Ridge Cap Damage: Why Ridge Shingles Crack, Blow Off and Leak

Field shingles stop just short of the peak on each side. The ridge cap is a row of shorter shingles folded over the peak and overlapped one on the next, covering the joint where the two slopes meet. On a hip roof, the same cap runs down each hip.

On many homes, the ridge also has a ridge vent: a slot is cut through the deck along the peak and a vent product is laid over it, then capped with ridge shingles. Warm, moist attic air leaves through the vent, and the cap shingles keep rain and snow out of the slot. When the cap fails on a vented ridge, water has a direct path into the attic.

West Michigan combines the two things ridge caps handle worst: cold and wind. Shingles installed in late fall may not self-seal until spring, which leaves new caps loose through months of winter wind. Cold also makes caps brittle, so the same gust that lifts a cap can snap it at the fold instead of just bending it.

Strong winds off Lake Michigan and from passing storm systems hit ridges first. Wind-driven snow is the other Michigan problem. Fine, dry snow can be pushed through ridge vents and under lifted caps, then melt on the attic insulation. That can look like a roof leak when the caps are actually intact.

The 2015 Michigan Residential Code requires asphalt shingles to meet wind classifications under ASTM D7158 or ASTM D3161 for the design wind speed where the house is (Section R905.2.4.1), and it requires roofing to be installed following the manufacturer's instructions. For the ridge, those instructions are specific.

GAF's Seal-A-Ridge instructions say to start at the bottom of a hip, or at the end of a ridge opposite the direction of the prevailing wind, nail each piece with two nails at set distances from the edges, and not to place nails in the sealant. In cold weather, GAF says to warm the ridge cap shingles before bending them and, when self-sealing may be delayed, to put quarter-sized dabs of asphalt plastic cement between cap pieces and press them firmly together. GAF's main shingle instructions also say to position ridge cap laps away from the prevailing wind.

On a vented ridge, the vent instructions add their own rules. GAF's Cobra ridge vent instructions say nails for the caps must be long enough to penetrate through plywood decks or at least 3/4 inch into wood planks, and recommend longer ring shank nails for uplift resistance on some models. The Michigan code's ventilation section (R806.2) sets the net free vent area the attic needs and, when relying on its 1/300 exception, the split between upper vents like a ridge vent and the intake at the eaves.

A few missing or cracked caps on an otherwise healthy ridge are a straightforward spot repair. When most of the caps along a ridge are cracked, or they are cut 3-tab pieces that are crumbling, replacing the full run of caps on that ridge is usually better value than chasing pieces after each storm.

If the ridge vent is the problem (crushed, letting in snow, or never balanced with enough intake at the eaves), the repair may include replacing the vent with a better baffled product and checking the soffits. If the attic has a ridge vent plus box vents or a power vent fighting it, that is a ventilation question as much as a roofing one.