Dry-out behaviour of cross-laminated timber (CLT) edge conditions in roof assemblies: A field study

Exposure to wetting is a concern during mass timber construction and in service. Mass timber roof assemblies are susceptible to moisture intrusion and sustained loading as surface ponding. Because wood is hygroscopic, crosslaminated timber (CLT) panels absorb and store moisture when exposed to bulk water. Moisture is rapidly absorbed parallel to the direction of wood grain, making the edge of CLT panels particularly vulnerable to sorption. This field study monitors the moisture of CLT panel edges to assess distribution patterns and dry-out behaviour. Field data was collected for 11 months from a mass timber building under construction in Toronto, ON. CLT roof assembly data was collected at ten locations, each location measuring the following data points: relative humidity (%) and temperature (degrees C) at the interior surface of the CLT, and moisture content (%) and temperature (degrees C) at three depths in each CLT panel: the interior wood layer, the center wood layer, and the exterior wood layer. The results of this field study demonstrate the volatility of the moisture behaviour at CLT edge conditions in mass timber roof assemblies, including: the impact of exposure to moisture prior to the direct application of an impermeable membrane to the exterior surface of the CLT. Two major outcomes of this research are: 1) the comparative analysis of dry-out rates based on MC monitoring location within the CLT panels (interior, center, or exterior wood layer), and 2) the observation of moisture sorption within the center layer of a CLT panel during the monitoring period. The results of this research demonstrate a significant increase in the dryout period of any wood layer measuring above 15 % MC, particularly at the exterior wood layer where the measured dry-out rates (%MC/hr) are on average approximately 1.5-2.5 times slower than those measured at the center and interior wood layers. An exception to this outcome was noted at the center wood layer of one of the monitoring locations where a positive dry-out rate was determined based on the collected data - indicating moisture sorption at this location during the monitoring period.