies, aligning design intent with manufacturing reality and site execution. This resource is intended for architects, engineers, manufacturers, contractors, and developers working with prefabricated and hybrid timber systems. By establishing a shared language around tolerances, it supports better coordination, reduced risk, and more predictable project outcomes.

Throughout history, wherever wood has been available as a resource, it has found favour as a building material for its durability, strength, cost-competitiveness, ease-of-use, sustainability, and beauty. Wood-frame and timber buildings have an established record of long-term durability. From the ancient temples of China and Japan built in the 1000s, and the great stave churches of Norway to the numerous North American buildings built in the 1800s, wood construction has proven it can stand the test of time. Although wood building technology has been changing over time, wood’s natural durability properties will continue to make it the material of choice. This website helps designers, construction professionals, and building owners understand what durability hazards exist for wood, and describes durability solutions that ensure wood, as a building material, will perform well for decades, and even centuries, to come. Durability Guidelines Wood structures, properly designed and properly treated, will last indefinitely. This section includes guidance on specific applications of structures that have constant exposure to the elements. Mass timber exteriors Modern Mass Timber Construction includes building systems otherwise known as post-and-beam, or heavy-timber, and cross laminated timber (CLT). Typical components include solid sawn timbers, glue-laminated timbers (glulam), parallel strand lumber (PSL) laminated veneer lumber (LVL) laminated strand (LSL), and CLT. Heavy-timber post and beam with infill walls of various materials is one of the oldest construction systems known to man. Historic examples still standing range from Europe through Asia to the long-houses of the Pacific Coastal first nations. Ancient temples in Japan and China dating back thousands of years are basically heavy timber construction with some components semi-exposed to the weather. Heavy-timber-frame warehouses with masonry walls dating back 100 years or more are still serviceable and sought-after as residences or office buildings in cities like Toronto, Montreal and Vancouver (Koo 2013). Besides their historic value, these old warehouses offer visually impressive wood structures, open plan floors and resultant flexibility of use and repurposing. Building on this legacy, modern mass timber construction is becoming increasingly popular in parts of Canada and the USA for non-residential construction, recreational properties and even multi-unit residential buildings. Owners and architects typically see a need to express these structural materials, particularly glulam, on the exterior of the building where they are at semi-exposed to the elements. In addition wood components are being increasingly used to soften the exterior look of non-wood buildings and make them more appealing. They are anticipated to remain structurally sound and visually appealing for the service life. However, putting wood outside creates a risk of deterioration that needs to be managed. Similar to wood used for landscaping, the major challenges to wood in these situations are decay, weathering and black-stain fungi. This document provides assistance to architects and specifiers in making the right decisions to maximize the durability and minimize maintenance requirements for glulam and other mass timber on the outside of residential and non-residential buildings. It focusses on general principles, rather than providing detailed recommendations. This is primarily focussed on a Canadian and secondarily on a North American audience. Click here to read more Disaster Relief Housing Shelter needs after natural disasters come in three phases: Immediate shelter: normally supplied by tarpaulins or light tentsTransition shelter: may be heavy-duty tents or more robust medium-term shelters.Permanent buildings: Ultimately permanent shelters need to be constructed when the local economy recovers. Immediate and transition shelters are typically supplied by aid agencies. Light wood frame is ideal for rapid provision of medium- to long-term shelter after natural disasters. However, there are challenges in certain climates for wood frame construction that must be addressed in order to sustainably and responsibly build them. For example, many of the regions which experience hurricanes, earthquakes and tsunamis also have severe decay and termite hazards including aggressive Coptotermes species and drywood termites. In extreme northern climates, high occupancy loads are common and when combined with the need for substantial thermal insulation to ensure comfortable indoor temperatures, can result in condensation and mould growth if wall and roof systems are not carefully designed. The desire of aid organizations to maximize the number of shelters delivered tends to drive down the allowable cost dictating simplified designs with fewer moisture management features. It may also be difficult to control the quality of construction in some regions. Once built, “temporary” structures are commonly used for much longer than their design life. Occupier improvements over the longer term can potentially increase moisture and termite problems. All of these factors mean that the wood used needs to be durable. One method of achieving more durable wood products is by treating the wood to prevent decay and insect/termite attack. However, commonly available preservative treated wood in Canada may not be suitable for use in other countries. Selection of the preservative and treatment process must take into account the regulations in both the exporting and receiving countries, including consideration of the potential for human contact with the preserved wood, where the product will be within the building design, the treatability of wood species, and the local decay and termite hazard. Simple design features, such as ensuring wood does not come into contact with the ground and is protected from rain, can reduce moisture and termite problems. Building with concrete and steel does not eliminate termite problems. Termites will happily forage in a concrete or masonry block buildings looking for wood components, furniture, cupboards, and other cellulosic materials, such as the paper on drywall, cardboard boxes, books etc. Mud tubes running 10ft over concrete foundations to reach cellulosic building materials have been documented. Indeed, termites have caused major economic damage to cellulosic building materials even in concrete and steel high-rises in Florida and in southern China. Timber bridges Timber bridges are an excellent way to showcase the strength and durability of wood structures, even under harsh conditions, when material selection, design, construction and maintenance are done well. They could also be critical infrastructure elements that span fast rivers or deep gorges. Consequences of failure of these structures can be severe

