How to Seal a Roof Penetration That Will Not Leak
Why this matters
Pipe boots and vent flashings are the most common roof leak in residential work, and most of them are installed by someone who is not a roofer: the plumber who added a vent, the HVAC tech who moved a flue, the solar or satellite crew. The leak shows up months later as a ceiling stain several feet from the pipe, the roofer who finds it blames the last trade on the roof, and they are usually right. A flashing that will not leak is not about sealant. It is about which shingle goes over which piece of metal.
First: get on and off the roof without falling
Roof repair is construction work, and OSHA's construction fall protection rule (29 CFR 1926.501(b)(13)) requires fall protection for residential work at 6 feet or more above a lower level: a guardrail, a safety net or a personal fall arrest system. A written alternative plan is allowed only where those are shown to be infeasible (1926.502(k)).
- The first person up is protected too. Set the roof anchor from the ladder at the eave, from a lift, or from staging that is already guarded. If nobody on the crew can reach an anchor point without being exposed first, that roof needs a crew equipped for it.
- Ladder: rails extending 3 feet above the eave, set at about a 1 to 4 angle, tied off at the top.
- Surface: do not walk a wet, frosty or mossy roof. On hot days, shingles scuff and tear underfoot; on cold days they crack when lifted. Work the boot in the cool part of a warm day.
- Hot vents: a gas vent or flue may be running. Shut the appliance off at its control and let the vent cool before handling it, and never fit a rubber boot to one. See related: Penetrations Near Flues and Chimneys.
Steps
Read the roof and the pipe before buying the part. Measure pitch as rise over 12 inches of run with a level and tape. Identify the covering: asphalt shingle, metal, tile, membrane. Measure the pipe's outside diameter and note whether it is hot. Why: boots are made for a pipe size and a pitch range, and asphalt shingles are not permitted below 2:12 under IRC R905.2.2 as adopted, with double underlayment required from 2:12 up to 4:12. Skip it and you will arrive with a boot whose collar is too loose or whose base is the wrong shape.
Choose the flashing that fits the pipe and the roof. For a cold plastic or metal vent on shingles: a boot with a base of metal or plastic and a flexible collar sized to the pipe, or a soft lead flashing whose top is rolled into the pipe. For a hot vent: the vent manufacturer's listed roof flashing and storm collar only. Why: a collar sized for a larger pipe never grips, and rubber on a hot vent cooks and cracks. Skip it and the collar is the leak.
Free the shingles above the old flashing. Break the sealant strips on the course or two above it with a flat bar, slide the bar under, and pull the nails that hold the flashing's upper flange. Why: the new flashing's top has to go under those shingles, so they have to lift without tearing. Skip it and you will be tempted to set the new boot on top of the shingles and caulk it, which is the classic installed-leak.
Look at the deck before covering it again. With the old boot off, press the decking around the hole and look at the underlayment. Why: a long-running leak leaves soft or delaminated deck. Skip it and you nail a new flashing into rotten wood and the fasteners never hold. Soft deck stops the job for a roof repair.
Weave the flashing into the courses. Slide it up under the shingles so the upper part of the base and the upper sides sit under the course above, and the lower part of the base lies on top of the course below. Why: water running down the roof then flows onto the flashing at its top and off it onto shingles at its bottom, without crossing an edge that faces uphill. Skip it and either the upper edge catches water and sends it under, or the lower edge is buried and dams water behind it.
Fasten only where the next course covers. Nail the upper corners and upper sides of the base, where the shingles above will hide the heads. Never nail through the exposed lower part. Put a dab of roofing sealant on each covered nail head. Why: an exposed nail is a leak waiting for its gasket or sealant to age. Skip it and you have put a hole in the one part of the flashing that is designed to be watertight.
Seal the right joints and leave the wrong ones open. Reseal the shingle tabs you lifted with spots of roofing cement so wind cannot lift them off the flashing. Do not caulk the lower edge of the flashing to the shingle below and do not run a bead around the base. Why: the lower edge is where water leaves; seal it and water that gets under the upper shingles is trapped against the deck. Skip the tab sealing and the first storm lifts the course and drives rain under it.
