With over 90% of single-family residences composed of light-wood framing materials, a large bias towards wood construction exists in the North American market. While traditional light-wood framing construction is only applicable to single-family homes and small-scale commercial buildings, the Wood Innovation and Design Centre (WIDC) (Figure 13.1) has demonstrated the feasibility of mass timber (MT) to build higher and provide a unique value proposition over conventional construction typologies.

FIG 13.1 Wood Innovation and Design Centre, Prince George, British Columbia by Michael Green Architects (2014). Layered CLT floor slabs create service zones unobstructed by downstand glulam beams to this mid-rise timber hybrid.

In North America, very little value is placed on the environmental benefits of using mass timber, and the top three decision-making criteria for the design of a project are typically costs, general performance criteria, and availability. Solid wood panels simply cannot compete with the value or air between studs in light-wood framing. This has shifted focus for mass timber in North America towards large-scale multi-family residential and office projects, driving expertise in larger on average projects than typically seen in other markets.

A single unified product standard PRG-320, published by the American National Standards Institute and APA (The Engineered Wood Association), exists for CLT to ensure all manufacturers adhere to the same panel production quality control requirements for inclusion in the building code. This standard is relatively rigorous in terms of glue bonding capability and is such that CLT panels produced in a vacuum press cannot pass the quality standard. As a result, small to mid-size CLT players as well as offshore groups have significant barriers to implementing their non-conforming products to projects in North America.

The early development and inclusion of these standards for stringent quality control has allowed code officials to revise building codes in both Canada and US to allow up to 12 and 18 storeys respectively for mass timber buildings in each country. This unified standard and code development has signalled a clear path for an all-out revolution in the construction industry for mass timber structures to be become a standardised staple of modern construction. There is much speculation throughout North America that the cost-driven environment paired with simplified and unified building code systems will allow mass timber construction to flourish and possibly pass volumes currently undertaken in Europe over the next five to ten years.

A major hub for mass timber construction in North America is in the Pacific Northwest, including the very rainy coastal areas of British Columbia, Washington and Oregon, where it is common for rainfall days on site to exceed 20 consecutive days. This has caused many concerns around moisture and risks associated with mass timber construction, as well as hard lessons learnt with other less dimensionally stable products such as Dowel Laminated Timber (DLT) and Nail Laminated Timber (NLT). These products require a moisture protection strategy and covering during construction to ensure they stay dry, monitoring the moisture content, and only sealing off any CLT when it is below 18% MC. If a project is left wet for two months or longer significant risks are involved, however prior to this the risk can be easily managed by closing in the building on time.

FIG 13.2 MEC flagship retail Store, Vancouver, 2020 by Proscenium Architecture & Interiors, Fast + Epp Structural Engineers and Structurlam.

Coordination of installation and tolerances

Hybrid construction systems for mass timber are very popular. The tolerances around concrete to mass timber interfaces and steel to mass timber interfaces vary greatly by trade and further standardisation in understanding these tolerances and design deliverables for these interface points need to be developed.

FIG 13.3 Brock Commons, British Columbia, 2016 by Acton Ostry.

FIG 13.4 Virtuoso apartments, British Columbia, 2018 by Rositch Hemphill Architects – the first North American condo scheme to use CLT.

Fire performance

Fire performance and safety has been a key concern for allowing greater heights and floor areas for mass timber buildings. Full-scale fire testing was performed on a variety of projects to demonstrate both structural capacity and the ability of the building to self-extinguish in the event that sprinklers failed, and fire services also failed to respond. A new PUR glue has been developed that exhibits the same char-rate of solid wood to ensure no char fall of or reignition capacity existing in large buildings. These enhanced fire performance glues are key to life and structure safety for large CLT projects.

Project insurance

Construction insurance and operational insurance can be sticking points for new mass timber buildings. While reference data points are not extremely plentiful, insurance providers tend to see these types of structures as a riskier project set. However, risks and hazards can be greatly mitigated from traditional construction methods with a few easy solutions. Operational insurance is based on current risk analysis, however the new code requirements and associated testing have very much proved the background of mass timber construction beyond testing that has been done on other building typologies. Due to the in-depth code requirements there is sense that these are some of the safest buildings on the market and thus may see some insurance benefits in the future. Currently that is not the case.

Environmental and Wellness Market Value

Leading Australian companies and institutions have adopted triple bottom line measures for their reporting. This has driven a considerable uplift in demand for environmentally rated projects which now have a ‘value’, defined through tools such as Green Star, WELL and One Planet. Such tools don’t yet fully recognise the benefit or reduction in embodied carbon and this can be a limiting factor in the decision-making process.