When mass timber gets wet: failures, court cases and the real cost

Mass timber is a superb structural material, until water is trapped against it and left. The failures that follow are rarely dramatic at the time; they unfold quietly, behind finishes, and surface years later as repair bills, programme delays and, increasingly, litigation. Understanding how it goes wrong is the clearest argument for getting moisture management right.

This is a companion to our pillar guide, Moisture management in mass timber and CLT: a structural engineer's guide.

The failure mechanism

The dangerous failure is not a one-off soaking. It is moisture trapped during construction that keeps the timber wet long enough to decay, often hidden behind the finishes until it becomes structural.

Short-term wetting is usually recoverable if the timber dries. The problem case is different. As the TDUK knowledge sheet explains, "fungal decay is therefore primarily a risk associated with moisture that is trapped during construction, and which progresses within the first months and years of the building's life." And crucially: "decay can continue behind other layers of the building fabric and may progress to a point of structural failure prior to detection." The classic trap is the roof. The Adohi Hall monitoring study found that while interior floors stayed dry, at roof level "moisture intrusions during construction were trapped in the CLT panels by waterproofing," taking about a year to dry below acceptable levels. Water gets in before the build-up is complete; the waterproofing then prevents it drying out; the moisture sits against the timber.

Documented failures: the CROSS safety reports

The UK's Collaborative Reporting for Safer Structures (CROSS) scheme has published anonymised reports on exactly this mechanism, including Safety Report 852, "Rotting of cross-laminated timber (CLT) roof panels" (2019), and Safety Report 1124, "Water ingress to cross-laminated timber structural frame" (2022). These are not isolated curiosities; they are the profession formally flagging a recurring risk.

When it reaches the courts

Moisture damage to mass timber is now generating large, well-documented UK disputes. Liability is fact-specific, but the recurring question is always whether the timber was protected, monitored and dried before it was closed up.

Sky Central — Sky & Mace v Riverstone [2023] EWHC 1207 (Comm)

Sky's global headquarters in west London features what has been described as Europe's largest flat timber roof, glulam beams supporting hundreds of timber cassettes. During construction the cassettes were exposed to substantial rainfall over several months without a temporary roof, then sealed without adequate ventilation; the timber swelled and decayed. The Construction All Risks insurance dispute that followed involved sums reported in the region of £200m. The court found the damage referable to the decision not to use a temporary waterproofing system during installation, and Sky was awarded an interim payment of nearly £39m, with the matter continuing on appeal.

Vitsoe v Waugh Thistleton Architects [2025] EWHC 850 (TCC)

A CLT roof at a headquarters building in Royal Leamington Spa was wetted by sustained rainfall around Christmas 2016, rotted, and had to be repaired or replaced, a remedial claim put at around £4.4m. The architect was found not liable. The court held that a "suitable and robust specification" had been provided, that close sequencing was a suitable method of moisture protection, and that the programming and protection of the roof works were not, on the facts of that contract, the designer's responsibility. The case is a reminder that whether a designer carries the moisture risk depends entirely on how responsibility was defined.

Sandal Magna eco-school — Sarah Wigglesworth Architects

A celebrated, RIBA-award-winning CLT primary school in Wakefield suffered roof leaks and, when the defective roof was being replaced, "extensive" timber rot and structural concerns were found in the main teaching block. The architect was reported to have been ordered to pay the local authority £1.3m.

Read together, these cases make one point unmistakable: the cause is consistently construction-phase wetting, the sums are large, and the outcome turns on who owned the moisture control plan and the protection of the timber.

The economics: prevention versus cure

Protecting timber costs a few pounds per square metre. Drying or repairing it costs hundreds to thousands.

Caldwell et al. (2025) provide the hard figures from a supervised four-storey project: protective covers, including all fastenings and weighting, cost around €3.80/m² (2022 prices). If active drying of a wetted CLT roof slab is required, "the cost of temporary enclosure and drying could [be] many hundreds of €/m²," and "in the event of moisture damage to the timber, the cost of a repair can be several thousand €/m²." The same chapter documents a real timber-frame wall "displaced by 5.2 cm due to a glued laminated timber slab with elevated moisture content," a tangible illustration of restrained swelling doing structural damage.

"The costs of such measures are, however, small compared to the potential time and monetary costs of active drying or of rectifying moisture damage." — Caldwell, Viereck, Aondio & Flexeder (2025)

The pattern, and how to break it

Every case above shares the same DNA: timber exposed too long, sealed too wet, with no continuous record of its condition and no clear owner of the moisture risk. The remedy is equally consistent: a moisture strategy set at the design stage, protection detailed for the roof and end grain, defined moisture limits and hold points, and continuous monitoring so trapped water is caught before it becomes decay, with the data to answer any question raised later in the building's life.

Don't become the next case study

Vector helps structural engineers and their clients take the moisture risk off the table: independent waterproofing and moisture-strategy review at RIBA Stage 3, and embedded leak-detection and moisture sensors designed into the build to monitor the highest-risk junctions continuously, with a 20+ year battery life and automated alerts. The evidence that the timber stayed dry, captured, timestamped, and available if it is ever needed.

Talk to Vector

References

1. Caldwell, M. (2024). Moisture management during construction (Timber Knowledge Sheet). Timber Development UK. (Citing CROSS Safety Reports 852 and 1124.)
2. 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.
3. Poblete, E., et al. (2022). Moisture Monitoring of a CLT Structure in a Southern Climate (Adohi Hall). Journal of Architectural Engineering (ASCE), 28(2).
4. Sky UK Ltd & Mace Ltd v Riverstone Managing Agency Ltd & Ors [2023] EWHC 1207 (Comm); appeal [2024] EWCA Civ 1567.
5. Vitsoe Ltd v Waugh Thistleton Architects Ltd [2025] EWHC 850 (TCC).
6. Architects' Journal (2022): "Sarah Wigglesworth told to pay £1.3m over leaking Yorkshire eco-school" (Sandal Magna Community Primary School).

Published by Vector (Vector Leak Consultants Ltd). General technical information, not legal advice. Case summaries are drawn from published judgments and contemporaneous industry reporting and are provided for general illustration; refer to the primary judgments for the full findings.

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