Ice Dams, Cold Weather, and a Ridge Vent With No Intake
It is November 18, the forecast is 34 degrees and falling, and the crew is 12 squares into an 18-square, 8:12 re-roof with laminated architectural shingles. The house is a 1962 split-level with a cathedral-ceiling living room framed in 2x8 rafters, a 24-inch eave overhang, and only two small gable-end vents. The homeowner's original complaint was ice dams and paint stains at the eave. Your proposal included self-adhering ice-dam membrane at the eaves, synthetic underlayment, and a ridge vent. The crew chief wants to stop the ice membrane 3 feet up because 'that part isn't over living space,' and plans to hand-seal only the ridge caps because of the cold.
1. Where must the ice-dam membrane be installed, and does it solve the homeowner's actual problem?
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Where: the code requires the ice barrier to start at the eave edge and extend up the roof to a point at least 24 inches inside the exterior wall line of the heated building. The crew chief's 'not over living space' argument is exactly backwards - the overhang distance is irrelevant, and the 24-inch-inside-the-wall dimension is what protects the wall plate from backup. Stopping 3 feet up on a 24-inch overhang leaves nothing inside the warm wall line.
Does it solve the problem? No. The membrane is a backup, not a cure. The ice dam is caused by warm air leaking into the cathedral assembly and melting snow above the eave, which refreezes. The durable fix is in the assembly: air-seal the ceiling plane, insulate to the code level, and provide balanced ventilation - or, where a 2x8 rafter bay cannot hold both insulation and a clear vent channel, rigid insulation above the deck. Because the rafters are shallow, confirm that baffles leave an unobstructed path; if they cannot, adding a ridge vent alone is decorative. Explain this to the homeowner in writing: the new roof will look right and may still ice if the attic side is not addressed.
2. What are the cold-weather installation requirements, and what should the crew chief be told?
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Tell him the plan is wrong in two places.
- Sealing: shingle self-sealing adhesive strips do not bond below roughly 40 degrees. Every shingle tab in the field must be hand-sealed with an approved asphalt cement, not just the ridge caps - otherwise the first spring wind event lifts a field the manufacturer will not warrant. Keep bundles in a heated trailer until they go up, and do not install below the manufacturer's published minimum temperature.
- Handling: cold shingles crack when bent and are slick underfoot; stage bundles over the ridge, not on the field, and limit foot traffic.
- Fastening: nails driven in the manufacturer's nailing zone - through the nailing line, not above the sealant strip and not below the common bond - heads flush with the shingle surface, never overdriven or high. A 6:12 or steeper roof in a wind-exposed area typically requires the enhanced fastener pattern; verify against the manufacturer's instructions.
- Underlayment: synthetic underlayment must be rolled tight and fastened per the manufacturer in cold weather; wrinkles left in cold product telegraph through the shingles.
Document the ambient temperature and the hand-sealing in the daily report.
3. The crew wants to install the ridge vent as planned. What is the ventilation problem and what are the options?
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The problem: a ridge vent with no intake path cannot work. Exhaust needs makeup air, and without soffit or eave intake the vent either does nothing or pulls air from along the ridge itself, short-circuiting the roof and increasing the negative pressure that draws warm interior air into the assembly - which is the ice-dam mechanism again. Net free area is roughly 1/150 of the attic area, reducible toward 1/300 only where a ceiling vapor retarder exists and intake and exhaust are balanced (confirm local amendment).
Options, in order of preference:
- Open the 24-inch soffit overhang and install continuous soffit intake vents with baffles that keep insulation out of the air path - then the ridge vent has a purpose.
- If the soffit is inaccessible at the cathedral section, use a different assembly there (rigid insulation above deck plus a vented or unvented detail approved for that assembly) rather than a ridge vent that vents nothing.
- Gable vents are not a substitute and should not be combined with a ridge vent in a mixed strategy that short-circuits the flow.