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The trouble with timber: What’s stopping the switch to wood?

Enlightened designers and local authorities often consider timber construction, but the same objections seem to crop up. So what’s stopping everyone? Asks Steve Webb, director of Webb Yates engineers

 

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Anna Freud Centre by Penoyre and Prasad uses a thin layer of concrete in the floor with timber
Anna Freud Centre by Penoyre and Prasad uses a thin layer of concrete in the floor with timber

 

If we were able to swap out a proportion of concrete and steel components for timber, that would make a meaningful reduction to construction’s carbon footprint. While in France the government has mandated that all public buildings be 50% timber, the UK government, in its laissez-faire fashion, has left it up to individual choice and responsibility, with property developers, building owners, contractors and designers relatively unsupported by regulation.

 

Both the Climate Change Levy and building regulations focus on reducing the energy use of new buildings in-use, not construction with a low-carbon frame – an approach that favours carbon released in future (from a hopefully decarbonised grid in 20 years’ time), over emissions from construction today.

 

In this vacuum of regulatory oversight, there is no coordinated approach or clear pathways to using timber – it’s left up to professionals to be responsible and use it where we can. Enlightened design teams often explore timber options, but the same objections seem to crop up. Some climate-conscious local authorities have even adopted policies or standing instructions against timber buildings. So what’s stopping everyone?

 

Cost is top of the list. Solid cross-laminated timber panels (CLT) have become as synonymous with timber construction as Hoover and vacuum cleaners. It’s a product with perceived quality, its thickness and finish inspire confidence and it benefits from an attractive procurement route where the fabricator completes the design and detailing of the product, making life easy for client-side designers. The attractiveness has led to designers offering a binary choice between concrete or CLT to the detriment of other timber typologies.

 

“While we have accepted the reality of global heating and the need to cut carbon emissions, and recognised our personal complicity in the sources of carbon, we haven’t always found the determination to change course”

 

Initially of comparable cost, CLT has become more expensive than concrete. The panels are predominantly manufactured overseas and are affected by fluctuations in the British Pound and post-Brexit trade barriers. CLT uses a lot more material by volume when compared to traditional timber loose-joist construction (a 200mm CLT floor uses about seven times more timber than a floor with joists) which means that the post-Covid hike in timber prices has hit CLT disproportionately. The limited number of UK suppliers has also emboldened some suppliers to pass on price hikes mid-construction.

 

CLT has other advantages that belie its material cost, and these may not be turned up in a quick like-for-like comparison. A CLT building weighs about 80% less than a concrete which results in a massive saving in the cost of a foundation. On site, CLT construction on site is faster than concrete or steel, saving on programme time. Avoiding unnecessarily long spans and accepting some glulam downstands can help to control costs, too. You could also choose a timber soffit finish, saving on the cost of a ceiling.

 

The carbon reductions achieved with timber can also improve BREEAM scores, meaning that other more expensive measures can be avoided. Timber, as we know, has a much lower carbon footprint than steel or concrete. Consider a 2m-long beam carrying a one-tonne point load; that beam in steel or concrete has two to three times the carbon footprint of its timber equivalent. Add sequestered carbon into the calculation (the carbon, the tree, or its replacement consumed while it grows) and the timber beam is as carbon negative as the concrete one is positive.

 

Hothouse in London Fields, designed by Ash Sakula, combines timber with a steel frame
Hothouse in London Fields, designed by Ash Sakula, combines timber with a steel frame

 

CLT isn’t the only timber option. Prior to the arrival of CLT, glulam and other timber frame options were more common and perhaps are due for a comeback. Projects such as Anna Freud Centre’s Kantor Centre of Excellence in Pentonville, a 3,200m2 new build and refurbishment by Penoyre and Prasad, use a composite veneer lumber and concrete floor construction. Using a thin layer of concrete in the floor in combination with a very strong beech veneer means the floor has 60% less concrete than a flat slab and 60% less timber than a CLT one, with added advantages to fire resistance and enough thermal mass to design out air conditioning.

 

Timber-frame construction has been the staple of cheap mainstream building, in the form of mass low-rise housing, and remains so today. Barratt-type houses are usually stick-built with brick skins. These are highly efficient, very quick to build and often use local materials in the form of lower grade C16 timber. The use of this typology isn’t limited to low-rise housing. Many taller, cassette-panel social housing projects have been built this way, but they would now fall foul of new fire regulations regarding flammable materials in facades. This doesn’t mean that joisted floors are out or that timber walls can’t be used in commercial buildings. Perhaps there is a place for a ‘heavy light frame’ option with close centred joists and thick planked floors in combination with steel frames: Ash Sakula’s Hothouse in London Fields is an example. These kinds of options need to be put back on the structural engineer’s menu for stage-1.

 

“Prior to the arrival of CLT, glulam and other timber frame options were more common and perhaps are due for a comeback”

 

On a normal project the structure is about 20% of a project’s budget. Floor elements might be half of that budget, so about 10% of total building cost, so even significant changes to the cost of the floor have a relatively minor impact on the total build budget. Would it be better to run with a climate-helpful timber frame and save additional costs by downgrading, say, the bathroom fittings or fancy reception desk?