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.

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

Wood is resistant to some of the chemicals destructive to steel and concrete. For example, wood is often the material of choice when exposed to: organic compounds, hot or cold solutions of acids or neutral salts, dilute acids, industrial stack gases, sea air and high relative humidity. Because of its resistance to chemicals wood is often used in the following applications: Potash storage buildings Salt storage domes Cooling towers Industrial tanks for various types of chemicals With thoughtful design and careful workmanship wood bridges prove to be remarkably durable. Throughout the world, there are numerous examples of long lasting wooden bridges – both historic and modern. Modern bridge decks are subjected to relentless attack of de-icing chemicals, and wood is gaining acceptance as a viable option for these applications. Pilings that are constantly submerged in fresh water have been known to last for centuries. Foundation piles under structures will not decay if the water table remains higher than the pile tops. Many of the world’s important structures are built on wood piles including much of the city of Venice and the Empire State Building in New York.

There’s no reason a wood structure can’t last virtually forever – or, at least hundreds of years, far longer than we may actually need the building. With a good understanding of how to protect wood from decay and fire, we can expect today’s wood buildings to be around for as long as we wish. While wood does not have the historical longevity of stone, there nonetheless remain standing some very old wood buildings. In Europe, wood was long a dominant building material dating back to the beginning of civilisation. Most of these ancient buildings are long gone, lost to fire, decay, or deconstruction for another purpose. In the early days of wood construction, the primary structural components were placed directly in the ground, which eventually leads to decay. It was not until sometime in the 1100s that builders began to use stone footings – thus our still-standing examples of wood buildings generally date from no earlier than that time. Perhaps the most famous ancient European wood buildings still in evidence today are the Norwegian stave churches, hundreds of which were built in the 12th and 13th centuries and of which 25-30 still remain today. Their exterior claddings have typically been replaced, but the structural wood is original.               In North America, the abundance of wood and the existing timber skills of early settlers led to widespread use of wood – wood has always been and still is the primary structural material for small buildings here. The oldest surviving wood homes in the US date to the early 1600s. Nearly 80 homes remain from this era in the New England states.               Many other North American wood buildings survive from the 18th century. Even in the demanding climate of Louisiana, where hot and humid conditions present a challenge for wood durability, one can still find some of the original French settlements dating to the first half of the 1700s. And of course, there are countless standing wood buildings from the 1800s and early 1900s, most of which are probably still occupied.               Japan has a well-known history of wood use and is the home of the oldest surviving wood structure in the world, a Buddhist temple near the ancient capital city of Nara. The Horyu-ji temple is believed to have been built at the beginning of the eighth century (c. 711) and possibly even earlier, as one of the hinoki (Japanese cypress) posts appears to have been felled in the year 594. This temple’s longevity is largely helped by careful maintenance and repair. This entire region of Japan has many other ancient wood buildings still standing.                 For modern buildings, we don’t normally require such exceptional longevity. The life of a typical North American house is no more than 100 years (the average is lower), and our non-residential buildings are usually demolished in 50 years or less. Wood is perfectly suitable for these lifetime expectations. Click here for survey data showing that wood buildings last as long, or longer than buildings made of other materials. Reference: Architecture in Wood: A History of Wood Building and Its Techniques in Europe and North America. Hans Jrgen Hansen, Ed., Faber and Faber, London, 1971.. Case Studies 1865 House, Vancouver BC         Irving House is a large, one and one-half storey plus basement wood-frame residence, designed in the Gothic Revival style, located on its original site at the corner of Royal Avenue and Merivale Street in the New Westminster neighbourhood of Albert Crescent. Irving House is remarkable for the extent to which its original exterior and interior elements have been maintained. Operated