How to Install a Plumbing Vent Pipe Boot on a Shingle Roof
The rubber collar cracks long before the roof does. Here is how to replace it properly, with the flange shingled in and no caulk required.
- Intermediate
- 30 to 60 minutes
- Updated
The short answer
A plumbing vent pipe boot is a flange with a rubber collar that seals around a vent pipe where it passes through a shingle roof. Match the boot to the pipe diameter (commonly 2, 3 or 4 inches), slide the upper half of the flange under the shingles above, leave the lower half over the shingles below, and nail only the upper corners. Cracked collars are the usual failure, and sealant is not a repair.
Every plumbing vent passes through the roof, and every vent needs a pipe boot: a metal or plastic flange with a rubber collar that hugs the pipe. The flange is shingled into the roof and does the work of shedding water. The collar only keeps water from running down the pipe. Sunlight, moss and age crack the rubber, and that is the most common leak in a roof that is otherwise fine.
The fix is to replace the boot, not to smear the crack with caulking. That means lifting shingles, sliding the new flange under them and nailing in the right places. It is a small job that fits into a reroof and often into a repair.
This guide covers standard rubber boots on shingle roofs. For lead boots, see lead pipe flashing. For gas vents and metal chimneys, see B-vent flashing. For the overall picture, see roof flashing types. Pipes on other roofs are flashed differently: see flashing a pipe on a metal roof, EPDM pipe flashing, TPO penetration flashing and torch-on penetrations.
Tools
- Flat pry bar
- Hook blade utility knife
- Roofing hammer
- Tape measure
- Wire brush or rag for cleaning the pipe
- Caulk gun, for a touch of sealant at fasteners
Materials
- Pipe boot sized to the pipe, with a flange at least 12 by 12 inches or as the boot specifies
- Roofing nails, 1-1/4 inch, with rubber washers or sealant on the heads
- Sealant rated for the boot and shingles, only for fasteners and edges
- Replacement shingles if any are torn
- Ice and water shield, for a flat or low-slope area
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury, and vent pipes are often on the far side of the roof.
- Do not work on a wet roof or one covered in moss. A slip is likely when you are bent over lifting shingles.
- Cold shingles crack when bent. Below about 5 degrees C, warm the shingles in the sun or work slowly.
Step by Step
Step 1: Identify the pipe and buy the right boot
The pipe size is stamped on the pipe or you can measure across it. Plastic ABS or PVC vent pipe is commonly 2, 3 or 4 inches nominal, and the outside diameter is a little larger. Choose a boot sized for that nominal pipe, and check the packaging.
Some boots have stepped collars that cut to size and some have a fixed fit. A collar that is too loose will leak, and one that is too tight will tear as it is pushed on.
Step 2: Remove the old boot and free the shingles
Lift the shingles gently with a flat bar, working from the top of the flange. Pull the nails and slide out the old boot. Cold or old shingles crack easily, so go slowly and warm them if you can. Check the deck for wet or soft spots. A rotted deck around a pipe has to be repaired before the new boot goes on.
Watch out: If the decking is soft or stained around the pipe, stop and repair it. A new boot over rotten wood just hides the problem.
Step 3: Clean the pipe and the area
The new collar must grip a clean, dry pipe. Wipe off dirt and old caulk. Pull any stray nails and check that shingle granules are not bunched under the flange area, since the flange must lie flat. On a low slope, add a square of ice and water shield around the pipe before the boot goes on.
Step 4: Slide the flange into place
This is the lapping rule applied to a flange. Water running down the roof from above goes over the top of the flange, and the lower half lies on top of the shingle courses below so water keeps running down. Never put the lower half under shingles, since that reverses the lap. See the lapping rule.
If the flange will not slide under the upper shingles, carefully lift more tabs. Do not force it or cut the shingle to fit.
Step 5: Check the fit and centre the boot
The collar should hug the pipe all the way round with no gap. Tilt or wrinkling in the flange means it is not seated. Press it flat. Mark the shingle tabs that will need to be nailed back.
