Cross-laminated timber and moisture - Research Summary

Sep 15, 2026

Research Summary documents the mechanisms and risks of moisture uptake in cross-laminated timber (CLT) when panels are exposed to prevailing weather during construction. It highlights how sealing, insulation overlays, and temporary coverings can restrict drying, leading to long-term wetting and fungal decay. The report outlines a two-phase research programme with Phase 1 evaluating temporary preventative strategies and Phase 2 investigating the most effective drying approaches for wet panels, informed by CLT project experience.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages18

Reading Time45 min

AccessFree to download

Introduction

CLT construction exposure and moisture risks

Research programme scope and phases

Phase 1: Preventative strategies

Water uptake assessment and risk reduction goals

Protection methods evaluated

Panel joint types and wetting regime considerations

Research Summary Cross-laminated timber and moisture While every effort is made to ensure the accuracy of the advice given, the company cannot accept liability for loss or damage arising from the information supplied. © 2022 Warringtonfire Testing and Certification Limited All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form, by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owners. All photographs and illustrations are © Warringtonfire Testing and Certification Limited unless otherwise credited. ISBN: 978-1-909594-92-0 Research Summary Introduction Over the past decade, cross-laminated timber (CLT) buildings have been making headlines in the UK and international construction press. Completed in 2009, Stadthaus in Murray Grove, London became the world’s tallest residential timber building at nine storeys, 29m tall. This was exceeded by Forté Living in Melbourne, Australia in 2013 at more than 32m tall, followed by Mjøstårnet in Brumunddal, Norway in 2019 at 85m tall. Others set to exceed this height are now under construction. At the other end of the spectrum, Dalston Works in London boasts the largest square meterage of CLT. These CLT building systems are now commonly being used in the construction of large commercial, retail and educational buildings. A common thread among all of these types of buildings is the length of time during construction that parts of the CLT structure are exposed to the prevailing weather; the larger and more complex the building, the greater the risk of wetting during construction. Through BM TRADA’s timber consultancy service we have been engaged on numerous projects where CLT roof and wall panels have become wet during construction. Once closed in with foil-faced insulation and overlaid with roof or other impermeable coverings, drying has been severely restricted, resulting in long-term wetting and the formation of fungal decay within the panels. During discussions with Stora Enso we agreed to undertake a programme of research work to: determine the mechanism and extent of water uptake when CLT panels are exposed to wetting evaluate the effectiveness of temporary preventative strategies to minimise risk investigate how panels dry when wet. The programme was divided into two phases: Phase 1 consisted of assessing ways to protect CLT panels from wetting during construction. Phase 2 consisted of investigating the most effective ways to dry panels if they get wet. Phase 1: introduction The purpose of Phase 1 was to: assess water uptake during construction investigate methods for reducing risk. Through discussions with Stora Enso, and our experience of commercial projects, we decided to investigate the: Figure 1: Horizontal CLT panel exposed to wetting during construction 2 Cross-laminated timber and moisture use of end-grain sealers to protect joints use of end-grain sealers to protect the face of panels use of adhesive tapes to protect joints use of a temporary protective membrane influence of joint type influence of wetting regime. Horizontal panel joint type The three main types of panel joint are butt, half-lap and spline. Different panel joints have differing surface areas and therefore potentially different risks of wetting. Stora Enso confirmed that the most common types of panel junction they produce are half-lap and butt joint so we selected these types for testing. Ifa third joint type had been tested, it would have exceeded the limit on the number of panels we could accommodate. Self-adhesive tape Depending on the CLT system being used, joints between panels are often sealed with adhesive tape to provide a line of airtightness to the structure. During construction this tape can limit the water ingress into joints. In test panel configurations where tape was used to

FAQs

Why does construction wetting create higher risk for CLT buildings?

During construction, CLT parts can be exposed to prevailing weather for varying lengths of time. Larger and more complex buildings increase the risk of wetting, which can lead to restricted drying once coverings are applied.

What problems were observed when CLT roof and wall panels became wet during construction?

After panels were closed in with foil-faced insulation and covered with impermeable coverings, drying was severely restricted. This resulted in long-term wetting and fungal decay within the panels.

What does Phase 1 of the research programme focus on?

Phase 1 focuses on assessing water uptake during construction and investigating methods for reducing risk by protecting CLT panels from wetting, including evaluating sealers, tapes, protective membranes, joint types, and wetting regimes.

Sep 15, 2026

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