Roof Leaking Around a Vent Pipe? Why Pipe Boots Fail and How to Fix Them

What a plumbing vent pipe is and why it goes through your roof

Every drain in a house is connected to a vent stack that runs up through the roof. The vent lets air into the drain lines so water flows freely and sewer gases exit above the house instead of inside it. On most homes you can spot one or more short pipes, usually white or black plastic or older cast iron, sticking up a foot or so above the shingles.

Because the pipe is open to the sky and passes straight through the roof deck, the seal around it has to be watertight for decades. That seal is called a pipe boot, pipe flashing or vent boot.

The types of pipe boots

Rubber (neoprene or EPDM) collar boots

The most common type: a flat metal or plastic base that sits under the shingles, with a flexible rubber collar that grips the pipe. They are inexpensive and easy to install. Their weak point is the rubber, which sits in direct sun and can dry out and crack, often well before the shingles around it are worn out.

Lead or soft-metal boots

A one-piece sleeve of lead or similar soft metal that slips over the pipe, with the top folded down inside the pipe opening. There is no rubber to crack, so they tend to last a long time. Their weak points: the soft metal can be dented or torn, and squirrels and other animals are known to chew it.

Retrofit collars

Slip-over rubber collars made to go on top of an existing cracked boot without lifting any shingles. They can stop a leak, but they depend on the old boot underneath, so treat them as a repair that buys time, not a permanent fix.

Metal flue pipes are different

A metal pipe from a furnace, water heater or exhaust fan is flashed differently. It usually has a metal flashing cone plus a storm collar, a metal ring clamped and sealed on the pipe just above the flashing. The collar sheds water away from the gap around the pipe. When the collar slips or its seal dries out, that pipe can leak just like a failed rubber boot. Do not use a rubber plumbing boot on a hot flue pipe.

Why vent pipe boots fail

• UV cracking. The rubber collar faces the sun all day, every day. Over time it hardens, shrinks and splits, opening a gap right at the pipe.
• Exposed or backed-out nails. The lower part of the flashing base sits on top of the shingles. If it was nailed through with exposed nails, those nail holes are an open path for water once the nails loosen or the sealant over them fails.
• Improper shingle weave. The upper half of the base belongs under the shingles above it; the lower half belongs on top of the shingles below. If a boot is installed with its top edge on top of the shingles, water runs straight under it.
• Wrong size. A boot made for a 3-inch pipe on a 1.5-inch pipe, or the reverse, never seals properly.
• Animal damage. Squirrels chew lead boots. Damaged metal lets water straight in.
• Physical damage. Ice sliding off a roof, falling branches or foot traffic can tear a collar or bend a base.

Signs your roof is leaking at a vent pipe

Inside the house

• A water stain on the ceiling below or near a bathroom, kitchen or laundry room, since those rooms sit under vent stacks.
• A stain that grows or darkens after a rain and does not change when nobody has used the plumbing.

In the attic

• Water trails or dark staining on the outside of the vent pipe where it comes through the deck.
• Stained or soft sheathing in a ring around the pipe.
• Damp or matted insulation directly below the pipe.

From the ground (use binoculars)

• A rubber collar that looks split, gapped or pulled away from the pipe.
• A flashing base that is lifted, bent, or missing nails or sealant.
• A torn or chewed lead sleeve.

One caution: not every stain under a bathroom is a roof leak. A leaking drain, supply line or toilet seal can leave the same mark. If the stain appears whether or not it rains, call a plumber first.

How to fix a leak around a vent pipe, from best to worst

1. 1Replace the boot. The shingles above the pipe are lifted carefully, the old boot is removed and a new boot of the right size is installed with its top under the shingles above and its bottom over the shingles below. Fasteners go where the shingles will cover them. On a roof in good condition, this is the correct repair.
2. 2Install a retrofit collar. If the base is sound and only the rubber collar has cracked, a slip-over collar can seal the pipe. It is quicker, but it still relies on the old base.
3. 3Seal it, temporarily. Roofing sealant around a cracked collar can slow a leak until a proper repair is done. Sealant dries out in the sun, so it is a patch, not a fix.
4. 4Replace the roof when the roof is done. If the shingles are near the end of their life, replacing just the boot only solves one of the problems. A full replacement re-flashes every pipe, vent and chimney at once. See how long a roof replacement takes.

Do not ignore a slow leak at a pipe. Water that gets past a boot soaks the deck around the pipe first, and that wood is often the first to rot. If it does, it gets replaced during the repair; we explain what that involves in our article on roof decking replacement.