as an historic house museum, it also includes a collection of many original furnishings from the Irving family. Irving House Location 302 Royal Avenue, New Westminster, B.C. Completion of Construction 1865 Other Information Original owner – Captain William and Elizabeth Jane Irving Current Status Heritage of New Westminster Construction Method Platform-Frame Style Gothic Revival style Framing 2-inch Douglas Fir lumber Cladding Wide lapped Redwood weatherboard siding and wooden trim Comdition No signs of decay on any framing members Major Repair 1880 By courtesy of New Westminster Museum and Archives, New Westminster, British Columbia Other link: http://www.flickr.com/photos/bobkh/297751638/in/set-72157594340707368/ 1912 House, Vancouver BC         This classic turn-of-the-century home was slated for demolition in 1990. It was already stripped back to the bare framing when it was purchased by a new owner who wished to convert it into apartments. At the new owner’s request, the building was inspected by Dr. Paul Morris of Forintek in 1991 for signs of deterioration. After 80 years in service there were no signs of decay on any of the framing members nor the window frames, most of which were original. 1912 House Location Vancouver Date of Construction 1912 (estimated) Original Records Water service 1909 On City File 1915 Other Information Original owner – Henry B. Ford Current Status Vancouver Heritage Resource Inventory Construction Method Platform-Frame Style Heritage, with multiple pitched roofs & wide overhangs Framing Rough green full 2-inch Douglas Fir lumber Sheathing Rough green Douglas Fir boards Building Paper Asphalt-impregnated paper Cladding Western Red Cedar shakes Western Red Cedar siding Roofing Western Red Cedar shakes (new in 1991) Condition No signs of decay on any framing members Temple at Nara, Japan The Horyuji Buddhist temple at Nara is probably the oldest wooden structure in the world. Nara became the first permanent capital of Japan in 710.           Horyuji Buddhist temple at Nara Location Nara, Japan Date of Construction 670 – 714 (Estimated) Original Records Built on site of original temple from 607 Other Information Original owner – Prince Shotoku Current Status World Cultural Heritage Building Construction Method Heavy Timber Style 2-inch Douglas-fir lumber Framing Hinoki (Durable – Japanese cypress) Roofing Multi-tiered roof with Clay tile Condition No signs of decay on any framing members Maintenance Schedule

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                               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 Journal, 61(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 journal, 66(5), 268-273.

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).

Canada has had a wood preservation industry for about 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 65 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 and CA.  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 or CA. 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.

Since depot treatment is localized, it is critical that it be placed in the right location, which requires an understanding of how moisture may get into the structure. This can only be done when construction is complete or very near completion. At that point the degree of protection by design can be assessed and any water traps can be identified and, where possible, eliminated. The treatment can then be applied in the right location to intercept moisture close to its point of entry. Depot treatments are an excellent choice for a few common design applications such as partially exposed beams. When a beam penetrates the building envelope, only a portion is exposed to moisture and it makes sense to just treat that part. Depot treatments are especially useful for products that are not well-suited to pressure treatment with waterborne preservatives, like glulam. Similarly, depot treatments are appropriate for exposed log ends in log homes – logs that extend beyond the protective roof overhang are at risk of decay. Solids Depot treatments most commonly use a solid form of preservative. Borate, copper/borate and fluoride rods are highly suited to this end use since they are easy to install and the active ingredients only become mobile if moisture entry occurs. Other formats Pastes can be packed into drilled holes or routed grooves – log home grooves are an appropriate application. Liquid injection is less common, as this involves drilling small holes, inserting a pin nozzle injector connected to a 70 -120 psi tank/pump, and forcing preservative along the grain under pressure. A series of such holes is required, particularly for large dimensions, to increase loading. Less suited to depot treatments, fumigants have not, to our knowledge, been used in these applications.

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.

