Decay
Wood is biodegradable – that’s a characteristic we normally consider to be one of the benefits of choosing natural materials. Organisms exist that can break down wood into its basic chemicals so that fallen logs in the forest can contribute to the growth of the next generation of life. This process – essential in the forest – must be prevented when we use wood in buildings. A variety of fungi, insects, and marine borers have the capability to break down the complex polymers which make up the wood structure. In Canada, fungi are a more serious problem than insects. The wood-inhabiting fungi can be separated into moulds, stainers, soft-rot fungi and wood-rotting basidiomycetes. The moulds and stainers can discolour the wood however they do not significantly damage the wood structurally. Soft-rot fungi and wood-rotting basidiomycetes can cause strength loss in wood, with the basidiomycetes the ones responsible for decay problems in buildings. With regard to insects, carpenter ants only cause problems in decayed wood, and significant subterranean termite activity is confined to a few southern areas of Canada. However, other parts of the world have a serious problem with termites. Decayed wood is the result of a series of events including a sequence of fungal colonization. The spores of these fungi are ubiquitous in the air for much of the year. Wood-rotting fungi require wood as their food source, an equable temperature, oxygen and water. Water is normally the only one of these factors that we can easily manage. This may be made more difficult by some fungi, which can transport water to otherwise dry wood. It can also be difficult to control moisture once decay has started, since the fungi produce water as a result of the decay process. The outer portion of this log is being attacked by a decay fungus. Note that the damage is held back at the line between heartwood and sapwood. To understand why, click here to read about natural durability. More Information Click Here for a 26-page paper on biodeterioration, including illustrations and bibliography. For answers to common questions on decay, visit the FAQ page
Depot Treatment
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.
Durability – FAQ
Please refer to the pdf documents below for Frequently Asked Questions pertaining to durability: General Wood Decay and repair Discoloration Finishing Mould Treated Wood
Durability by design
“Durability by design” is the most important aspect of durable solutions. It starts with using dry wood, storing it appropriately to ensure it stays dry, and then designing the building to protect the wood or, if the wood will be exposed, designing to not accumulate moisture. It includes ensuring the building envelope is appropriately designed to shed bulk water, mitigating water and vapour from getting into the envelope, and draining water that does leak into the envelope.
Durability by nature
For outdoor applications of wood, we have a strong tradition here in North America of using our naturally durable species: Western red cedar, Eastern white cedar, yellow cypress and redwood. These are familiar choices for decks, fences, siding and roofing. These species are resistant to decay in their natural state, due to high levels of organic chemicals called extractives. Extractives are chemicals that are deposited in the heartwood of certain tree species as they convert sapwood to heartwood. In addition to providing the wood with decay resistance, extractives also often give the heartwood colour and odour. Only the heartwood has these protective deposits. The sapwood of all North American softwoods is susceptible to decay and must be protected by other means when decay resistance is required. Sapwood is the newer part of the tree, closer to the bark. It needs no decay protection in the live tree because wound responses keep out any invading organisms. The heartwood is the inner, older part of the tree and is no longer alive. Heartwood is often visibly distinguishable from sapwood by colour (heartwood is generally darker), but not in all species. However, even if you’re sure you have heartwood of a durable species, you may not have the level of resistance you think. Decay resistance is often highly variable, and may be lower in plantation-grown trees. There is currently no way to reliably estimate the durability of a piece of naturally durable heartwood. More Information Click Here for a table showing natural durability rankings of common softwood species.
