Roof Caps for Exhaust Vents: Sizes, Dampers and Installation
Updated · By the Roof to Footing editors
A roof cap, also called a rooftop cap or roof jack, is the metal hood that ends an exhaust duct where it passes through the roof. It keeps rain, snow, birds and backdrafts out while letting a range hood, bathroom fan or dryer blow air outside. Picking one is mostly about matching the duct size, choosing a cap with a working damper, and flashing it into the roof so it doesn't leak.
People also say "roof cap" when they mean the cap over the roof peak. If that's what you're after, see our guide to the roof ridge cap.
Sizes by appliance
Match the cap's collar to the duct. A cap smaller than the duct chokes airflow, and caps vary a lot in how freely air passes through them.
| Appliance | Typical duct | Cap collar |
|---|---|---|
| Bathroom fan | 4 or 6 inch | 4 or 6 inch |
| Clothes dryer | 4 inch | 4 inch, made for dryers |
| Range hood, up to about 400 CFM | 6 inch round or 3¼ x 10 rectangular | 6 inch or matching rectangular |
| Range hood, 600 CFM and up | 8 or 10 inch | 8 or 10 inch, high-flow design |
For bigger hoods, check the hood maker's duct size and pick a cap they list as compatible. Low-profile caps look nice but can cut a powerful hood's airflow noticeably. Our duct CFM chart shows how much air each duct size carries.
Dampers and screens
- Backdraft damper: a flap that opens when the fan runs and closes when it stops. It keeps cold air, rain and insects from coming down the duct. Check that it swings freely after install.
- Bird guard or screen: fine for bath fans and range hoods. Not allowed on dryer exhaust. The International Residential Code (M1502.3) says dryer duct terminations get a backdraft damper but no screen, because lint clogs screens and creates a fire risk.
- Many dryer manufacturers discourage roof terminations altogether because lint builds up in a long vertical run. A wall termination is usually better if you have the choice.
Where the exhaust can't go
Under the IRC (M1501.1), exhaust air must go outdoors, and it can't be dumped into the attic, a soffit, a ridge vent or a crawl space. A bath fan duct that just lies in the attic, or ends under a ridge vent, puts warm, moist air right where it causes mold and frost on the roof deck. A roof cap fixes that.
Installation basics
- Find the spot from the attic, between rafters, keeping the duct run short with few bends.
- Cut the hole the size of the cap's collar, not the flange.
- Slide the cap's flashing flange under the shingles above and at the sides, and over the shingles below, so water always runs over the flange, never behind it.
- Seal nail heads and the shingle edges with roofing sealant. Don't rely on sealant alone.
- In the attic, connect the duct with metal tape (not cloth duct tape) and insulate the duct in cold attics to stop condensation.
On a metal roof, use a cap with a flexible base or a flashing boot made for metal panels, and keep it out of the panel flats where water collects.
Materials
Galvanized steel is the budget option and can rust at cut edges. Aluminum doesn't rust and comes painted. Copper looks good on copper-trimmed houses but must not drip onto aluminum or steel below. Painted caps can be matched to shingle color so they disappear from the street.
Signs your roof cap needs attention
- Damper stuck open or missing (you feel drafts at the fan)
- Rust streaks below the cap
- Daylight around the collar from the attic
- Bird nests or lint buildup blocking the opening
- Shingles lifting around the flange