 

After cost, fire is a major concern blocking the use of wood. Counterintuitively timber has very good inherent fire resistance. When the outer layers of timber char, they protect the core for long fire durations and timber beams will, in fact, outperform unprotected steel by long periods. Since Grenfell, fire engineers and regulators have rightly become far more cautious about fire compliance. There are now several new regulations prohibiting flammable materials in facades of residential buildings over 18m (soon to be 11m), but this does not include commercial buildings and does not prohibit timber construction inside residential buildings.

 

Fire engineers tend to shy away from the use of timber, so if you want to build in timber, you’d better find a fire engineer who knows it and feels positively about it. The wrong fire engineer could needlessly quash your timber scheme or unexpected additional costs could arise later in the design process.

 

Buildings are designed to survive a set fire duration, and expected to outlast the total combustion of their contents, so the fire brigade can theoretically wait for the fire to burn itself out with confidence that the building won’t collapse. In order to achieve this, timber needs to self-extinguish in the way a log can’t burn in isolation on a fire.

 

While relaxed about more traditional timber beam construction, fire engineers rightly demand detailed certification of the performance of laminated timber products, such as CLT, with specific concerns about glue lines failing, allowing layers of lamination to fall off, which not only adds fuel to the fire, extending fire duration, but also exposes fresh timber to the flames reducing the protection from charred layers. An exposed timber frame building might require sprinklers or a mist system. The local water supplier may not guarantee flow rates and so tanks or a flow test will be needed. Flame retardants to prevent the surface spread of flame for internally exposed timber is standard. A fire strategy for the construction condition is also required.

 

“If you want to build in timber, you’d better find a fire engineer who knows and feels positively about it”

 

Difficulty in getting insurance is also perceived as a major obstacle, however Steve Wallace of dRMM, an architecture practice that has long been at the forefront of promoting timber use, says they have no problem obtaining insurance for any of their buildings. Wallace says that they seek to engage the insurer right after planning and that there has never been strong pushback on structural timber, which proves these kinds of buildings can be built to conform with the insurer’s build regulations.

 

Not everyone has had the same experience, however, and insurance markets are tightening. Oliver Schofield of insurance broker RISCS describes the global insurance market as “distressed” (too many claims and catastrophes but not enough money). He gives this as a reason why insurers are increasing premiums and withdrawing cover from areas of perceived risk. His organisation is in the process of establishing a mutual insurance scheme as a vehicle for providing insurance specifically for timber buildings with a better-informed underwriting team. A number of major UK developers have committed funding for its establishment. Presumably some of the catastrophes that have hardened the insurance market are themselves climate related and so it seems unfortunate that the insurer’s algorithm, while silent if I buy a rickety timber Victorian house, is totally allergic to the state-of-the art CLT office block.

 

Mortgage eligibility is another matter. An acquaintance recently tried to buy a flat in an exposed CLT block and found it impossible to obtain a mortgage. Mortgage lenders frequently ask the reductive question, “Is the building of traditional brick and mortar construction?”. What does this mean? Almost all new residential buildings have load-bearing timber components, yet because they are decorated with brick, they are deemed low risk. Rumours about the un-mortgage-ability of timber housing has served to quash many timber schemes. Are the people who are setting these guidelines aware of the consequences. Do they really understand where risks lie?

 

“It seems unfortunate that the insurer’s algorithm, while silent if I buy a rickety timber Victorian house, is totally allergic to the state-of-the art CLT office block”

 

Finally, timber schemes can be dropped in favour of other materials simply because people are unfamiliar with the typology, while others go because developers have long standing trust-based relationships with contractors, who in turn have longstanding relationships with subcontractors who make steel or concrete frames. Everyone in the construction industry has been brought up in a high-risk environment, battered by the consequences of bad decisions. They naturally want to stick to doing what they know, with people they trust. While we have accepted the reality of global heating and the need to cut carbon emissions, and we have recognised our personal complicity in the sources of carbon, we haven’t always found the determination to change course.

 

Timber as a primary default building material is a really good short-term solution to cutting carbon emissions now. Design teams and contractors need to surmount the technical challenges with enthusiasm and help developers find financially sustainable ways of making timber work. The government and local authorities need to help create a framework where timber buildings are the norm. They need to change carbon levies and building regulations to give credit for low carbon frames over long term energy reductions.

 

The construction sector uses huge amounts of energy in material production. Meanwhile the UK consumer and taxpayer carries the very heavy financial toll of building nuclear power plants to decarbonise that energy. Wouldn’t it be better to spend that money reducing the energy demand at source? Put simply, if you build in concrete and steel, don’t be surprised if someone builds a great big nuclear power station on a coastline near you. Whereas, if you build in timber, you might get a sawmill instead. Personally, I’d rather our kids lived near – and worked in – a timber factory than a nuclear power plant. But the government is about to spend £20bn and probably a lot more on Sizewell C instead. Surely a smarter choice would be subsidising CLT factories in the UK.

 

Steve founded Webb Yates Engineers with Andy Yates in 2005

 

 

 

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