Mass timber buildings are popping up everywhere in New Zealand and Australia, and for good reason. Substituting concrete and steel with Cross-Laminated Timber (CLT) saves on embodied carbon and offers much lighter structures. The real holy grail is construction speed.
Consider the massive Bunker Hill project in Massachusetts. They used a clever “kit of parts”—including 62-foot CLT floor panels—to minimize on-site labor and massively accelerate their schedule. Enclosing a building rapidly, a whole floor at a time, is brilliant because it limits the time the timber is exposed to the weather.
But here in the real world, CLT sometimes gets left sitting out. Hoping to avoid the pouring rain. Really – this is a real site in New Zealand. Maybe it didn’t rain while they were building?
While mass timber can handle a bit of moisture, repeated wet-dry cycles are a recipe for disaster. If it can’t dry out, you’re looking at intolerable risks: excessive swelling, structural decay, and rampant microbial growth. It’s a fantastic way to ruin a perfectly good building.
The bottom line is that you have to keep the timber dry before and during construction. RDH Building Science has just updated their excellent Moisture Risk Management Strategies for Mass Timber Buildings guide, and it’s mandatory reading if you are working with CLT.
Here are the core takeaways to avoid a soggy disaster:
- Assess the risk early: Know your timber. CLT absorbs water primarily at the end-grain.
- Make a plan: Develop a solid moisture management plan during the design phase, not when the trucks arrive.
- Actively manage water: Sweep it off, drain it away, and never trap moisture under vapour-impermeable materials.
- Test it: Get your pin probe out and ensure the wood is strictly below 16% moisture content before closing it in.
You don’t have to reinvent the wheel. Read the guides, plan ahead, and for goodness’ sake, keep your timber dry.
Abstract & References
Critical Mass: Scaling Passive House at Bunker Hill outlines how Leggat McCall Properties used a mass timber kit of parts to achieve economies of scale and rapid enclosure for a massive multifamily Passive House project. Link
Moisture Risk Management Strategies for Mass Timber Buildings by RDH details a three-step process to assess moisture risks and execute a robust management plan on-site. Link