Technical Resource
Safe moisture content for CLT: thresholds, fibre saturation and service classes
Almost every moisture decision in mass timber comes back to a number: the moisture content of the timber. This reference explains the thresholds that matter to a structural engineer: what is safe, what triggers decay, what the fibre saturation point and service classes mean, and how much a panel actually moves as it dries.
This is a companion to our pillar guide, Moisture management in mass timber and CLT: a structural engineer's guide.
Moisture content vs relative humidity
Moisture content (MC) is the weight of water in timber as a percentage of its oven-dry weight; relative humidity (RH) describes the surrounding air. They are linked but not interchangeable.
Because water can weigh more than the dry wood that holds it, MC can exceed 100%. Every threshold here is an MC figure on the oven-dry basis unless stated otherwise. The link between the two is equilibrium moisture content: the MC a piece of timber settles at if left long enough in air of a given RH and temperature.
| Relative humidity (at 20°C) | Equilibrium moisture content |
|---|---|
| 30% | ~7% |
| 50% | ~10% |
| 60% | ~12% |
| 70% | ~15% |
| 90% | ~23% |
Source: Danish Technological Institute (2025). The implication is direct: leave CLT in persistently humid site conditions (70%+ RH) and it will climb toward 15% MC and beyond, before any rain even touches it.
The thresholds that govern CLT
Below ~16% for dry internal service; below ~17% for high-glue panel products; below 20% to avoid fungal decay; mould risk begins around 15 to 18%.
| Threshold | Figure | Basis |
|---|---|---|
| Target for dry internal service | ≤ ~16% | DTI (2025) |
| Surface mould risk begins | ~15 to 18% | TDUK (2024); DTI (2025) |
| Wood-based panels (high glue content) | ~17% | DIN 68800-2 via StGHB (2025) |
| Fungal (rot) decay risk begins | > 20% | TDUK (2024); DIN 68800; Caldwell et al. (2025) |
| Fibre saturation point (softwood) | ~26 to 34% (spruce ~30%) | StGHB (2025); DTI (2025) |
The 20% line is the one to design around. German practice (DIN 68800) limits in-service timber moisture to 20% to avoid wood-destroying fungi, and is explicit that the figure already "considers the inaccuracy of moisture meters as well as the uncertainty of having measured the wettest point in a member." It is a threshold with a safety margin built in: not the point of failure, but the point of action. Wood-based panels with high adhesive content carry a lower limit, about 17%. Mould is a separate, faster, lower-threshold phenomenon, starting around 15% MC: a health and indoor-air-quality issue rather than a structural one, but a clear signal that conditions are wrong.
What the fibre saturation point tells you
Below fibre saturation point the timber shrinks and swells; above it, it does not move further but is at risk from decay and staining fungi, which need moisture above this level.
For softwood the fibre saturation point sits at roughly 26 to 34%, around 30% for spruce. Below it, water is bound in the cell walls and any change in MC produces dimensional change. Above it, extra water is "free" water in the cell cavities: the panel does not swell further, but this is the regime in which decay and blue-stain fungi become active. This is why "more than four months above fibre saturation point" is the conservative European marker for decay risk.
Service classes: what your timber is warranted for
Most engineered timber is only warranted by its manufacturer for Service Class 1 or 2, dry or occasionally humid internal conditions. The construction phase routinely breaches those conditions.
Eurocode 5 (BS EN 1995) and BS EN 335 define three service classes by the timber's expected in-service moisture: broadly, Service Class 1 (heated internal, equilibrium around 12% MC), Service Class 2 (covered/unheated, up to around 20% MC) and Service Class 3 (exposed, persistently above 20% MC). Most CLT and glulam is supplied for Service Class 1 or 2 in the permanent case; elements intended for permanent Service Class 3 are not covered by the standard timber-protection guidance. The point for designers: a panel rated for dry internal service can be exposed to Service Class 3 conditions for weeks during the build, which is exactly the mismatch a Construction Stage Moisture Control Plan exists to manage.
How much does CLT actually move?
Across the grain, solid softwood moves about 0.25% per 1% change in moisture content, but CLT's cross-lamination restrains in-plane movement to roughly a tenth of that. The trade-off is that restrained swelling builds stress, which can show up as cracking, cupping or, at worst, displacement.
For solid softwood, a swing from 8% to 18% MC produces a tangential change of about 4 mm per 100 mm; for CLT the in-plane change is roughly one-tenth of that thanks to cross-lamination. Perpendicular to the plane, however, CLT still moves at around 0.24% per 1% MC. These small percentages add up over real spans: the StGHB bulletin works an example of a 10 m-wide glulam member rising from 10% to 15% MC and deforming by 125 mm, noting "such deformations cannot usually be absorbed by the element joints." Caldwell et al. (2025) document a real timber-frame wall "displaced by 5.2 cm due to a glued laminated timber slab with elevated moisture content."
This is why drying must be controlled, not just achieved: dry a wet panel too fast and the differential between surface and core causes splitting and potential delamination at the glue lines. A practical close-up rule from the DTI guide is to wait until the difference between surface and core moisture content of solid timber falls below about 3% before sealing the element.
Designing the numbers in, not just hoping for them
Knowing the thresholds is one thing; proving the timber stays within them through the build is another. Vector designs and commissions embedded moisture and leak-detection sensors that monitor CLT continuously at its highest-risk points, with a 20+ year battery life, so the moisture content is a measured fact, not an assumption. We also review waterproofing and moisture strategy independently at RIBA Stage 3.
References
- Danish Technological Institute (2025). Moisture Management for Timber Construction: A Practical Guide. ISBN 978-87-91461-97-2.
- Caldwell, M. (2024). Moisture management during construction (Timber Knowledge Sheet). Timber Development UK.
- Studiengemeinschaft Holzleimbau e.V. (2025). Bulletin: Moisture protection of mass timber and skeleton buildings during the construction phase, 1st edition. (Citing DIN 68800-1/-2 and DIN EN 1995-1-1.)
- Caldwell, M., Viereck, L., Aondio, P. & Flexeder, N. (2025). Moisture management during construction. In: Holistic Design of Taller Timber Buildings, Springer, pp. 127-142. DOI: 10.1007/978-3-032-02098-7_11.
Published by Vector (Vector Leak Consultants Ltd). General technical information, not project-specific advice. Service-class moisture associations are indicative; refer to BS EN 1995-1-1 and BS EN 335 and manufacturer data for design values.
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