The Roofer Who Shingled Over the Plumbing Vent
Why this matters
A plumbing vent is the one pipe on a roof whose job is to stay open. It lets air into the drain system so that water running down the pipes does not pull the water out of every fixture's trap, and it carries sewer gas up and away from the house. A reroof is the one event in a house's life when someone removes the flashing around that pipe, replaces the roof around it, and has to remember it is there. Miss a short one under a fold of underlayment and the house gets drains that gurgle and a sewer smell that comes and goes, and the homeowner calls a plumber, not a roofer. This case follows the plumber who first found a real cause, fixed it correctly, and watched the problem come back. How the drain, waste and vent system works is in Drain, Waste and Vent for Roofers, Framers and HVAC Techs.
Safety first
- A strong sewer smell in the house: open windows, run bathroom fans, and if anyone feels sick or dizzy, everyone goes outside. Sewer gas can carry hydrogen sulfide, which deadens the sense of smell as it gets stronger, so a smell that fades is not proof it has gone.
- Attics: step only on framing, not between it; in summer an attic can be hot enough to cause heat illness within a short time, so go in early, carry water and keep it short. Unidentified loose fill, especially gray-brown granular material, is not disturbed; vermiculite can contain asbestos, and disturbing it puts the fibers in the air you breathe.
- On the roof: fall protection under 29 CFR 1926.501 for the roofing work, which is construction and triggers at 6 ft.
The first call, and a correct diagnosis
Three weeks after a tear-off and reroof, the homeowner called a plumber. Two complaints: the hall bath sink and tub gurgled when the upstairs toilet was flushed, and there was a sewer smell in the basement most days and in the hall bath some days.
The plumber started where sewer smell usually starts, at traps that have lost their water. A trap is the U-shaped bend under every fixture and in every floor drain that holds a plug of water to block sewer gas. Traps that go unused dry out by evaporation. The basement floor drain had not had water down it in months and was dry. The plumber filled it, explained trap evaporation, suggested pouring water in monthly, and left.
That diagnosis was right. A dry floor drain trap does cause a basement sewer smell, and it was dry. The basement smell eased. It did not account for the gurgling, and the plumber noted the gurgling as unexplained and asked the homeowner to call back if it continued.
The callback
Five days later: the gurgling had not stopped, the hall bath smelled after every upstairs flush, and the floor drain was dry again.
That last detail is what turned the case. Evaporation takes weeks to empty a trap, not five days. A trap that empties in days with no one using it is being pulled empty, which happens when air pressure in the drain pipe moves the water out of it.
How the plumber reasoned the second time
What gurgling means. When a toilet flushes, a slug of water drops down the stack, the main vertical drain pipe. Behind it, the pipe goes to lower pressure, and something has to let air in. Normally air comes down the vent from the roof. If the vent cannot supply it, the drain pulls air from the next easiest place, through the traps of other fixtures, and the water in those traps is drawn out with a gurgle. Ahead of the slug, the pressure rises and can push sewer gas back out through a weakened trap. A drain system that gurgles on every flush from one fixture into others is short of vent air.
What could cause that. The candidates for a vent that will not supply air are a blockage in the vent pipe (a bird's nest, a ball, debris from roofing work), a vent pipe that was never connected right in the first place, or a vent terminal that has been covered. A house that drained quietly for years makes a vent that was never connected right unlikely, and gurgling that started three weeks after a reroof points at the roof.
Why it was this bad here. This house had one vent through the roof, serving every fixture through the stack. In a house with two or three vent terminals, or an air admittance valve (a one-way valve that lets air into a drain under a sink), covering one terminal starves only the fixtures that depend on it and the symptoms are milder and more local, which makes them easier to miss and harder to trace.
Why the attic before the roof. From the attic, the plumber could follow the vent pipe itself from the ceiling of the hall bath up to where it met the roof, see whether it was connected and intact, and see what it ended at, without walking a new roof and without anyone cutting into it.
