Assessing the effectiveness of cross-laminated timber (CLT) roof assembly moisture control and mitigation strategies: A field laboratory

Exposure to moisture and wetting is a concern in mass timber construction and service. Understanding the moisture conditions and the drying response of mass timber during construction and service can impact durability of the building material and occupant health because of the potential for biological degradation and mould growth, respectively. Cross-laminated timber (CLT) used in roof assemblies is particularly susceptible to high moisture loading from exposure to driving rain and extended periods of ponding and water retention on roofs during construction. The field research conducted in this study utilises a purpose-built CLT test facility in Toronto, ON to test and assess the impact of three specific moisture control strategies towards both mitigating moisture and increasing the dry-out rate of CLT used in roof assemblies. The moisture control strategies tested were: (1) the integration of an air cavity directly above the CLT within the roof assembly, (2) the protection of CLT during exposure and moisture loading periods on site, and (3) the permeability of membranes installed in the roof assembly and during site exposure. The CLT roof panels underwent four months of exposure to exterior environmental conditions followed by two months of bulk water inundation at the CLT test facility site immediately prior to installation and enclosure. Moisture content was monitored at evenly distributed depths in the CLT panels throughout exposure and inundation periods and for one year of simulated service conditions, post-enclosure. Comparative analysis of the moisture control strategies tested during this field laboratory demonstrate that the presence of an air layer above the CLT in the roof assembly consistently doubled the dry-out rate compared to the assemblies without an air cavity. The results also found that unprotected CLT panels took approximately 2.5 times longer to dry-out to below 15 % MC than protected panels and finally, that in comparing the results of using impermeable vs. permeable air/vapour barrier membranes on the surface of exposed CLT, that the impermeable membrane slowed the dry-out rate of wetted CLT by approximately 20 %. The results of this research support the implementation of specific moisture mitigation and control strategies during design and construction towards promoting rapid dry-out of CLT in roof assemblies in service. • Integrated air cavity in CLT roof assemblies promotes drying response. • Protection of CLT on construction sites reduces initial peak moisture content and dry-out period. • Impermeable membranes applied to CLT trap moisture and slow drying response. • Moisture content at the exterior wood layer is primarily influenced by liquid water transport due to rainwater exposure and ponding water.