Back Up Your Vent Pipe Flashing
Use flashing tape and additional underlayment to backstop your flashing boot.
Vent pipes that penetrate through the roof of a house are common places for roof leaks. The flashing boots used to seal around these pipes often don’t last as long as the shingles. When the thin rubber collar on these boots cracks or gets damaged, there’s typically no additional seal to keep water from infiltrating around the vent pipe. The roofing cement many builders use to seal the shingles around the vent pipe also doesn’t always last, creating another pathway for leaks. Rhode Island builder Mike Guertin takes a few extra steps to backstop vent flashing boots.
After shingling up to and around the vent pipe, Guertin uses 6-in.-wide strips of uncured EPDM flashing tape with a butyl adhesive backing to flash around the vent pipe. Mike likes Protecto Wrap’s Form Flash 2 EPDM for this application. Cut the pieces at least 6 in. longer than the diameter of the pipe.
It’s helpful to mark where the top of the flashing boot’s rubber collar will seal against the pipe. Apply the tape so that it’s about 1/2 in. below this mark. Start at the bottom, folding the tape roughly in half, and center it on the downslope side of the pipe. Carefully wrap it around and adhere it to the pipe. Try to get all of the air bubbles out of the tape, and press it firmly into the shingle, the pipe, and the underlayment. Then do the same thing with another piece of tape on the upslope side of the pipe.
A lot of roofers use roofing cement to seal the bottom of the flashing boot’s pan to the shingles. But over time, this cement can dry and crack. Mike prefers to use 6-in.-wide asphaltic flashing tape at this joint to create a dam against wind-driven rain. Cut a piece about an inch shorter than the width of the pan, and score the backing lightly down the center. Remove half of the backing, center the tape on the underside of the flashing pan’s lower leg, and stick it to the pan. The 1/2-in. gaps on either end let out any water that might make it under the pan. Fold the other half back, remove the rest of the backing, and fit the boot over the vent pipe. Make sure that the sticky side of the tape is down so that it sticks to the shingle, and press it down firmly for good adhesion.
Then use more strips of asphaltic tape to flash the sides and top of the flashing pan, sort of like flashing a window. Keep the side strips above the shingle’s self-seal strip so the next course of shingles can seal down, but make sure they extend a bit beyond the edge of the flashing pan. Then add the top piece of flashing so that it laps over the side pieces.
After that, counterflash with an additional piece of underlayment. Slip this piece under the next lap up in the roof’s underlayment, and make sure it extends down onto the pan of the flashing boot. Then staple it in place, and continue shingling.
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