A kitchen exhaust fan that vents through the roof is a short pipe with a cap on top, and the cap is the part people look at. The leak almost never starts there. Water usually gets in where the pipe passes through the roof covering, at the flashing that is supposed to shed rain down the slope and around the pipe. If you have a stain on a ceiling near an exhaust run, or a drip that appears only in wind-driven rain, this is a repair question you can start answering before anyone climbs up.
Where the water actually gets in
A roof vent for a range hood usually has a metal or plastic sleeve running up through the roof deck, a flange that lies on the shingles, and a cap. The flange is sealed to the roof by flashing, which is either a formed metal collar tucked under the shingles above and lapped over them below, or a rubber boot on a smaller pipe. When the upslope shingles lap correctly over the flange, rain runs off. When they do not, or when the sealant around the flange has cracked, water runs under the flange and down along the pipe into the attic.
Freeze and thaw is the usual cause in this region. Sealant around a flange expands and contracts through a St. Louis winter, and after several seasons it pulls away at the top edge, where the exposed bead meets the cap. Water then gets under the flange in the next thaw. A thick bead laid over a gap does not last as long as flashing that was tucked properly into the shingles, so a bead alone is a poor repair for a flange that is lifting.
What to check from inside first
Go to the attic on a dry day with a flashlight. Find the vent pipe where it comes up through the decking. Look for water staining on the underside of the sheathing around the pipe, on the pipe itself, and on the insulation beneath it. A dark ring on the wood that follows the pipe's edge points to the flange. A stain that runs along the top of the pipe and down toward the ceiling joist suggests water traveling down the pipe. Note which side of the pipe the stain sits on, because the fix depends on where the water came from.
Check the pipe joints inside the attic too. Duct tape is common on older exhaust runs, and tape fails. A disconnected section lets grease-laden kitchen air and moisture into the attic, which can stain the decking and soak the insulation even when the flashing is intact.
What to check from the roof
From the ladder, look at the upslope shingles first, not the cap. Shingles above the flange should overlap the top of the flashing and run over it. If they stop short of the flange, or if the flashing sits on top of them, water is being directed into the joint. Next, check the sealant. Cracked, shrunken or missing sealant at the top edge of the flange is an easy sign. Then look at the cap. A cap that has been knocked sideways or has a broken screen tells you the vent has been disturbed, but it is rarely the source of a leak into the house.
Do not walk the roof while it is wet, frosted or still warm from a storm that has just passed. Shingles are slick, and granules come loose under foot. If the pitch is steep or the only access is a second story window, leave this part to the crew.
What a proper repair includes
A repair that stops the leak replaces or properly reseats the flashing, not just the sealant. The crew lifts the shingles above the flange, slides the upper flashing edge under the course above it, and sets the lower flange over the shingles below. Fasteners go where the flashing and shingle instructions call for them, and they are sealed where they penetrate the flange. Sealant is a secondary line of defense at the flange edge, not the main one. If the flashing is corroded, cracked or too small for the pipe, it gets replaced.
If the exhaust run itself is loose, the repair includes reconnecting the duct with proper clamps or metal tape, sloping it so condensation drains away from the house rather than back toward the roof, and checking that the termination cap sits at the right height above the roof surface. A run that pitches toward the roof collects condensate at the joint, and that water eventually finds the flange.
What to ask before work starts
- Will the flashing be replaced or only resealed, and if replaced, what material and gauge?
- Which shingle courses will be lifted, and will any shingles be replaced because they were cut or have gone brittle?
- Will the duct joints inside the attic be reconnected and sealed, and is the run sloped away from the roof?
- Will the crew photograph the flange before and after, and the attic pipe with the old stain visible?
- Does the written scope name the vent, the flashing and the duct as separate lines, so you can see each one?
Keys Roofing documents the damage, puts a written scope with photos and measurements in your hands, and attends as the contractor while you deal with your own insurer. If a vent leak is part of a larger storm claim, the claim belongs to you and your insurer. Missouri law (RSMo 407.725) forbids a roofing contractor from representing or negotiating your insurance claim, and forbids saying anything about paying, waiving or covering your deductible.
Call (314) 220-2333 if the ceiling stain sits near an exhaust run and you want someone to look at the flange and the attic side before the next freeze.