Treatability of Major North American Softwoods Some wood is easier to treat than others. The particular structure of the cells for a given piece of wood will determine how permeable the wood is to chemicals. This table describes the permeability of common softwoods used in North America. The permeability ratings are: 1 – Permeable 2 – Moderately Impermeable 3 – Impermeable 4 – Extremely Impermeable Tree Permeability Permeability Predominant in the Tree   Sapwood Heartwood   Douglas Fir 2 4 Heartwood  Western Hemlock 2 3 Heartwood Eastern Hemlock 2 4 Heartwood White Spruce 2 3-4 Heartwood Engelmann Spruce 2 3-4 Heartwood Black Spruce 2 4 Heartwood Red Spruce 2 4 Heartwood Sitka Spruce 2 3 Heartwood Lodgepole Pine 1 3-4 Heartwood Jack Pine 1 3 Heartwood Red Pine 1 3 Sapwood Southern Pine 1 3 Sapwood Ponderosa Pine 1 3 Sapwood Amabilis Fir (Pacific silver fir) 2 2-3 Heartwood Alpine Fir 2 3 Heartwood Balsam Fir 2 4 Heartwood Western Red Cedar 2 3-4 Heartwood Eastern White Cedar 2 3-4 Heartwood Yellow Cypress 1 3 Heartwood Western S-P-F 2 3-4 Heartwood Eastern S-P-F 2 4 Heartwood Hem-Fir 2 3 Heartwood Western Larch 2 4 Heartwood Tamarack 2 4 Heartwood Incising We can improve the penetration of preservative into impermeable wood by making little cuts in the wood. A series of small, shallow slits are cut into the wood by an incising machine. This is an effective way of increasing the treatability of lumber pieces which are predominantly heartwood. Species with heartwood permeability ratings of higher than 3 require high density incising (over 7,500 incisions per square meter). Incising does reduce the strength of lumber and this effect must be taken into account in engineering calculations. Drying to Maximise Treatabilty Unless the purchaser can be assured that lumber for treatment will be air dried to less than 30% moisture content, the specification of KD lumber for preservative treatment is strongly recommended. The problem with treating lumber which is not kiln dried is that the practicalities of production and delivery lead to the potential for poor product quality. The durability of treated Canadian lumber relies on a shell of preservative treatment preventing access by wood-rotting fungi to the untreated core. If the treated shell fails to prevent penetration by checks or abrasion or if the wood-rotting fungus is already in the untreated core, premature failure can result. There are four major pitfalls in treating green lumber: saturated sapwood, frozen lumber, check development and pre-treatment infection. Saturated Sapwood In order for the preservative to penetrate the wood cells, they must be empty of water, that is, the wood must be below 30% moisture content. In green lumber the sapwood cells may be too full of sap to accept any preservative. The sapwood is the part most susceptible to decay and most in need of preservative penetration. Partial air or kiln drying to between 20 and 30% moisture content is ideal, but there is seldom the time or the conditions necessary to do this. Purchasing commercial KD material (maximum 20%) is normally the only option to ensure the sapwood will accept treatment. Frozen Lumber The overwhelming majority of production is treated over the winter to prepare for the spring and summer outdoor construction season. With the exception of coastal British Columbia, most regions of Canada will be dealing with frozen wood at this time. Many treating plants do not have dry kilns, thus material is treated in the condition it is delivered to the plant. Preservative will not penetrate through ice until it is fully thawed. This typically occurs in contact with the treating solution. Frozen green lumber contains a lot of ice and there is insufficient time for this to thaw during typical commercial treating cycles. The residual moisture (12 – 20%) in kiln-dried lumber is in the cell walls and will not impede preservative penetration even if it is frozen. Check Development Checks only develop when the moisture content of wood drops below about 28%. If lumber is treated green and then dries, checks will penetrate the treated zone exposing the untreated core. If lumber is kiln-dried to the in-service moisture content, typically around 16% in exterior exposure, the checks will be largely developed prior to treatment. This means that the checks will be lined with a treated zone and the shell of treatment will remain intact. Pre-treatment Infection A lesser problem than the above three, but still of some concern, is the potential for survival through the manufacturing process of wood-rotting fungi that may have infected in the tree, log or lumber storage stages. At worst, this might only apply to 10% or fewer of pieces. Nevertheless, we have seen examples where treatment of green lumber without application of heat (60°C or more) fails to kill wood-rotting fungi already in the product, leading to premature failure in service. This can occur in as little as 4 years. CCA treatment is a cool process, but most kiln-drying schedules will kill all wood-rotting fungi.