Step 6: Nail the upper corners only
Nails at the top of the flange are covered by the shingle course above and never see water. Nails along the lower or sides of the exposed flange are in the path of water and become leaks. If the manufacturer requires more fasteners, cover each exposed nail head with a dab of sealant. Use rubber washer nails or sealant on any nail that stays exposed.
Do not nail through the collar or in the pipe area.
Step 7: Replace the shingle tabs
The shingles above and to the sides of the boot should lie flat over the flange's upper edges. If a tab is cracked, replace it. See how to replace a damaged shingle.
Step 8: Check the attic side
Insulation around the pipe should be in place. Staining on the deck or pipe can show an earlier leak, and it is worth monitoring the spot after the next heavy rain. See how to trace a leak from the attic.
Why Rubber Boots Crack
Ultraviolet light degrades the rubber collar over time. Pipe movement, heat and cold cycles and moss growth speed it up. A crack at the collar does not mean the roof has failed, but it does let water run down the outside of the pipe and into the attic or wall. Many roofers replace boots on every reroof for this reason. Some boots have a metal or plastic collar over the rubber, or a replaceable gasket, which lasts longer.
The No-Caulk Approach
Sealant around a cracked collar is a short-term patch. It cracks as the rubber moves and gives a false sense of security. The repair that lasts is a new boot shingled in correctly. There are retrofit collars that fit over an existing boot, which may be an option when a full replacement is not practical, but follow the maker's instructions. See roofing sealants for where sealant does help.
Fraser Valley Notes
Moss and needle debris collect around vent pipes on shaded roofs and keep the collar wet, which shortens its life. Gently remove debris each fall, and look at each boot after a windstorm.
Common Mistakes
- Putting the lower flange under the shingles, a reverse lap that sends water under the boot.
- Nailing the lower or side flange so nail holes sit in the water path.
- Smearing caulk over a cracked collar and leaving it, which fails within a season or two.
- Using the wrong size boot, giving a loose collar that leaks or a tight one that tears.
- Forcing the flange under cold shingles and cracking them.
- Ignoring a soft deck around the old boot and covering it with a new one.
When to Call a Professional
If the pipe is on a steep or high roof, if the deck is soft, or if the boot is part of a cluster of vents, leave it to a roofer. A leak at a pipe can also be a sign of a bigger problem in the wall or the plumbing stack, and a roofer can check.
If you are unsure what the pipe is, treat it with care. A pipe with a metal sleeve and a storm collar may be a gas vent or chimney, and those need different flashing. See B-vent flashing.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Install a Plumbing Vent Pipe Boot: Questions
How do you replace a pipe boot on a roof?
Lift the shingles around the old boot, pull the nails and slide it out, clean the pipe and deck, then slide a new boot over the pipe. The upper half of the flange goes under the shingles above and the lower half lies over the shingles below. Nail only the upper corners, press the tabs back down and seal any exposed nail heads.
Why does a plumbing vent boot leak?
The rubber collar cracks from sun, age and moss, which lets water run down the outside of the pipe. A flange that was nailed along its lower edge, installed with a reverse lap or set over a soft deck can also leak. Cracked collars are the most common, and are fixed by replacing the boot.
Can you caulk a cracked pipe boot?
Only as a temporary measure. Sealant on a cracked collar fails quickly as the rubber moves and ages. A lasting repair is to replace the boot, or in some cases fit a manufacturer's retrofit collar over it. Sealant has a place at exposed nail heads, but not as the main protection.
How long does a rubber pipe boot last?
Often about 10 to 15 years, sometimes less in strong sun or where moss holds moisture against the rubber. The boot commonly fails before the shingles do, which is why many roofers replace pipe boots when reroofing. Collars with a metal or hard plastic outer ring, or lead boots, generally last longer.
Do you nail through a pipe boot flange?
Yes, but only near the upper corners where the next shingle course covers the nails. Do not nail the lower or side edges, which sit in the water path. If a fastener must remain exposed because the manufacturer requires it, use a rubber washer nail or cover the head with a dab of compatible sealant.
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This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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