Bathroom, Dryer and Kitchen Vents Through the Roof: Done Right
The moist air your house exhausts has to exit somewhere, and “somewhere” is too often the attic, where it becomes mold, rot and mystery leaks. Here is how appliance exhausts properly exit a roof, and the shortcuts that rot houses.
The attic-venting sin
Open attics across America are quietly rotting because a bath fan blows into them: warm shower air condenses on cold sheathing all winter, mold blooms across the deck, insulation mats wet, nails frost and drip (“attic rain”), and the eventual repair bill reads like a roof leak that never was. Venting into the attic, straight, at the soffit (where the intake re-inhales it), or “toward a vent”, violates code and physics equally: exhaust must run in a duct to its own dedicated exterior termination. The attic passthrough is the single most common defect our inspections photograph, and every mold-remediation quote it eventually causes cost more than the duct that would have prevented it.
Bathroom fans through the roof
The right assembly: smooth or quality insulated flex duct, sized to the fan (4-inch standard), run as short and straight as the framing allows, insulated wherever it crosses unconditioned attic (uninsulated duct condenses the shower inside itself and rains back into the fan, the classic “my fan drips” call), sloped subtly toward the roof so incidental condensate exits outward, and terminated in a dedicated roof cap with damper and screen, never a ridge vent, never a soffit, never the general attic airflow. The cap is a roofing component: base flange woven into the shingle courses like any flashing, upper courses over, lower course under, sealed at the sides only. Wall exits, where the floor plan allows, spare the roof a penetration, but roof caps done right serve decades.
Dryer vents: the strict one
Dryer exhaust carries lint, which is fuel, and the rules reflect it: rigid smooth metal duct only (flex foil is a lint trap and code violation on concealed runs), length limits with deductions per elbow, no screen on the termination (screens catch lint into a mat; dampers alone, cleanable), and a cap engineered for lint passage. Roof terminations are legal and common on interior laundry rooms, with two honest costs: lint accumulates at the cap (annual cleaning stops being optional, per the maintenance habit) and the run often grows long and elbowed, so wall exits win where possible. A lint-caked roof cap is a dryer that dries slowly, a heat trap, and a genuine fire statistic, the least glamorous and most consequential item in this whole guide.
Kitchen and plumbing exhausts
Range hoods follow dryer logic scaled up: smooth metal duct, grease consideration, dedicated caps with dampers, and never a recirculating hood pretending to be ventilation where real cooking happens. Plumbing vent stacks, the pipes already poking through your roof, are a different system entirely (they breathe the drains, not the house) and belong to the pipe-boot craft of our flashing guide; their one rule here is that nothing else may tie into them, a bath fan ducted into a plumbing stack (a real and recurring find) sends sewer gas rules and moisture both the wrong way. Each exhaust gets its own duct, its own termination, its own damper: the house breathes in dedicated channels or it breathes into itself.
The roofing half of the job
Every termination is a roof penetration, and the roofing rules of this site apply at small scale: caps flashed into the courses (flange under above, over below), ice-and-water membrane collaring the cut in cold climates, dampers checked at inspection time (stuck-open dampers are winter drafts and wasp doors; stuck-closed ones are moisture backups), and old abandoned caps properly removed and decked over rather than left as future leaks. When we re-roof, the exhaust audit rides along free: ducts that end in the attic get flagged, undersized or crushed runs noted, and terminations rebuilt into the new roof correctly, the cheapest moment those fixes will ever have, across our service areas.
Roof exhaust vent FAQs
Can a bathroom fan vent into the attic?
No, it is a code violation and a slow-motion disaster: shower moisture condenses on cold sheathing, molding the deck, wetting insulation and creating “attic rain” that mimics roof leaks. Bath fans must duct to a dedicated exterior termination, roof cap or wall exit, insulated across unconditioned space. The attic passthrough is the most common defect inspections find.
Why does my bathroom fan drip water?
Condensation in the duct: warm shower air crossing cold attic in an uninsulated duct condenses and runs back into the fan. The fix is insulated duct (or an insulation sleeve), the shortest straightest run available, subtle slope toward the exit, and a proper damper cap, not fan replacement, which changes nothing about the cold pipe above it.
Can a dryer vent go through the roof?
Yes, legally and commonly for interior laundry rooms, with strict rules: rigid smooth metal duct, length-and-elbow limits, a lint-rated cap with damper and no screen, and committed annual cap cleaning, lint accumulates at roof terminations faster than wall ones, and a caked cap is a fire risk and a slow dryer. Wall exits win where the floor plan allows.
Why should dryer vents never have screens?
Screens catch lint into a felt mat that chokes airflow within months, overheating the dryer and building the fire hazard the vent exists to prevent. Dryer terminations use dampers alone, cleanable flaps that close against weather and pests, which is why dryer caps are their own product, not shared with bath-fan caps that legitimately carry screens.
How are roof vent caps installed without leaking?
As flashing: the cap flange weaves into the shingle courses, upper courses lapping over the flange, lower course under it, sealant at the sides only, with ice-and-water membrane collaring the cut in cold climates, the same water-sheds-over-metal logic as every penetration on the roof. Caps surface-mounted on top of shingles with caulk are the leak version.
Fan dripping, dryer slow, or ducts ending in the attic?
Exhaust audits, insulated runs and caps flashed like they belong to the roof, because they do. Get a free assessment.
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