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.

The treated wood you buy at your local building supplies store will have an end tag to help you choose the right product. The tag identifies the type of preservative used, the amount retained, the appropriate use for this piece of wood, and the treatment plant name and location. The most important information to look for is the use class. If the piece is going in the ground (e.g. a fence post), you need the piece to be treated for “ground contact.” All other uses (such as fence boards, deck boards and shingles) can be labeled “above ground.” The piece may also be tagged with consumer safety information. You might also find this information in the store, either posted or as a technical brochure.

The National Building Code of Canada (NBC) contains requirements regarding the use of treated wood in buildings and the CSA O80 Series of standards is referenced in the NBC and in provincial building codes for the specification of preservative treatment of a broad range of wood products used in different applications. The first edition of CSA O80 was published in 1954, with eleven subsequent revisions and updates to the standard, with the most recent edition published in 2015.

The manufacture and application of wood preservatives are governed by the CSA O80 Series of standards. These consensus-based standards indicate the wood species that may be treated, the allowable preservatives and the retention and penetration of preservative in the wood that must be achieved for the use category or application. The CSA O80 Series of standards also specifies requirements related to the fire retardance of wood through chemical treatment using both pressure and thermal impregnation of wood. The overarching subjects covered in the CSA O80 Series of standards also include materials and their analysis, pressure and thermal impregnation procedures, and fabrication and installation.

Canadian standards for wood preservation are based on the American Wood Protection Association (AWPA) standards, modified for Canadian conditions. Only wood preservatives registered by the Canadian Pest Management Regulatory Agency are listed.

The required preservative penetrations and loadings (retentions) vary according to the exposure conditions a product is likely to encounter during its service life. Each type of preservative has distinct advantages and the preservative used should be determined by the end use of the material.

Processing and treating requirements in the CSA O80 Series are designed to assess the exposure conditions which pressure treated wood will be subjected to during the service life of a product. The level of protection required is determined by hazard exposure (e.g., climatic conditions, direct ground contact or exposure to salt water), the expectations of the installed product (e.g., level of structural integrity throughout the service life) and the potential costs of repair or replacement over the life cycle.

The technical requirements of CSA O80 are organized in the Use Category System (UCS). The UCS is designed to facilitate selection of the appropriate wood species, preservative, penetration, and retention (loading) by the specifier and user of treated wood by more accurately matching the species, preservative, penetration, and retention for typical moisture conditions and wood biodeterioration agents to the intended end use.

The CSA O80.1 Standard specifies the following Use Categories (UC) for treated wood used in construction:

• UC1 covers treated wood used in dry interior construction;
• UC2 covers treated wood and wood-based materials used in dry interior construction that are not in contact with the ground but can be exposed to dampness;
• UC3 covers treated wood used in exterior construction that is not in ground contact;
  • UC3.1 covers exterior, above ground construction with coated wood products and rapid run off of water;
  • UC3.2 covers exterior, above ground construction with uncoated wood products or poor run off of water;
• UC4 covers treated wood used in exterior construction that is in ground or freshwater contact;
  • UC4.1 covers non-critical components;
  • UC4.2 covers critical structural components or components that are difficult to replace;
• UC5A covers treated wood used in Coastal waters including; brackish water, salt water and adjacent mud zone.

This CSA O80 Series of standards consists of the following standards, as follows:

1. CSA O80.0 General requirements for wood preservation; specifies requirements and provides information applicable to the entire series of standards.
2. CSA O80.1 Specification of treated wood; is intended to help specifiers and users of treated wood products identify appropriate requirements for preservatives for various wood products and end use environments.
3. CSA O80.2 Processing and treatment; specifies minimum requirements and process limitations for treating wood products.
4. CSA O80.3 Preservative formulations; specifies requirements for preservatives not referenced elsewhere.
5. CSA O80.4 has been withdrawn.
6. CSA O80.5 CCA Additives — Utility Poles; specifies requirements for preparation and use of CCA preservative/additive combinations for utility poles permitted by CSA O80.1 and CSA O80.2.

For further information, refer to the following resources:

www.durable-wood.com

CSA O80 Wood preservation

Wood Preservation Canada

National Building Code of Canada

Pest Management Regulatory Agency

American Wood Protection Association

ISO 21887 Durability of wood and wood-based products — Use classes

Click here for more information on performance tests done with treated wood.

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.

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.

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.

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.

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.

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.

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.

Wood Storage And Handling

Wood is a long-lasting, economical and renewable resource that is the building material of choice for North American housing. This is due, in large part, to the proven performance of properly designed and constructed wood-frame buildings that have provided strong and lasting housing for many people. Although wood can withstand much, attention must be paid to storage and handling in order for the material to perform according to expectations. Managing moisture in structural wood products is essential to controlling the swelling and shrinkage of the wood and in the prevention of problems directly associated with such contact.

On Site Moisture Management

Moisture management during construction has become more and more important with the increase in building height and area (which potentially prolongs the exposure to inclement weather), and the overall increase in the speed of construction which may not allow adequate time for drying to occur. In addition, the drying capacities of modern assemblies may have reduced resulting from increased insulation levels to meet more stringent energy efficiency requirements, or the use of membrane or insulation products with low vapour permeance. Adequate on-site moisture protection is challenging given the range of possible moisture exposure conditions and the inevitable site and cost constraints of a construction project.

More Information

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.

Since remedial treatment is intended to solve a known insect or decay problem, the first thing to do is investigate the extent of the problem and, if necessary, provide temporary structural support. The investigation phase should also identify the causal factors so that these can be eliminated, where possible. Also during the investigation, the parts of the wood that have lost strength may be removed. Be aware that a wood decay fungus may have penetrated well beyond the boundaries of the visibly rotted wood. Since deterioration is underway, a rapid response is normally required. This means that where the deteriorated and infected wood cannot be removed and replaced with sound wood, the remedial treatment must be capable of rapidly penetrating the wood and killing the fungi or insects.

Solids

Since solids take time to dissolve and move, they are commonly supplemented by liquid treatments for more rapid eradication of the decay fungus or insect. Borate and copper/borate rods are the only solid remedial treatment method available to the homeowner.

Liquids, Pastes and Gels

Liquids, pastes and gels work rapidly as they do not have to rehydrate or dissolve to start moving and working. Since all visibly decayed wood should be removed wherever possible, these treatments are often used primarily to kill and contain any residual infection inadvertently left behind. Brush or spray applications are quite appropriate for this use. Gels are commonly applied to paint cracks in window joints and to the bottom of door frames, locations where moisture may get into the wood. Where decayed wood is present inside poles and timbers and cannot be removed, liquids, pastes or gels must be inserted deep into the wood for rapid action.

Fumigants

Gases move the most rapidly and therefore have a faster eradicant action.

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.

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.