5 Thoughts on Mass Timber

“After a cave, timber is our oldest building material,” says senior director, William Johnston, who leads WSP’s global timber network. “Examples of its strength, versatility and durability are seen in historic buildings around the world. With the advent of concrete and steel, the use of timber in construction declined, and it was mainly used to create architectural features or in the stick frame structures of housing. But over the past twenty years the mass timber products, such as CLT (Cross Laminated Timber) and Glulam (Glue Laminated), have opened up new opportunities for the use of timber as a construction material for different buildings across a wide range of market sectors.”
Here are five thought in using timber.

1. Is timber carbon-friendly?

In the drive to decarbonisation, mass timber may offer a way to reduce embodied carbon. The reason is twofold: when trees grow, they absorb CO2 from the atmosphere, and using timber in construction means replacing carbon-intensive materials, such as concrete or steel, with a more carbon-friendly alternative. 
Bas de Leijer, project manager at WSP in The Netherlands, sees a growing appetite for the use of wood, particularly in residential buildings because of its environmental properties. “Timber is renewable, recyclable, waste efficient, biodegradable and carbon positive. It’s a growing market, especially for dwellings, because people want to live in more ecological buildings. We are also seeing more timber offices and we use it extensively in sports facilities as well.”
Embodied carbon is typically less than half of that for concrete floor plate solutions.
Its is also relatively light weight compared to concrete. Not only does this reduce the overall weight of the building, but it also allows for more efficient transportation onsite. You can fit 30 timber elements in a truck from factory to construction site, compared to three to five concrete elements.  That means 60-70% fewer trucks coming onto a construction site, resulting in less carbon emissions from transportation during construction.

2. Is timber good for you?

William Johnston cites the biophilic properties of timber as an important benefit. In a recent research for Forestry Innovation Investment, an independent organisation set up by the province of British Columbia in Canada to promote BC wood products, educate on forest practices and provide research around all aspects of growing and using wood, WSP found that wood has a number of material and immaterial benefits. “It is hypoallergenic, has antimicrobial properties, has excellent sound absorption and reduced off-gassing emissions; benefits which collectively decrease negative physiological effects on occupants.” 

For William Johnston, the fact that timber buildings support the health and wellness of their occupants adds significant value in the long term by creating natural, welcoming and healthy environments. “Patients in hospitals recover more quickly, businesses experience more engaged staff with higher retention, children concentrate better in their classrooms. Biophilia is proven and real,” he says.

William Johnston also believes it is vital that we start to re-evaluate what constitutes successful design. “We’ve learnt that long term operational costs of the building systems significantly trump the initial capital costs. But the financial implications of the human implications are even greater – financial benefits of increased productivity, engagement and retention dwarf all other costs. Human centric design is fundamental to the long-term performance of a facility and it needs to be measured as such in the feasibility stage.”

Wood also naturally absorbs humidity, buffering it. This has the effect of regulating the indoor relative humidity to a comfortable level between 30-70%, for a healthier indoor climate. It helps reduce the spread of bacteria in humid weather, and viruses when it’s dry and cold. And, because there’s less dust than with concrete, building with wood can also ease human respiratory issues, such as asthma.

3. Is timber versatile?

Timber offers many advantages, and versatility is very important. “Its thermal insulation properties make it energy efficient, and it is economical and fast to construct,” says Ove Morten Bergane, head of structures for WSP in Norway. “It is ideal for prefabrication, simple and quick to install, and very flexible.

Thanks to timber, WSP’s structures team in Norway overcame a number of challenges in the design of a student apartment building in Haugesund, which has distinctive architecture with slope CLT walls, slices and different shapes. The ground floor is for business areas with open spaces. The floor above is made in concrete and CLT above this level. The project meets passive house standards and is an infill project in the centre of the municipality.

“The light weight of timber makes it ideal for over-building as our cities get ever denser and space is at a premium,” adds Phil Gardiner, principal director based in Melbourne, Australia. “We’ve designed a residential building on top of an office tower; and an office tower above a shopping centre, which would require almost no strengthening of the structures on the trading floors. We are currently designing a ten-storey CLT hotel in Melbourne that will be built over a railway line.”

Furthermore, wood is easy to handle on the construction site. There’s no hard drilling, which makes the construction process quieter, and cleaner, for everyone.

4. Is timber design friendly?

Timber is extremely easy it is to work with, especially when offsite construction is part of the equation. Cross laminated timber (CLT) panels – or ‘2D elements’ – consist of three to nine layers of timber, each glued perpendicular to the one below. These 2D elements work like concrete, but are quicker, easier and safer to manufacture offsite. With digitalisation, the different components can be precisely fabricated according to specifications, and easily assembled on-site.

“We can use a hybrid floor plate that has steel columns and beams but replaces a composite deck slab with a cross-laminated timber slab manufactured off-site. The timber makes an aesthetically pleasing ceiling for people on the floor below, services can still run as they would in a traditional solution and – with no waiting to fix reinforcement, pour concrete and allow it cure – construction is faster”, explains Dan Hagan, technical director and Modern Methods of Construction Lead for WSP in the UK. 

In addition, by moving construction into a controlled offsite manufacturing setting, we can address reliability, quality issues and provide certainty in design programming and cost. 

5. Does timber make economic sense?

Last but not least, cost. Sure, mass timber can be more costly when compared to steel or concrete on a per unit basis. However, when looking at the whole picture, timber makes the cut.

- Thanks to its light weight, timber allows for smaller foundations and fewer support structures.
- Manufactured components and ease-of-use means timber necessitates smaller crews, leading to reduced labour costs. 
- Using timber can speed up construction time potentially resulting in schedule savings of 20-50% than if traditional materials are used.

Dan Hagan adds that timber is an excellent option for retrofit projects that involve adding floors to existing buildings. “A multi-storey mass timber building has a substructure up to 85% smaller than a traditional alternative. This reduction in substructure material relates directly to a similar saving in costs, programme, and importantly, embodied carbon”, he says. 
And ultimately, timber buildings have other financial benefits that are more difficult to calculate, such as improved health and wellbeing for occupants and higher staff retention (or lower staff turnover) thanks to attractive work environments.  

Timber might be one of the oldest building materials, but we are still learning how to optimise its potential. WSP experts across the globe collaborate to increase our understanding of the challenges and opportunities for timber, and explore its use as a sustainable, healthy, safe and efficient alternative to concrete and steel.