Prove it from the attic. Ideally in real rain. Otherwise run a hose test: one person in the attic with a light, standing and kneeling only on framing, one on the roof, tied off, running water gently, starting below the flashing and working upslope in zones, a few minutes per zone. Why: starting low means that when a drip appears, the zone you are running is the one that caused it. Skip it and you hand the owner a repair you hope works.
Worked example: a cracked boot on a plumbing vent
A 3 inch PVC plumbing vent on a 6/12 roof, architectural shingles at about a 5-5/8 inch exposure. The owner reports a ceiling stain in the hall, roughly 4 feet downslope of where the vent sits. The crew is tied off to an anchor set from the ladder at the eave.
Reading it. In the attic, the stain on the ceiling drywall lines up with a wet streak on the underside of the deck that runs down the rafter from just below the vent. Water entering at a penetration usually runs downslope along the deck or a rafter before dripping, which is why the stain was 4 feet away from its source. On the roof, the boot's neoprene collar is split around half its circumference from sun, a common failure on older boots.
The replacement. Pitch measured at 6 inches rise over 12, inside the boot's stated range. New boot with a collar for 3 inch pipe. The tech broke the seals on the two courses above, pulled four nails from the old upper flange, and slid the old boot off.
What the old boot hid. The new boot's base was slightly narrower than the old one, which exposed two old nail holes in the shingle above, now uncovered by any flashing. Each is a straight path through the shingle to the underlayment. The tech filled both with roofing sealant and slipped a small piece of flashing metal under that shingle, over the holes, so they sit under a lap and not just under a dab of sealant.
The deck. Firm around the hole, underlayment stained but intact. Proceeded.
Weave and fasten. The base slid up under the course above until its upper edge sat a full exposure, about 5-5/8 inches, up under that course, with the lower part of the base lying over the course below. Four nails at the upper corners and sides, each under shingle cover, each sealed. Tabs above resealed.
Hose test. Three zones: below the boot, at the sides, then above, about five minutes each, with the attic observer watching the deck at the old streak. Dry throughout. The tech then wetted the deck streak spot from inside with a sponge so a fresh drip would be visible against it on the next rain, and asked the owner to check after the first storm.
What changes on a metal roof
- Exposed-fastener panels take a retrofit boot with a flexible base that is formed to the panel's profile, bedded in sealant and fastened at the close spacing its instructions give. Put the pipe in the flat of the panel. A wide base sitting across the major ribs dams the water running down the flats, and water that piles up behind a boot finds the fasteners.
- Standing seam panels must not be fastened through the seams, and the boot must sit on a flat. A penetration that lands on a seam usually means moving the pipe, not the panel.
- Movement. Metal panels expand and contract along their length. The boot's flexible base absorbs that; a pipe rigidly fixed to the panel tears its own seal.
- Larger penetrations need an upslope diverter or a cricket so water goes around rather than into them.
What changes on a low-slope roof
Below about 2:12 the roof is usually a membrane: TPO, PVC, EPDM or modified bitumen. Pipe flashings there are membrane-specific, either prefabricated boots or field-fabricated from the same membrane, heat-welded for TPO and PVC and adhered with primer and seam tape for EPDM. Bring the wrong product and it will not bond. Most membrane roofs also carry a manufacturer's warranty that requires a certified installer for any penetration. On a commercial building, or a home with a newer membrane roof, the right first move is to call the roofing contractor of record. See related: How to Coordinate a Penetration With the Trade That Owns the Assembly.
How to verify you got it right
Before you leave the roof, look at the flashing from directly upslope. You should see shingles over the top and upper sides of the base and the base over shingles at the bottom, with no exposed nail heads anywhere below the course line and no sealant bead along the lower edge. In the attic, the deck around the pipe should be dry after the hose test. Photograph the woven flashing before the tabs go back down; it is the only evidence anyone will have of the laps. Write the penetration, the part and the test result into the job record. See related: The Roof Penetration Closeout SOP.
References
- 29 CFR 1926.501(b)(13) and 1926.502 (fall protection in residential construction) and 29 CFR 1926.1053 (ladders).
- IRC R905.2.2 (asphalt shingle slope limits) and R905 generally, as adopted by your jurisdiction.
- Boot, flashing, shingle, metal panel and membrane manufacturers' installation instructions.
- See related: The Control Layers a Penetration Goes Through; Penetrations Near Flues and Chimneys; How to Move Through an Attic Without Stepping Through a Ceiling.