What is the standard roof vent size?
Short answer. There is no single standard, but common sizes are shingle-over ridge vents in 4 foot sections, box vents rated around 50 square inches each, turbines with 12 or 14 inch throats, soffit vents 16 inches long and 4 to 8 inches wide, bath fan caps for 4 inch ducts, and plumbing vent pipes of 1.5 to 4 inches. For attic ventilation, the net free area rating matters more than the outside dimensions.
Exhaust vents
- Ridge vent: rigid sections commonly 4 feet long, or rolls, with net free area listed per linear foot.
- Box or louver vents: listed by net free area per vent, many around 50 square inches, with a cut opening specified by the manufacturer.
- Turbines: 12 and 14 inch throat sizes are common.
- Powered fans: rated in cubic feet per minute, with a required intake area.
Intake vents
- Rectangular undereave vents: 16 by 4, 16 by 6 and 16 by 8 inches are common.
- Round plug vents: roughly 1 to 4 inches across.
- Continuous strip vents: sold by length.
Other roof penetrations
- Plumbing vents: 1.5, 2, 3 and 4 inch pipe, with boots sized to match.
- Bath fan caps: most fans use 4 inch duct; some use 3 or 6 inch.
- Kitchen hood caps: often 6 to 8 inches, larger for high output hoods.
Why size alone misleads
Two box vents with the same footprint can have different net free area depending on louver and screen design. When replacing a vent, match or exceed the old one's rating and keep the total in line with your attic's requirement. See roof ventilation basics and pipe boot leaks.