60Sometimes 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.

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.

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 (Figure 1). Ancient temples in Japan and China dating back thousands of years are basically heavy timber construction with some components semi-exposed to the weather (Figure 2). 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 (Figure 3). In addition wood components are being increasingly used to soften the exterior look of non-wood buildings and make them more appealing (Figure 4). 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.

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Disaster Relief Housing

Shelter needs after natural disasters come in three phases:

1. Immediate shelter: normally supplied by tarpaulins or light tents
2. Transition shelter: may be heavy-duty tents or more robust medium-term shelters.
3. 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.

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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 in loss of life and loss of access to communities. Durability is as critical as engineering to ensure safe use of timber bridges for the design life, typically 75 years in North America.

There are numerous examples of old wood bridges still in service in North America (Figure 1). The oldest are traditional covered bridges (Figure 2), three of which are around 190 years old. In Southeast China, Fujian and Zhejiang provinces have numerous covered bridges that are almost 1000 years old (Figure 3). The fact that these bridges are still standing is a testament to the craftsmen that selected the materials, designed the structures, built them, monitored their condition and kept them maintained and repaired. They would have selected the most durable wood species available, likely Chestnut or cedars in North America, china fir (china cedar) in southeast China. They would have adzed off the thin perishable sapwood exposing only the naturally durable heartwood. The fact the covered bridges around today all look similar is because those were the tried and tested designs that worked. They clearly designed those bridges to shed water with a wood shingle roof, vertical siding projecting below the deck and structural elements sheltered from all but the worst wind-driven rain. Any rain that did not drip off the bottom of the vertical siding and wicked up the end grain would also dry out reasonably rapidly. Slow decay that did occur at the bottom of these boards was inconsequential because it was remote from connections to structural elements. Construction must have been meticulously performed by experienced craftsmen. Those craftsmen may well have been locals that would continue to monitor the bridge over its life and make any repairs necessary. Of course, not every component in those ancient bridges is original, particularly shingle roofs that typically last 20-30 years depending on climate. These bridges have all been repaired due to decay and in some cases dismantled and re-built over the years for various reasons (e.g., due to changes in traffic loads, arson, flooding, fire, hurricanes, etc.). The Wan’an Bridge in Fujian is known to have been built in 1090, refaced in 1708 and rebuilt in 1845, 1932 and 1953. The apparently increasing frequency of rebuilding may suggest a loss of knowledge and skills, but all repairs and reconstruction prior to 1845 may not have been recorded.

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Permanent Wood Foundations

A permanent wood foundation (PWF) is a strong, durable and proven construction method that has a number of unique advantages over other foundation systems for both the builder and the homeowner. The first Canadian examples were built as early as 1950 and are still being used today. PWFs can also be designed for projects such as crawl spaces, room additions and knee-wall foundations for garages and mobile homes. Concrete slab-on-grade, wood sleeper floors and suspended wood floors can all be used with PWFs.

A permanent wood foundation is an in-ground engineered construction system designed to turn a home’s foundation into useable living space. A below-grade stud wall constructed of preservative treated plywood and lumber supports the structure and encloses the living space. PWFs are suitable for all types of light-frame construction covered under Part 9 (Housing and Small Buildings) of the National Building Code of Canada, under clauses 9.15.2.4.(1) and 9.16.5.1.(1). This includes single-family detached houses, townhouses, low-rise apartments, and institutional and commercial buildings. In addition, the recently revised CSA S406 standard, Specification of permanent wood foundations for housing and small buildings, allows for three-storey construction supported by PWF.

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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.

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.

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.

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

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.