Ice Dam and Condensation on a Cathedral-Ceiling Metal Roof
A homeowner has a 7/12 cathedral-ceiling roof over a 2x10 rafter frame with fiberglass batt insulation and no ridge or soffit ventilation. The roof has a 24-in. eave overhang and a heated second floor below. The existing asphalt shingles are being removed to 1x6 spaced sheathing; the contractor proposes to lay 30-lb felt directly over the spaced sheathing, install steel shingles, and add a 3-ft strip of self-adhered ice dam membrane at the eave. Homeowner reports icicles and water stains at the top plate after a 2-ft snowfall, even though flashing was replaced. The contractor says the leak is from a failed ice dam membrane and wants to add a second layer of felt. There is no ventilation and the bathroom exhaust terminates in the attic.
1. Diagnose the likely cause of the icicles and top-plate staining. Explain why replacing the ice dam membrane alone will not solve the problem.
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The reported icicles and top-plate staining point to air leakage and warm moist air reaching the cold roof deck, not simply a failed strip of membrane. A cathedral ceiling with fiberglass batts, no ventilation, and a bathroom exhaust terminating in the attic is a classic condensation and ice-dam setup. The exhaust adds moisture; warm air bypasses the insulation and melts snow at the ridge or middle of the roof; refreeze at the eave creates an ice dam; meltwater then backs up under the shingles. The 24-in. overhang is colder, so the dam forms there. Replacing flashing or adding felt will not stop it. The first step is to air-seal the ceiling plane, remove the exhaust from the attic and terminate it outside through the wall or roof, and provide a continuous insulation and ventilation path. For a cathedral ceiling, use baffled channels from soffit to ridge, add soffit and ridge vents, and keep insulation in contact with an air barrier but not blocking the channel. A blower-door test before and after helps verify the air sealing.
2. Specify the deck preparation, ice-dam protection, and underlayment for the metal shingle roof. Include the required ice-barrier extent for the 24-in. overhang.
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Steel shingles need solid sheathing unless the manufacturer specifically approves spaced sheathing; 1x6 spaced boards with only 30-lb felt allow the metal to deflect, distort the slots, and tear the underlayment. Remove the spaced sheathing or overlay it with minimum 7/16-in. OSB or plywood screwed to the rafters, then install underlayment per the manufacturer. The ice-dam membrane should extend from the eave edge to at least 24 in. inside the exterior wall line. With a 24-in. overhang, that is a minimum of about 48 in. from the eave edge, and many manufacturers require more in severe areas; a 3-ft strip is short. Use a self-adhered membrane approved for metal roofs and compatible with the metal and underlayment; if the membrane is granulated or slippery, use the manufacturer’s slip sheet. Cover the membrane with a suitable underlayment for the field. Do not add a second layer of felt over a failed ice barrier as a substitute; correct the deck, the membrane extent, and the ventilation.
3. What sequencing and verification steps would you require before the roof is closed up?
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Sequence the repair: remove the old roofing and sheathing, inspect the rafters and deck for rot, install new solid sheathing, air-seal and correct the exhaust, then install ice-dam membrane, underlayment, and metal shingles. Verify ventilation openings are not blocked by insulation or membrane; the intake and exhaust areas must balance. Before closeout, check the eave detail: drip edge over the membrane, membrane turned up the wall, and no fasteners through the membrane below the waterline. Confirm the eave overhang is not so cold that the ice barrier is the only defense; ventilation and insulation are the primary fix. After the first snow, inspect the attic for moisture and the eave for icicles; if they return, the air sealing or ventilation is still wrong. Document the membrane product, extent, and fastening, and keep the manufacturer’s installation instructions on site.