On a recent renovation project, Steven Baczek Architect was faced with a mid-1970s low-sloped improperly vented roof. The long side of the roof measured 30 ft. in length, and the framing was 2x10s 24 in. oc. with multiple intermediate bearing points. The roof was vented with typical vent chutes held tight to the underside of the roof sheathing, and the rafter cavity was packed with batt insulation. At numerous locations the cavities were interrupted by blocking and skylights, which left many of the cavities unable to vent.
Batt insulation is air permeable—a condition exaggerated at the end of the rafter cavities, where they meet the soffit overhang. As a result, on this house wind washing at the soffit led to annual ice damming on the roof and associated water leakage. After the demo was complete, we discovered large areas covered in mold, especially at the cavities where venting was restricted. Despite all good intentions, this roof assembly failed at water, vapor, and thermal control.
The project scope consisted of removing all the interior drywall, batt insulation, and vent chutes. The existing roof rafters and sheathing remained. On the exterior, the roof shingles and building paper were removed. A new strategy to prevent wind washing, control vapor, and manage thermal conditions was put in place. In areas of low mold risk, the mold was simply scrubbed and the plywood sheathing was left to dry. In the areas with a lot of mold, the plywood was replaced.
Fixing an improperly vented roof
The most important aspect of this retrofit was developing an effective ventilation strategy. The new detail at the soffit uses the existing fascia/soffit vent assembly. A 4-in.-wide gap running the full length of the soffit was cut into the existing sheathing. This provides an air path through the soffit…