How wide should roof vent openings be?
Short answer. There is no single width; each opening is cut to the vent manufacturer's template or spec, and the number of vents comes from how much net free area the attic needs. Typical figures are a ridge slot of about 2 inches total on truss roofs, box vent openings cut to the template printed on the vent, turbine openings matching a 12 or 14 inch throat, and soffit vents commonly 4 to 8 inches wide.
Opening by vent type
- Ridge vent: commonly about 1 inch on each side of the peak on truss roofs; on roofs with a ridge board, about 1 inch beyond each face of the board, up to the maker's maximum.
- Box vents: the opening is cut to the size printed on the vent or its template. Cutting larger leaves the flange too little overlap; cutting smaller chokes the vent.
- Turbines: the hole matches the throat, commonly 12 or 14 inches.
- Powered fans: per the fan's template, usually larger.
- Soffit vents: rectangular undereave vents commonly come 16 inches long and 4, 6 or 8 inches wide; the cut opening is slightly smaller so the flange covers it.
Why bigger is not better
- An opening wider than the vent covers is a leak.
- A wide ridge slot cuts away the solid deck the vent and cap nails need.
- Exhaust larger than intake pulls air from the house.
Screens and openings in the vent
The code limits the vent openings themselves to between 1/16 and 1/4 inch, or screens to that range, to keep out pests without clogging. That is about the louvers and mesh, not the hole in the deck.
Getting the total right
Work out the net free area the attic needs, divide by each vent's rating, and cut each opening to spec. See roof ventilation basics and ridge vent vs box vents.