Durability of Mass Timber

As mass timber systems develop and gain a foothold in the built environment, demonstrating their durability, particularly where elements are hard to access or replace during the asset life, is critical to building trust in these elements as a viable long-term, sustainable alternative to concrete and steel.

The durability of mass timber, as with any manufactured wood product, is influenced by various factors. This article explores key factors to consider when designing and detailing mass timber structures and the importance of considering element manufacturing processes, short-term exposure, and longer-term exposures to project success.

Natural Durability of Wood

The natural durability (resistance to decay) of wood varies by species of tree, and the lumber sourc – sapwood or heartwood.

Heartwood is typically more durable than sapwood, and hardwoods are typically more durable than softwoods. 

However, relying solely on the natural durability rating of a wood species to provide for long-term durability in any application can have unintended consequences. These can include an increase in the material’s emissions profile, increased wastage if appearance defects are rejected, and ultimately a more expensive project.

Wood Solutions’ advice to designers is: focus on timber detailing rather than wood species’ durability ratings.

Moisture

Timber, whether conventional sawn sections or engineered products such as GLT, LVL or CLT, is susceptible to biological decay if moisture content is not managed. This can lead to dimensional instabilities, persistent mould within a building, corrosion of fasteners and loss of structural integrity.

Typically, timber will be free of decay where the moisture content is less than 20%.

When moisture content exceeds 22%, prolonged exposure can lead to the commencement of decay. 

Where moisture content exceeds 28%, rapid biological decay by fungi occurs. This is also happens to be the threshold for corrosion of metal fasteners – upon which modern timber structures heavily rely – see this article by Filip Cirovic on materials selection for screws.

In practice, much lower moisture contents are required for sawn structural timber and engineered wood products that use adhesives to bond timber lamella into GLT,  LVL or CLT (see Table 1).

Exposure During Manufacture

Most, if not all, engineered wood products will be fabricated from seasoned or kiln-dried timber. Some suppliers can provide their laminated products using thermally modified or acetylated timber feedstocks. These processes reduce the moisture content and modify the timber’s microstructure further enhancing durability.

Regardless of the feedstock, the moisture condition has an impact not only on the durability of the timber fibres, but also durability of adhesives and the longer-term integrity of adhesive bonds.

Table 1. Moisture content limits for engineered timber products and adhesives

With the majority of engineered timber manufacture occurring indoors, moisture exposure should not be an issue, but as with any engineering specification, minimum performance requirements must be clearly defined and expectations set to ensure all parties are aligned, and durability is not compromised.

Exposure During Construction

Once the engineered product has left the manufacturer and is accepted on site, the manufacturer may have limited control over exposure. 

Potential exposure scenarios  – The primary exposure of timber to moisture during construction is rainfall. Depending on the site’s microclimate, short-term exposure is unlikely to present a significant concern (for periods up to 3 months). However, continued exposure to intense rainfall, extended periods of rainfall, or inundation that can lead to excessive moisture content, leading to warping, cracking and decay.

Weak points – Weak points vary depending on the timber substrate of concern. For engineered timber products such as CLT, GLT, LVL or finger jointed composite members, it is the ability of glue lines to resist the effects of moisture exposure – with phenolic and resorcinol resin-based glues expected to be more resistant to persistent moisture exposure compared to melamine urea formaldehyde or cross-linked PVA glues.

Fastener penetrations can also serve as moisture ingress points, particularly on horizontal surfaces where water cannot drain from the surface.

Unprotected edges, such as the edges of plywood, if not treated or provided with adequate space between adjacent elements to facilitate drying, can absorb moisture into the element by capillary action.

Mitigations – In the construction case, careful planning of material delivery schedules, construction scheduling to avoid significant rainfall periods, and providing adequate protected storage on site are key mitigations. Of course, weather can be difficult to predict, inner city sites can be tight, and there are often scenarios beyond the control of the manufacturer or constructor that could see timbers exposed to excessive moisture. Nick Hewson of Aboralis has been focusing on the development of moisture management plans for prefabricated timber products and advocates for designers and clients to consider moisture in design, the same way safety in design is approached. Using a risk-based approach to assessment to identify likelihood and consequence and flag requirements for mitigations – whether that be drainage, membranes and or coatings – and monitoring with embedded sensors to alert asset owners when there is a leak that could impact the integrity of the structure.

Spec Toolbox partner Rothoblass have a novel solution for CLT manufacturers – a self-adhesive membrane that provides up to 12 weeks of protection to panels, is transparent and enables handling and site operations (including installation) to proceed.

Temporary water-based acrylic coatings are also available to provide similar protection to mass timber during shipping and installation.

Long-term exposure

The extent to which engineered timber products are exposed long-term is influenced by the form and function required of a particular structural member. For example, internal elements such as floor plates and columns that form the primary structural frame of a building are unlikely to see long-term exposure to uncontrolled environments. 

External walls, columns, stairs, joists, railing, exposed rafters – all of these applications have the potential to expose timber products to cycling wetting and drying, UV exposure and potentially significant diurnal temperature variations.

The key to good moisture control and durability is in detailing. For moisture control, providing drainage planes, damp proof courses, air gaps, or physical barriers is critical.

For exterior exposures, control against wetting and drying, heating and cooling and UV exposure are critical. This can be achieved using coatings, shielding and considered detailing for shrinkage. Selection of corrosion-resistant fasteners and specifying installation patterns that reduce the likelihood of moisture ingress and stresses on adjacent members is also critical for exterior applications.

With timber structures built in the 7th century still in use, it is clear that durable timber buildings can be developed and delivered.

There is a vast array of resources available to designers globally on detailing timber structures for durability. In addition to the footnotes in the article, some key references are linked in Table 2.

Table 2: A selection of global durability references