How to Flash Pipes and Vents on an EPDM Roof
A pipe boot is a small part with a big job. Sizing, flange laps, the clamp and the top edge decide if it lasts.
- Advanced
- 20 to 45 minutes per pipe
- Updated
The short answer
To flash a pipe on EPDM, cut the field membrane to the pipe, slide a pre-molded EPDM pipe boot over it with its flange lying flat, prime and splice-tape the flange to the membrane with at least a 3 inch lap all round, then seal the top with a stainless hose clamp and a bead of sealant. Use a field-fabricated flashing or a pitch pocket only when no boot fits.
Every pipe that comes through a flat roof, whether a plumbing vent, an exhaust or an electrical conduit, is a place where the membrane is cut and something moves inside the hole. Pipes expand and contract with temperature, the roof moves, and rubber and rigid pipe do not bond to each other. A pipe flashing has to shed water over its flange and seal around the pipe with a joint that can tolerate movement.
On EPDM roofs, the standard solution is a pre-molded pipe boot, a cone of cured rubber with a flat flange that is taped into the membrane and clamped at the top. They are quick, consistent and durable, and come in sizes for common pipe diameters. Where a boot does not fit, such as a large, square or irregular penetration, the options are a field-fabricated wrap from uncured flashing, a pipe clamp or a pitch pocket.
Pipe flashing follows the field membrane in how to install a fully adhered EPDM roof and uses the same tape method as in how to seam EPDM with seam tape.
Tools
- Utility knife
- Scissors
- Hand roller
- Primer brush or pad
- Screwdriver or nut driver for clamps
- Caulking gun
- Tape measure
Materials
- Pre-molded EPDM pipe boot sized to the pipe
- Stainless hose clamp
- Splice tape and primer
- Uncured EPDM flashing for field-made collars
- Sealant compatible with EPDM
- Pitch pocket or pipe clamp where needed
Safety first
- Primers and sealants are flammable. Keep flames, sparks and heaters away, and ventilate.
- Do not work on or around vent pipes carrying gas or flue gases without confirming what they are. Hot flues need high temperature products and clearances. See how to flash a B-vent.
- Roofs around penetrations are often rotten from old leaks. Probe before you kneel, and stay clear of unprotected edges. WorkSafeBC fall protection applies at 3 metres (about 10 feet) and wherever a fall of any height could cause serious injury.
Step by Step
Step 1: Check the pipe and choose the flashing
A pipe that is loose or bent will tear any flashing. Fasten it, and check that it is straight and vertical. Measure the outside diameter. A pre-molded boot has a cone that stretches to fit a range of diameters, and the boot should grip the pipe firmly, without being stretched to its limit.
For square, oval or very large penetrations, or for bundled pipes, plan a field-fabricated flashing or a pitch pocket instead.
Step 2: Keep penetrations clear of walls and each other
A boot flange is commonly 8 to 12 inches across or more, and it has to lie flat on the membrane with laps on every side. If the pipe is closer to a wall than the flange, flash it as part of the wall flashing, or move the pipe if it can be moved by a plumber.
Keep seams in the field membrane away from the boot, at least 6 inches where possible.
Step 3: Cut the field membrane to the pipe
After the field membrane is adhered, cut a hole for the pipe with scissors or a knife. The hole should be about the diameter of the pipe, with no ragged edges. Do not cut a large hole, because the boot is what seals around the pipe.
Keep the membrane adhered right to the pipe base, and avoid leaving a gap between the membrane and the pipe that could collect water.
Step 4: Slide the boot over the pipe and set the flange
If the pipe has an open top, the boot can slide over it. If not, a split boot or a field-made wrap is needed. Lubricate the pipe with a little water or a manufacturer approved lubricant if the boot is tight, but never use petroleum products or oils.
Press the flange flat and mark its outline on the membrane with a marker. Lift the flange, and clean and prime the marked area.
Step 5: Prime and tape the flange
Apply primer in a band about 3 inches wide around the flange on the membrane, and to the flange underside where required, and let it dry to a slight tack. Lay splice tape around the flange, press the flange down onto it, peel the film and roll firmly. Overlap at least 3 inches onto the membrane.