Durability by treatment
Treating Methods There are two basic methods of treating: with and without pressure. Non-pressure methods are the application of preservative by brushing, spraying or dipping the piece to be treated. These are superficial treatments that do not result in deep penetration or large absorption of preservative. Their use is best restricted to field treatment during construction (for example, when a pressure-treated piece of lumber must be field cut), to cases where only part of a piece is to be treated, to manufacturing processes for strand-based wood products, to surface protection against moulds or to remedial treatment of wood in place. For example, mixtures of borate and glycols are used to treat sound wood left in place during repair of decay problems. The glycol helps the borate to penetrate dry wood, arresting the activity of any fungus which contacts it. The penetration of the preservative is still limited, and the most important function is to prevent undetected fungus left in place from spreading to sound wood. Deeper, more thorough penetration is achieved by driving the preservative into the wood cells with pressure. Various combinations of pressure and vacuum are used to force adequate levels of chemical into the wood. Pressure-treating preservatives consist of chemicals carried in a solvent. The solvent, or carrier, is either water or oil. Oilborne preservatives are largely used for treating industrial products such as railway ties, utility poles and bridge timbers, and for protection of field cuts. Waterborne preservatives are more widely used in residential markets due to the absence of odour, the cleaner wood surface and the ability to paint or stain the wood product. When a wood product will be used in an application known to present a risk, for example outdoors, pressure-treatment is recommended. Types of Preservatives The mostly commonly used wood preservatives in North America for residential construction are waterborne copper-based systems, including alkaline copper quaternary (ACQ), copper azole (CA) and micronized copper azole (MCA). Wood treated with these preservatives has a natural green hue, though this may be masked by the use of colourants that most often give the treated wood a mid-brown colour. Copper is the primary biocide in these systems. ACQ also contains quaternary ammonium compounds that act as a co-biocide to protect against copper-tolerant organisms. Similarly, CA and MCA contain tebuconazole to protect against these organisms. Chromated copper arsenate (CCA) was heavily used in residential construction until 2004 when its use in most residential applications was phased out. It is now largely limited to industrial applications, but can still be used in a few residential applications such as shakes and shingles and permanent wood foundations. Ammoniacal copper zinc arsenate (ACZA) can also be used in most of these applications, but is primarily favoured for treating Douglas-fir and for marine applications. Borates are another class of waterborne preservative used in North America. Their use is currently limited to applications which are protected from rain and other persistent sources of water. These include framing in termite areas and repair of decayed framing in leaky buildings where the main moisture source has been eliminated. Borates are also used as part of a dual treatment in conjunction with a creosote or copper naphthenate shell to protect railway ties. Metal-free waterborne preservative systems such as PTI and EL2 contain carbon-based fungicides and insecticides. Wood treated with these systems is used in residential construction in the United States, and is restricted to above-ground applications. Oilborne preservatives include creosote, pentachlorphenol, and copper- and zinc-naphthenate. Creosote is the well-known black oily wood preservative, the oldest type of preservative still in modern use. It’s now used in Canada almost exclusively for railroad ties, where its resistance to moisture movement is a key advantage. Pentachlorophenol in oil is mainly used for utility poles where the surface softening characteristics of the oil are useful in pole climbing. Copper naphthenate and zinc naphthenate are two common preservatives used for treating field cuts. Copper naphthenate is also used to treat ties and timbers in the United States. Thermal Modification The properties of wood are altered when it is exposed to high temperatures (160-260°C) under reduced oxygen conditions. Thermal modification kilns use much higher temperatures than drying kilns, and use steam (or other oxygen-excluding media) to protect the wood from degradation at these high temperatures. The resulting thermally modified wood generally has a darker colour, increased dimensional stability, and increased decay resistance. Thermal modification may reduce some mechanical properties and does not protect wood against insects. Thermally modified wood is typically used in non-structural, above-ground applications, such as siding, decking and outdoor furniture. More information from Producers of Wood Preservative Products Lonza Wood Protection Timber Specialties Viance LLC Genics Inc. Kop-Coat Rio Tinto Minerals Nisus Creosote council KMG Chemicals Wood Preservation Canada
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 (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. 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 tents Transition 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. Click here to read more 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
Durability Hazards
Moisture, Decay, and Termites Wood is a natural, biodegradable material. That means certain insects and fungi can break wood down to be recycled via earth into new plant material. Decay, also called rot, is the decomposition of organic material by fungal activity. A few specialized species of fungi can do this to wood. This is an important process in the forest. But it is obviously a process to be avoided for wood products in service. The key to controlling decay is controlling excessive moisture. Water by itself doesn’t cause harm to wood, but water enables these fungal organisms to grow. Wood is actually quite tolerant of water and forgiving of many moisture errors. But too much unintended moisture (for example, a major wall leak) can lead to a significant decay hazard. If a wood product is to be used in an application that will frequently be wet for extended periods, then measures need to be taken to protect the wood against decay. Various types of insects can damage wood, but the predominant ones causing problems are termites. Termites live everywhere in the world where the climate is warm or temperate.
