Mass timber deserves the attention it is getting. Cross-laminated panels are fast, light, dimensionally accurate, and pleasant to be near. The embodied carbon case is genuine, even after the accounting arguments are stripped out.
What it does not deserve is the tone. Timber has become a position rather than a specification, and positions do not get interrogated. That is a problem, because the material has three characteristics that need managing carefully and are routinely discussed last.
Water, during construction
The most common serious problem on a mass timber project is not fire and not deflection. It is rain in weeks four to twelve.
CLT arrives dry and must stay reasonably dry. Panels left exposed accumulate water at joints and in the screw holes, and once moisture content rises above about 20 per cent in a panel that cannot dry from both faces, you have a decision to make about a structural element that is already installed. Cutting drainage holes in a floor panel is a real and standard remedy, which tells you how common the situation is.
The projects that go smoothly plan for this explicitly: a sequence that gets the roof on early, temporary drainage at every floor level, and someone whose actual job is checking moisture content with a meter and writing it down. The projects that struggle treated weather protection as a contractor’s problem to solve later.
Acoustics, in housing
A concrete floor slab is acoustically forgiving because it is heavy. A timber floor is not, and the difference is most audible in exactly the impact frequencies that cause neighbour disputes.
The solutions are known and they work — a floating screed, a resilient layer, sometimes a suspended ceiling, sometimes added mass in the form of a screed or gravel fill. They also cost money, add depth, and add weight, which erodes several of the reasons timber was chosen.
This is not a case against timber housing. It is a case for costing the acoustic build-up at the same moment the structure is chosen, because a scheme that has been priced on a bare panel and detailed later will lose either its budget or its performance.
Ask early What is the assumed floor build-up, in millimetres, from the top of the panel to the finished floor? If nobody can answer, the acoustic strategy does not exist yet.
Fire, honestly
Timber burns and chars, and charring is predictable enough to design with: a known char rate, a sacrificial depth, and a residual section that carries load for the required period. This is well-established engineering and it is not the interesting question.
The interesting questions are about the fire load of exposed timber in a compartment, about connections — where steel plates conduct heat into the timber they are bolted to — and about what happens after the fire brigade leaves, since a charred and soaked panel is not a repairable element in the way a concrete slab often is.
Regulators in several jurisdictions have moved to encapsulation requirements above certain heights, which resolves the technical question and removes the exposed soffit that half the design intent depended on. Any project betting on visible structure needs to know which side of that line it sits on before the concept is signed off, not after.
What it is genuinely better at
Having listed the difficulties, the advantages are substantial and specific.
Speed and sequence. A superstructure erected in weeks rather than months, dry, with no curing, in weather that would stop a concrete pour. On a constrained urban site this is often the whole business case.
Weight. Roughly a fifth of an equivalent concrete frame. On poor ground, or over an existing structure, or in an extension on a difficult foundation, this can be the only viable option.
Accuracy. Panels arrive cut to a tolerance concrete cannot match, with openings already formed. Following trades can set out from the structure rather than correcting it.
Carbon, with the caveats. Substantially lower embodied emissions than a comparable concrete or steel frame in almost every credible study, provided the sourcing is certified and the transport distance is sane. Shipping panels across an ocean recovers a good part of the saving.
Those four reasons are enough to specify timber on a great many projects. They are also arguments, not virtues, and they can be checked. Which is more than can be said for the version of this conversation that treats the material as evidence of good character.
