Essential to manage risks from moisture in CLT buildings Failure from decay is expensive

15 July 2024 by Jason Quinn

It's essential to manage risks from moisture in CLT buildings

There are some advantages to being laggards in adopting new building products and methods. CLT is more common in Europe and has a longer history of use there. Damage and failures have occurred even in buildings less than five years old. It’s making insurers nervous and some are applying higher premiums to CLT constructions. Some key lessons about protecting against excess moisture are now evident and New Zealand designers/specifiers need to study them well. 

I recommend you read a recent technical paper by a UK CLT expert, Nick Clifford, that reveals the major moisture sources CLT has to deal with and details the design/construction changes needed to reduce the decay risk. There are three major risks:

  1. CLT getting wet during construction and being closed in before dried; 
  2. previously common design choices like terraces and flat roofs, on which infrastructure is often installed; and
  3. plumbing leaks.

It's essential to manage risks from moisture in CLT, including during construction.

Notably, CLT in Europe is generally not treated with timber preservatives. In Aotearoa, CLT is generally treated—Red Stag’s CLT plant treats with boron or CCA depending on application, XLAM offers to supply CLT with Hyne T3 Plus treatment—but don’t let this make you complacent. Treatment, especially boron treatment, only reduces the speed of onset of decay. It doesn’t provide immunity.  

Here’s my cheat sheet (but do read the full paper; it is readable, horrifying and extremely informative):

  • Keep CLT dry during construction and/or monitor and actively dry out before closing it in.
  • Horizontal panels (roof, terrace) are more at risk than horizontal panels and mid-floors.
  • Consider not using flat roofs or terraces/balconies – even with warm roof insulation. Or if you must have a flat roof or terraces, keep a minimum 10 degree pitch for the CLT and add drainage (yes, inside the warm roof build up) for leaks.
  • Early moisture monitoring as part of a warm roof build up is important. Add drainage to take care of any leaks.
  • Plumbing leaks go undetected for longer in a CLT construction so consider how to design ongoing monitoring.
  • CLT can be harder and more expensive to repair compared to other types of construction; take this into account when weighing up risk.

The author provides a great list of references for the issues discussed at the end of his paper.

Reference

Nick Clifford, “Achieving durability with cross laminated timber (CLT)”, Timber 2024 Industry Yearbook

Published by the author here

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