Overlay Over Skip Sheathing
A contractor is bidding a tear-off of 28 squares of asphalt shingles on a 1948 house. The roof is 6:12, with 1x6 tongue-and-groove boards laid diagonally over 2x8 rafters at 24 in. o.c. The boards have 1/8-3/8 in. gaps, several knots, and two soft areas near a valley. The attic has bath exhaust discharging onto the sheathing and only two gable vents. The homeowner wants a 50-year architectural shingle and added attic insulation. Local code requires solid sheathing for asphalt shingles and R-49 ceiling insulation. The contractor proposes nailing 7/16 in. OSB directly over the existing boards without removing them, using 8d common nails, then installing 30-lb felt.
1. Is the proposed OSB overlay acceptable? What conditions would force removal or replacement?
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- An overlay can be acceptable only if the existing boards are sound, flat, and securely fastened, and if the new panels are fastened into the rafters, not just into the boards.
- The two soft areas near the valley must be opened and repaired or replaced. Knots and gaps do not necessarily prevent an overlay, but they can telegraph and can leave unsupported fastener locations.
- Because rafters are at 24 in. o.c., verify the selected panel thickness and fastener schedule against the code and shingle manufacturer. A 7/16 in. panel may be marginal depending on support and fastener spacing; do not assume it is adequate.
- 8d common nails are about 2-1/2 in. long. Through 7/16 in. OSB plus 3/4 in. boards, about 1-5/16 in. remains for the rafter, which is adequate only if the nail actually hits the rafter. With diagonal boards and 24 in. spacing, layout control is essential; ring-shank nails or screws may be more reliable.
2. Evaluate the underlayment and ice-dam detailing. What would you specify?
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- 30-lb felt is acceptable in many steep-slope applications, but a synthetic underlayment is lighter, stronger, and more slip-resistant; confirm manufacturer acceptance for the shingle and deck.
- In a cold climate, install an ice-dam protection membrane at eaves, valleys, and other susceptible edges, extending above the interior wall line as required by code. Do not rely on felt alone.
- Use corrosion-resistant fasteners and appropriate laps. The underlayment must be compatible with the shingle warranty and must not be perforated excessively.
- Because the attic has moisture problems, the underlayment alone will not solve condensation; ventilation and exhaust corrections are required.
3. How should ventilation and insulation be coordinated with the roof replacement?
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- R-49 ceiling insulation cannot be added without addressing attic ventilation. The bath fan must exhaust outdoors, not onto the sheathing; this is a likely moisture source.
- Two gable vents are unlikely to provide balanced intake and exhaust. Add soffit intake and ridge exhaust, and verify net free area after insulation is installed.
- Keep insulation from blocking soffit vents with baffles. Air-seal the ceiling plane before adding insulation.
- If ventilation cannot be corrected, consider an unvented assembly only with an approved design and moisture control, not as an afterthought.