What size gap should be cut for a ridge vent?
Short answer. Most ridge vents call for a gap of about 1 inch on each side of the peak, roughly 2 inches total, on truss roofs; on roofs with a ridge board, the cut is typically about 1 inch beyond each face of the board, and one major manufacturer caps the total opening at 3 1/2 inches. Some vents specify less, such as 3/4 inch per side, so the vent's own instructions are the final word.
Why the gap is limited
- The vent has to cover it. Each vent is designed to span a certain opening with its filter and baffle in the right place. A wider slot can leave the filter edge over open air and let rain in.
- More gap is not more airflow. Once the slot meets spec, the vent's net free area rating limits the flow.
- Deck strength: a wide slot leaves less wood under the cap nails.
Measuring correctly
- Find the true peak: where the truss top chords meet, or the ridge board on a stick framed roof.
- Measure out from the peak, or from each face of the ridge board, as the instructions specify.
- Snap chalk lines along the full run.
- Set the saw depth to the deck thickness so it does not cut framing.
Checking an existing roof
Many older roofs have slots cut too narrow or only partway. If a ridge vent seems to do nothing, look from the attic: you should see a consistent opening of the specified width along the run, stopping short of the ends. See ridge vent vs box vents and roof ventilation basics.