Mold in Vented Wood-Frame Roofs in the Coastal Pacific Northwest

Sloped roofs over ventilated attics are one of the most common, if not the most common, roof configuration in the United States and Canada. This type of roof assembly is typically associated with single family houses, but they are also commonly used on townhouse complexes and large architecturally complex multi-unit residential buildings. Unfortunately, in coastal areas of the Pacific Northwest, moisture problems including mold growth on roof sheathing are common in vented wood-frame roofs. Often this type of damage is attributed to rain water ingress through the roof assembly, inadequate ventilation of the attic space, and condensation associated with airborne moisture from air leakage, duct leakage, or re-entrainment of humid exhaust air into the attic space. However, despite recent improvements in ceiling airtightness, reduced duct leakage, and code compliant attic ventilation rates, issues of fungal growth and mold on roof sheathing are still common throughout coastal areas of the Pacific Northwest.

This RDH Technical Bulletin looks at causes of vented wood-frame roof moisture problems, suggests possible solutions, and provides additional resources for further information.