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.
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.
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.
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.
