How to Flash Pipes and Penetrations on a Torch-On Roof
Every pipe, stack and curb is a hole in the membrane and a path for flame. Sleeves, wraps, pitch pockets and a strict fire plan.
- Professional
- 30 minutes to 2 hours per penetration
- Updated
The short answer
To flash a pipe on a torch-on roof, set a pre-formed sleeve or a flashing boot over the pipe on the base ply, wrap it with a membrane strip or collar that extends at least 4 inches onto the field, lap the cap ply over it, and seal the top with a clamp or sealant. Never aim the torch at combustible pipe, vent or wall material. Use self-adhered membrane or mastic there.
Pipes, vents, stacks, conduit, supports and curbs all interrupt the membrane, and every one of them moves slightly differently from the roof. The pipe expands with hot water, the roof shrinks and swells with the seasons, and the flashing between them has to tolerate both. That is why penetrations are a leading cause of flat roof leaks, and the reason many specifications say to keep them few and to group them.
The fix is to make each penetration a separate, simple detail: a base sleeve or boot, a membrane wrap that laps onto the field, a cap ply lapped over that, and a sealed top edge. On torch-on roofs, the detail is also a fire plan. Pipes pass through the deck into the building, and the gap around them is a chimney for flame and heat.
For shingle roofs, pipe flashings are different; see how to install a plumbing vent pipe boot. For metal flues, see how to flash a B-vent. For curbs, see how to flash a curb on a flat roof. For the main membrane see how to torch a cap sheet.
Tools
- Propane torch with hose and regulator
- Noncombustible torch shield
- Round-tip trowel
- Hook blade utility knife
- Caulking gun
- Hose clamp driver
- Fire extinguishers
Materials
- Pre-formed pipe sleeve or flashing boot sized to the pipe
- Base flashing membrane and cap flashing membrane
- Self-adhered membrane for collars
- Asphalt mastic
- Pitch pocket or pipe clamp and sealant
- Stainless hose clamps
- Flashing metal for curbs and stacks
Safety first
- Penetrations are the highest fire risk on a torch-on roof because flame and heat run down pipes and through the gap around them into wall and ceiling cavities. Plastic, insulation and framing are all close.
- Do not aim a torch at a plastic vent, a combustible pipe wrap, a B-vent or a gap around any pipe. Use mastic, self-adhered membrane and a noncombustible shield.
- Check the clearances of hot penetrations, such as metal flues, with the vent manufacturer and the building code before flashing.
- Keep an extinguisher at each penetration while it is being worked, and continue the fire watch after work ends. Penetrations can smoulder out of sight.
Step by Step
Step 1: Locate, group and support the penetrations
A pipe that moves or is loose will tear any flashing. Fasten it, brace it if needed, and confirm that it is in the right place before anything is lapped around it. Many specifications call for at least 12 inches of clearance between a penetration and a wall, curb or other penetration so there is room to lap the membrane.
Where several small pipes sit close together, consider a single pitch pocket or a platform instead of individual flashings.
Step 2: Protect combustibles and plan the fire safety
Look at what each penetration is made of and what is around it. Plastic vent pipe, insulated ducts, B-vents with clearance requirements, and wood blocking are all fire hazards under a torch. Close gaps around pipes with a noncombustible material or backing before any heat is used nearby.
The detail is built so the flame is on the membrane on the roof surface, never at the pipe. Shields between the torch and the pipe are standard, and in many cases self-adhered collars replace the torch entirely.
Watch out: Never torch around a plastic pipe or into the gap around any pipe. Heat and flame follow the pipe into the building.
Step 3: Install the base sleeve or boot
Pre-formed flashings are sized to the pipe diameter and come in rubber, lead, metal or modified bitumen. The flange sits on the base ply, bedded in mastic, and is fastened or adhered per the manufacturer. The sleeve should fit snugly, with no more than a small gap to the pipe.
Where no pre-formed piece fits, a field-fabricated collar is cut from membrane and wrapped around the pipe. Keep laps facing downslope and keep seams away from the upslope side.
Step 4: Wrap with membrane and lap onto the field
A collar of base or cap flashing membrane is cut as a square with a hole, slit to the pipe, wrapped and lapped onto the field so it extends at least 4 inches from the sleeve in every direction. It is adhered with mastic or self-adhesive at the pipe and heat only on the field side, with a shield.
