Mass timber construction is about more than just storing carbon

It also can put people back to work and save our forests.

TreeHugger has been covering the mass timber scene for a dozen years, starting with Waugh Thistleton's timber tower in Hackney. Now Tim Smedley of the BBC talks to Andrew Waugh and writes a really thorough article that looks at the benefits of building with wood. He starts, as we do, with the carbon footprint, and the fact that trees are the best form of carbon capture and storage. Waugh says:

“The machines being created for locking carbon in and burying are not as efficient as trees”, he enthuses. “Just grow more trees!”

Smedley and Waugh visit Dalston Lane, as TreeHugger did a few years ago. At the time, it was the largest CLT building in the world. Waugh explains how the building is much lighter than concrete (important when you are built on top of a train line) and how much carbon it stores.

Dalston Lane Plan© Waugh Thistleton

“There’s only two tonnes of steel in this whole building”, says Waugh, as we gaze up at Dalston Works, “about the same as a VW van. All the internal [CLT] walls are structural. It is like a honeycomb — the parting walls and principle walls are made of [structural] CLT, about 4,000m3 of timber, 3,225 trees, housing 800 people, so about three trees per person in the building. That’s about the equivalent of 200 years of carbon savings [compared to a traditional concrete and steel construction].”

To really draw down CO2, all the trees that are cut have to be sustainably harvested and replaced with new planting. When I have complained to Waugh that as much as half of the mass of the tree is left behind in roots and slash, he responded: “Plant two trees!” For mass timber to really work the way it is promised, that is the kind of analysis that will have to be done- how much planting is necessary to not only replace the trees that were cut but also the CO2-releasing parts left behind.

There are other benefits to harvesting wood that go beyond the straight CO2 calculation. Smedley writes:

CLT is now taking off in the United States too. The large forests that once served the dying newspaper industry have fallen into disrepair across America, fuelling the wildfire crisis. According to Melissa Jenkins of the U.S. Forest Service, her department is now actively promoting mass timber. She told an Energy Study Institute briefing that many planted forests are now “too dense, especially with small-diameter trees, creating conditions that fuel intense wildfires… Mass timber creates an economic incentive to use forests sustainably while leaving them intact, making communities safer while also developing local economies.”

Andrew Waugh Andrew Waugh in front of CLT building/ Lloyd Alter/CC BY 2.0

Small diameter trees are just fine for making CLT. Waugh continues on this theme: “At the same time as solving climate change, making better buildings, we can help rural economies… These vast forests are basically rotting and burning down.” It’s as if, he says, “we kept on breeding cows but we stopped eating beef”.”

There are going to be bumps on the way; in the UK they have banned combustible exterior walls on buildings over six stories in the wake of the Grenfell disaster, even though that building had plastic windows and plastic insulation and cladding and CLT doesn't burn in the same way.

But no matter how you calculate it, the upfront carbon emissions of making mass timber are a fraction of those of making steel and concrete. Those industries are pushing back hard and even pushing out life cycle analyses demonstrating that over 50 years their buildings are not much worse. But we don't have a lifecycle, we have to worry about what we are emitting now, and in the next ten years. If we are going to build at all, we have to do it in wood.

It also can put people back to work and save our forests.