Vent Flashing Done Right
Back up roof vent boots with flashing, counterflashing, and proper shingling to prevent leaks.
Synopsis: Mike Guertin describes his method for flashing roof vent boots, a main source of roof leaks. Because the vent flashing boot’s rubber collar is typically thin and seals only to the vent pipe, Guertin adds an additional seal with EPDM flashing tape with a butyl adhesive backing. In a series of step-by-step photographs, he shows how to flash the pipe, how to flash over and under the pan, and how to finishing by shaping the shingles.
Roofs generally don’t leak in the field of shingles unless there is some kind of damage. Other than ice dams, leaks usually occur at flashings around chimneys, roof-to-wall connections, and penetrations like exhaust vents. Of all the calls to investigate roof leaks I’ve received in the last few years, there’s been one main culprit: plumbing vent flashing boots. Often I find the rubber collar on the boot has split or been damaged by a falling branch or a critter chewing at the rubber. In some cases, the collar is fine, but the leak is due to how the shingles were detailed around the flashing pan.
Whatever the cause, the leak can lead to damage. When the roof has a vented attic below, the leak usually evidences itself as a stain in the ceiling beneath the attic, with little damage other than to the drywall and paint. Where the leak can be more devastating is on roofs where the rafters or trusses have been insulated with spray foam applied directly to the roof sheathing. There, the leak can persist for years before it’s noticed, all the while rotting the surrounding sheathing and framing.
Rubber collar: the weak link
Today’s roof shingles are much longer lasting than earlier generations. Many vent flashing boots, on the other hand, haven’t improved to…