Some boots have an adhesive flange and only need primer. Follow the product data sheet.
Step 6: Clamp and seal the top
The top of the boot is where water runs down the pipe. A stainless hose clamp holds the boot against the pipe, and a bead of sealant compatible with EPDM closes any gap. Tighten the clamp snugly but not so far that it cuts the rubber.
Sealant is maintenance, not flashing. Plan to check it yearly and renew it when it cracks.
Step 7: Use a field-fabricated flashing or pitch pocket when a boot will not fit
A field-fabricated flashing is made from a strip of uncured EPDM wrapped around the pipe and a patch on the flat, all sealed with primer and tape. It is used when a boot cannot be fitted. Keep the wrap free of wrinkles, and cover the vertical seam with tape.
A pitch pocket is a flanged collar filled with pourable sealer, set over the penetration and flashed into the membrane. It is for bundles and irregular shapes, and needs yearly checks and top-ups, because the sealer shrinks. See how to flash pipes and penetrations on torch-on for how pitch pockets are built on other membranes.
Step 8: Probe and test
Run a probe along the flange edge. Any lift is reprimed, retaped and rolled. Flood the area around the pipe with a hose and check the ceiling below if you can access it.
Check again after the first heavy rain.
Boot Problems to Watch For
- Cracked top edge: sun and movement crack old boots and sealant at the top, which lets water run down the pipe.
- Loose clamp: the clamp relaxes with movement and lets the boot slip.
- Flange lifting: dirt under the flange or a cold install leaves an open lap.
- Pipe movement: a loose or flexing pipe tears any flashing.
Fraser Valley Notes
Fir needles and moss collect around pipes on flat roofs and hold water against the flange. Clear the area when you clean the roof, and look at the boot top and sealant each autumn before the heavy rains.
Common Mistakes
- Wrong size boot. A boot stretched to its limit cracks, and one that is too loose leaks down the pipe.
- Flange laps under 3 inches. The tape has no margin and peels.
- Petroleum lubricant on the pipe. Oil attacks EPDM.
- Sealant as the only flashing. It cracks and leaves no lap to fall back on.
- No clamp. The boot slides and opens.
- Pipes too close to walls or each other. There is no room to lap, so the flashing is a patch on a patch.
When to Call a Professional
If the pipe is a flue, a gas vent or a large penetration, or if several penetrations are grouped tightly, call a roofer. Gas and hot flues have clearances and product requirements that must be followed.
If an existing boot is leaking, see how to repair an EPDM roof.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
How to Flash Pipes on EPDM: Questions
How do you flash a pipe on an EPDM roof?
Cut the field membrane to the pipe, slide a pre-molded EPDM pipe boot over it, prime the membrane and boot flange, seal the flange to the membrane with splice tape and a lap of at least 3 inches all round, then fasten a stainless hose clamp at the top and seal it with a compatible sealant. Check the top yearly.
What size pipe boot do I need for EPDM?
Measure the outside diameter of the pipe, then choose a boot whose cone is designed for that diameter. A boot stretches within a range, so pick one that grips firmly without being at the limit of its stretch. Common plumbing vents are about 1.5 to 4 inches in diameter, but confirm the boot manufacturer's size chart.
Why does my EPDM pipe boot leak?
Common causes are a cracked or aged boot top, a loose or missing clamp, sealant that has failed, a flange with dirt under it or a lap that never adhered, and a pipe that moves. Water then runs down the pipe or under the flange. Replace or retape the boot, clamp it, and renew the sealant.
Can you use roof cement around a pipe on EPDM?
Asphalt roof cement is avoided on EPDM because petroleum attacks rubber, and it fails over time. Use a sealant, primer and tape that the membrane manufacturer approves. Sealant is maintenance at the top of the boot, not the main flashing, and the boot flange and tape laps do the waterproofing.
Related Guides
All EPDM Rubber Roofing guides|The full How-To Library
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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