How much of a gap do you cut for a ridge vent?
Short answer. For most shingle-over ridge vents, the slot is about 2 inches wide on a truss roof, roughly 1 inch on each side of the peak, and on a roof with a ridge board it is cut about 1 inch beyond each face of the board. The slot usually stops at least 6 inches short of the rake ends, and the exact widths and limits come from the specific vent's installation instructions.
Typical slot dimensions
- Truss roofs (no ridge board): one major manufacturer specifies 1 inch on each side of the ridge, for a total opening of 2 inches.
- Ridge board roofs: the same manufacturer calls for a 1 inch slot beyond the ridge board on each side, with the combined width of slots and board not exceeding 3 1/2 inches. Another major maker allows a wider cut on each side but sets its own maximum total opening.
- Some products are narrower: at least one vent maker limits truss roof slots to about 1 1/2 inches total. That is why the instructions for your specific vent are the final word.
Where to stop the slot
- At least 6 inches from the rake edge or any end wall, so wind driven rain cannot enter at the ends.
- Farther back from hip and ridge intersections and chimneys; one manufacturer specifies 12 inches.
Gaps between vent sections
Some instructions call for a small gap, about 1/8 inch, between vent sections installed in cold weather so they can expand in summer heat without buckling.
Why the width matters
Too narrow, and the vent cannot deliver its rated net free area, so the attic is under ventilated. Too wide, and the vent may not cover the opening fully, inviting leaks, and you lose nailing surface for the vent and caps. Cutting into the rafters or trusses is a structural mistake, so the saw is set just deep enough for the decking. See ridge vent vs box vents and roof anatomy explained.