Durability Research and Development
FPInnovations has been field testing the performance of treated wood products for years. Click one of these categories for performance data from our field tests. Borate-treated Wood vs. Termites 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.
Durability Solutions
Wood has been a valuable and effective structural material since the earliest days of human civilisation. With normal good practice, wood can deliver many years of reliable service. But, like other building materials, wood can suffer as a result of mistakes made in storage, design, construction, and maintenance practices. How can you ensure long life of a wood building? The best approach is always to remember that wood meant for dry application must stay dry. Start out by buying dry wood, store it carefully to keep it dry, design the building to protect the wood elements, keep wood dry during construction, and practice good maintenance of the building. This approach is called durability by design. If wood won’t stay dry, you have two choices in approach. Because wet wood is at risk of decay, you must select a product with decay resistance. One choice is to choose a naturally durable species like Western red cedar. This approach is called durability by nature. Most of our construction lumber is not naturally durable, but we can make it decay resistant by treating it with a preservative. Preservative-treated lumber is more reliably resistant to decay than naturally durable lumber. This approach is called durability by treated wood. The level of attention you give to durability issues during the course of design depends on your decay hazard. In other words, the more that your circumstances put wood at risk, the more care you must take in protecting against decay. In outdoor applications, for example, any wood in contact with the ground is at high risk of decay and should be pressure-treated with a preservative. For wood that is exposed to the weather but not in direct ground contact, the degree of hazard correlates with climate. The fungi that harm wood generally grow best in moist environments with warm temperatures. Researchers have developed hazard zones in North America using mean monthly temperature and number of rainy days. This map in particular shows the rainfall hazard and applies to exposed uses of wood such as decks, shingles and fence boards. A high degree of hazard would indicate a need to carefully choose a wood species or preservative treatment for maximum service life. In the future, building codes may provide more specific directives as a function of decay hazard. For wood not exposed to weather, such as framing lumber, this map is only moderately useful. This is because the environmental conditions in the wall may be substantially different than those outdoors.
Encapsulated mass timber construction
In addition to combustible, heavy timber and noncombustible construction, a new construction type is presently being considered for inclusion into the National Building Code of Canada (NBC). Encapsulated mass timber construction (EMTC) is proposed to be defined as the “type of construction in which a degree of fire safety is attained by the use of encapsulated mass timber elements with an encapsulation rating and minimum dimensions for the structural timber members and other building assemblies.” EMTC is neither ‘combustible construction’ nor ‘heavy timber construction’ nor ‘noncombustible construction’, as defined within the NBC. EMTC is required to have an encapsulation rating. The encapsulation rating is the time, in minutes, that a material or assembly of materials will delay the ignition and combustion of encapsulated mass timber elements when it is exposed to fire under specified conditions of test and performance criteria, or as otherwise prescribed by the NBC. The encapsulation rating for EMTC is determined through the ULC S146 test method. In order for structural wood elements to be considered ‘mass timber’, they must meet minimum size requirements, which are different for horizontal (walls, floors, roofs, beams) and vertical (columns, arches) load-bearing elements and dependent on the number of sides that the element is exposed to fire. EMTC construction in Canada is expected to be limited to a height of twelve-storeys, that is, the uppermost floor level may be a maximum of 42 m (137 ft) above the first floor. An EMTC building must be sprinklered throughout according to NFPA 13 and it is likely that some mass timber will also be able to be exposed in the suites. All EMTC elements are expected to have a minimum two-hour fire resistance rating and the building floor area to be limited to 6,000 m2 for Group C occupancy