What is the purpose of a ridge cap?
Short answer. The ridge cap's purpose is to seal the peak of the roof, where two slopes meet and no regular shingle can cover the joint. It overlaps both sides to shed water, covers the nails in the top course, protects the ridge vent opening while letting attic air out, and finishes the roof line visually.
The jobs a ridge cap does
- Seals the joint: field shingles on each slope stop just short of the peak. The cap bends over the ridge and laps down onto both slopes so water runs off either side.
- Covers fasteners: the nails in the top course of shingles would be exposed without a cap.
- Protects the ridge vent: on vented roofs, a slot is cut along the peak for attic exhaust. The vent product sits over the slot, and the cap covers the vent, keeping rain and insects out while air escapes.
- Resists wind: wind speeds up over the ridge, so caps are sealed and nailed to resist uplift at the most exposed line of the roof.
- Finishes the look: a straight, even ridge line is one of the first things people notice on a roof.
How it is installed
Cap pieces are laid one over the next with a set exposure, each nail covered by the piece above it. Installers typically lay them so the exposed edges face away from the prevailing wind. Hips, the sloped ridges on hip roofs, get the same treatment working from the bottom of each hip up to the main ridge.
How ridge caps fail
- Wind lifts or tears off pieces, often the first damage after a storm.
- Cut 3-tab caps on thick architectural roofs crack along the fold.
- Nails back out or were too short to hold through the vent.
- Hail cracks or bruises the caps, which sit at the highest point.
For how the cap fits with the rest of the roof, see roof anatomy explained and ridge vent vs box vents.