Overlap the slit by at least 3 inches and face it away from the prevailing slope. Hold the shield between the flame and the pipe at every pass.
Step 5: Lap the cap ply and seal the collar
The cap ply is cut around the sleeve and lapped over the flange so the upstream side covers the joint. Keep the lap and bleed-out consistent, and trowel the edge. A joint that shows a gap at the flange is opened and reheated.
Keep the cap ply seams away from the penetration by at least 12 inches where possible.
Step 6: Seal the top of the sleeve
The top of the flashing is where water runs down the pipe and into the gap. A stainless hose clamp, a storm collar or a compatible sealant bead closes it. Sealant is maintenance, not flashing: check it each year and renew it when it cracks.
For hot penetrations, such as metal flues, use a high temperature product and the manufacturer's clearance.
Step 7: Use a pitch pocket only when nothing else fits
A pitch pocket, also called a pitch pan, is a flanged collar set over the penetration and filled with a pourable sealer. It works for irregular supports, cable groups and bundled pipes. The flange is flashed into the roof with membrane like any other flashing, and the pocket is filled with the sealer specified, leaving a slope at the top so water sheds.
Pitch pockets shrink and crack with age and need checks every year. Many roofers prefer pre-formed flashings or clamps wherever they can be used.
Step 8: Probe, test and keep the fire watch
Check each lap with a trowel and probe after it cools. Run water around the penetration and look for drips inside if the space is accessible. Refit a loose clamp or rework an open lap.
The fire watch at penetrations should include the room or space below. Look for a hot pipe, smoke odour or a warm ceiling.
Curbs and Larger Penetrations
Curbs, such as skylight curbs, mechanical curbs and roof hatches, are flashed like a small parapet: a cant, a two-ply upstand, a termination and a counter flashing. See how to flash parapets and upstands. Keep at least 12 inches between curbs and walls so the membrane can lap and water can flow around.
Fraser Valley Notes
Exhaust fan caps, plumbing vents and bathroom ducts on flat roofs are where moss and needle debris collect. Keep the area around a penetration clear, because debris holds water against the lap and leaves the first seam to fail.
Common Mistakes
- Flame around plastic or combustible pipe. It melts the pipe and can ignite framing or insulation inside the gap.
- Collars that are too small. Less than 4 inches onto the field leaves no lap margin.
- Penetrations too close together. Less than about 12 inches makes flashing impossible to lap cleanly.
- Relying on sealant alone. It cracks in a few seasons and leaves no lap to back it up.
- Loose pipes. Movement tears the collar, and the leak is at the same spot every time.
- Forgetting the top clamp. Water runs down the outside of the pipe and behind the sleeve.
When to Call a Professional
Penetrations combine waterproofing, plumbing and fire safety. Any flue, exhaust stack, gas vent or electrical service needs a roofer who knows the clearances and, where regulated, a licensed trade. Do not use an open flame near a gas vent or fuel line.
If a leak appears around a pipe on a torch-on roof, start by checking the top clamp and the collar laps. See how to repair a torch-on 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 and Penetrations on Torch-On: Questions
How do you flash a vent pipe on a flat torch-on roof?
Set a pre-formed sleeve or boot over the pipe with its flange bedded in mastic on the base ply, wrap it with a membrane collar that extends at least 4 inches onto the field, lap the cap sheet over the collar, and seal the top with a stainless clamp or sealant. Keep the torch off the pipe and use mastic or self-adhered membrane there.
What is a pitch pocket on a flat roof?
A pitch pocket, or pitch pan, is a flanged metal or formed collar set over an odd-shaped penetration and filled with a pourable sealer. It is flashed into the membrane like any other flange. It is used for irregular supports and bundled cables, but the sealer shrinks and cracks, so it needs an annual check and top-up.
Why are roof penetrations a fire risk when torching?
Heat and flame can follow a pipe or the gap around it down into the wall, ceiling or insulation, and smoulder unseen for hours. Plastic pipe and insulation also melt or burn. Roofers shield the pipe, use mastic or self-adhered membrane close to it, close the gap, and keep a fire watch that includes the space below.
How far apart should roof penetrations be?
Many specifications call for about 12 inches between penetrations, and between a penetration and a wall or curb, so the membrane can lap and water can flow around each one. Closer spacing makes clean flashing difficult. Where pipes are close together, a single pitch pocket or a platform is often better than several small flashings.
Related Guides
All Torch-On 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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