Steep-Slope Re-Roof with Concealed Fastener Metal Over Existing Shingles
A contractor is bidding a re-roof on a 1970s two-story house with 5:12 gable roof, 34 squares of existing three-tab asphalt shingles over 15-lb felt on 1x6 skip sheathing with 2x6 rafters at 24 inches on center. The homeowner wants a standing-seam metal roof with concealed clips and a 40-year finish warranty. The attic has no ridge vent and only two gable-end louvers; the contractor measured 92°F attic temperature on a 78°F day. The homeowner also asked for the existing shingles to stay in place to save disposal cost. The contractor plans to install 30-lb felt over the shingles and screw clips through everything into the rafters. The metal manufacturer's published installation instructions require solid decking and specify clip attachment to structural framing with a minimum pull-out value.
1. Should the contractor install the concealed-fastener metal directly over the existing shingles as planned? What must change before the bid is submitted?
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No. The manufacturer's written instructions control, and they require solid decking; a skip-sheathing assembly with shingles over it does not meet that requirement, and clip pull-out through layered shingles and felt into 24-inch-on-center rafters is unreliable. The contractor must either (a) remove the shingles and install new structural decking (e.g., minimum thickness per the metal manufacturer and local code) fastened to the rafters, or (b) if the manufacturer explicitly permits a retrofit over existing roof, obtain written approval and follow their prescribed substrate and fastener schedule. The bid must carry the cost of tear-off, decking, and disposal. Leaving shingles in place to save money is not a valid basis for deviating from the manufacturer's installation instructions, which also govern the finish warranty.
2. The attic ventilation is clearly inadequate. How should the contractor address this as part of the re-roof, and why does it matter for the metal roof?
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The contractor should calculate the required net free ventilation area using the code ratio (commonly 1/150 of the vented ceiling area, or 1/300 with a vapor retarder and balanced intake/exhaust) and install a balanced system - continuous ridge exhaust plus soffit intake - rather than relying on two gable louvers. The 92°F attic reading indicates trapped heat and moisture. For a metal roof, inadequate ventilation raises attic temperature, increases thermal movement and condensation risk on the underside of the panels, and can void finish warranties. The contractor should include the ventilation upgrade in the scope and document the calculation in the proposal.
3. What documentation should accompany the bid to protect the contractor from a dispute over the substrate and ventilation changes?
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Attach the metal manufacturer's published installation instructions and the specific substrate requirement to the proposal; state in writing that the existing skip sheathing and shingles do not meet the manufacturer's requirement and that tear-off and new decking are included. Include the ventilation calculation and the specified vents. Note that the finish warranty is contingent on installation per the manufacturer's instructions. Have the homeowner sign an acknowledgment of the scope change, and keep the manufacturer's literature and any written approval in the job file.