What size roof vent do you need for a range hood?
Short answer. Use a roof cap that matches the duct size in the range hood's installation instructions, never smaller. Many residential hoods use 6 inch round duct, while more powerful hoods often call for 7, 8 or 10 inch round or a rectangular duct, and the cap should be a kitchen exhaust cap with a damper and no fine screen.
Where the size comes from
Hood manufacturers specify the duct size needed to deliver the rated airflow. Reducing the duct, or using a cap with a smaller throat, cuts airflow, raises noise, and leaves grease and smoke in the kitchen. As a rough guide, many hoods up to about 400 CFM use 6 inch duct and higher airflow hoods step up from there, but the installation sheet is the authority.
Code points that affect the roof cap
- The duct must discharge outdoors, never ending in the attic or a soffit.
- Duct must be single-wall galvanized steel, stainless steel or copper, smooth inside, with airtight joints.
- A backdraft damper is required, and many roof caps include one.
- Hoods able to exhaust more than 400 CFM need makeup air under the residential code, so a big hood can mean extra work inside the house as well as on the roof.
Choosing and placing the cap
- Use a cap designed for kitchen exhaust, with a larger hood and no screen fine enough to catch grease.
- Keep the duct run short and straight, with as few elbows as possible.
- Place the cap away from valleys, skylights and other openings.
- Flash it in like any vent: upper flange under the courses above, lower flange on top of the courses below.
- Expect grease to collect on the cap over time, and include it in routine maintenance.
A roof cap over a busy kitchen is a long-term penetration, so it deserves the same care as a skylight. See roof flashing explained and common roof leak causes.