and 7,200 m2 for Group D occupancy. There are restrictions on the use of exterior cladding elements in EMTC, as well as other restrictions on the use of; combustible roofing materials, combustible window sashes and frames, combustible components in exterior walls, nailing elements, combustible flooring elements, combustible stairs, combustible interior finishes, combustible elements in partitions, and concealed spaces. If any encapsulation material is damaged or removed, it will be required to be repaired or replaced so that the encapsulation rating of the materials is maintained. Additionally, requirements related to construction site fire safety are to be applied to construction access, standpipe installation and protective encapsulation. EMTC and its related provisions are anticipated to be included in the NBC 2020. NBC definitions: Combustible means that a material fails to meet the acceptance criteria of CAN/ULC-S114, “Test for Determination of Non-Combustibility in Building Materials.” Combustible construction means that type of construction that does not meet the requirements for noncombustible construction. Heavy timber construction means that type of combustible construction in which a degree of fire safety is attained by placing limitations on the sizes of wood structural members and on thickness and composition of wood floors and roofs and by the avoidance of concealed spaces under floors and roofs. Noncombustible construction means that type of construction in which a degree of fire safety is attained by the use of noncombustible materials for structural members and other building assemblies. Noncombustible means that a material meets the acceptance criteria of CAN/ULC-S114, “Test for Determination of Non-Combustibility in Building Materials.” For further information, refer to the following resources: ULC S146 Standard Method of Test for the Evaluation of Encapsulation Materials and Assemblies of Materials for the Protection of Mass Timber Structural Members and Assemblies Fire performance of mass-timber encapsulation methods and the effect of encapsulation on char rate of cross-laminated timber (Hasburgh et al., 2016) CAN/ULC-S114 Test for Determination of Non-Combustibility in Building Materials NFPA 13 Standard for the Installation of Sprinkler Systems
Energy Code
The National Energy Code of Canada for Buildings (NECB) aims to help save on energy bills, reduce peak energy demand, and improve the quality and comfort of the building’s indoor environment. Through each code development cycle, the NECB intends to implement a tiered approach toward Canada’s goal for new buildings, as presented in the “Pan-Canadian Framework on Clean Growth and Climate Change”, of achieving ‘Net Zero Energy Ready’ buildings by 2030. The NECB is available for free online; published by the National Research Council (NRC) and developed by the Canadian Commission on Building and Fire Codes in collaboration with Natural Resources Canada (NRCan). CWC maintains ongoing participation in the development and updating of the NECB. The NECB sets out technical requirements for energy efficient design and construction and outlines the minimum energy efficiency levels for code compliance of all new buildings. The NECB applies to all building types, except housing and small buildings, which are addressed under Clause 9.36 of the National Building Code of Canada. The NECB offers three compliance paths: prescriptive, trade-off and performance. The most cost-effective time to incorporate energy efficiency measures into a building is during the initial design and construction phase. It is much more expensive to retrofit later. This is particularly true for the building envelope, which includes exterior walls, windows, doors and roofing. The NECB addresses considerations such as air infiltration rates (air leakage) and thermal transmission of heat through the building envelope. Considering the different climate zones in Canada, the NECB also provides requirements related to maximum overall (effective) thermal transmittance for above-ground opaque wall assemblies and effective thermal resistance of assemblies in contact with ground, e.g., permanent wood foundations. In addition, the NECB specifies the maximum fenestration and door to wall ratio based on the climate zone in which the building in located. As energy efficiency requirements for buildings are increased, wood is a natural solution to pair with other insulating and weatherizing materials to develop buildings with high operational energy performance and provide consistent indoor comfort for occupants. For further information on the NECB, visit the Codes Canada at the National Research Council Canada.