What the attic showed
The plumber traced the 3 in plastic vent stack up from the top plate of the hall bath wall to the roof deck. It was continuous and intact, and it passed up through its hole in the roof deck. A flashlight into the gap around the pipe at the deck showed the underside of new underlayment where the edge of an opening and some daylight should have been. Nothing around the pipe let light in.
Then the plumber did the cheap check from the ground: walked the yard looking at the back slope. There was no vent pipe sticking out of the roof anywhere above the hall bath.
The roofer's own photos, which the homeowner asked for, settled it. The pre-tear-off photos showed three penetrations on the back slope: the plumbing vent, a bath fan cap and a kitchen exhaust cap. The completed-roof photos showed two.
How it happened
The vent had been cut short at some earlier repair, to barely above the old shingles. With the old layers off it stood about 1 in above the bare deck, as the roofer measured when the roof was reopened. During this tear-off, the crew removed its old boot (the rubber or metal flashing that seals around a pipe). The underlayment went on next, in wide rolls, and the stub went under a fold of it without the installer seeing it. A small bump under underlayment looks like every other bump. The shingle crew followed the underlayment and saw a roof with two penetrations. When the roof was reopened, the end of the pipe still carried a ring of sealant from the old boot.
What the roofer owed
The roofer's contract warranted its workmanship, and covering a vent that was open when they arrived is a workmanship defect. What a roofer owes under a particular contract, and under a state's rules for contractor work, is a question for that contract and, if it comes to it, for the shop's own attorney; this is not legal advice. In this case the roofer did not dispute it, and what it took on was:
- Reopening the roof around the vent: removing shingles and underlayment over it, cutting a clean opening, and installing a new boot sized for 3 in pipe, flashed into the new shingles the way the shingle manufacturer specifies.
- The vent extension, by a plumber. At about 1 in above the deck, the stub would end up barely above the new shingles and well short of any required height. The IRC, at section P3103.1, requires an open vent through a roof to terminate a minimum height above the roof; the model text leaves the figure for each jurisdiction to fill in, 6 in is the common one, and many cold-climate jurisdictions raise it for snow. Check the number adopted where you work. Extending a vent pipe is plumbing work in most jurisdictions, so the roofer paid the plumber to add a coupling and a length of pipe before the boot went on.
- The plumber's two visits. The first visit's floor drain refill was a real repair, but the visit would not have been needed in this form, and the second visit was entirely the vent.
Confirming the fix
With the vent extended to 8 in above the new roof and booted, the plumber went back into the attic and looked up the pipe: daylight. Then flushed the upstairs toilet several times with the hall bath sink and tub full of water to the trap level and watched them. No movement, no gurgle. The floor drain was refilled and checked a week later: still full. The smell did not return.
The order mattered. The plumber looked for daylight from the attic and tested the fixtures before the roofer sealed the last shingles around the boot, so anything still wrong could be found with the roof open rather than after it was closed again.
The vent check the roofer added to closeout
The roofer rewrote its reroof closeout around a count, because a count is what would have caught this one.
Before tear-off: photograph every penetration on every slope, and write the count and the type on the job sheet: plumbing vents, exhaust caps, flues, attic vents, electrical masts, solar mounts. Measure each plumbing vent's height above the existing roof.
After tear-off, before underlayment: find every penetration on the sheet on the bare deck. Any plumbing vent shorter than the finished roof plus its required height is flagged for a plumber now, before underlayment goes over it, not after the roof is done.
After shingles, before the crew leaves: count again against the sheet. Every plumbing vent is open (look for daylight from the attic or look down it from the roof), stands at least its required height above the finished roof, and has a new boot that fits its size. Any count that does not match the before-count stops closeout until it is found.
The roofer's office now attaches the before and after penetration counts to every reroof invoice. The next time a homeowner calls about a sewer smell after a reroof, the first thing anyone checks is whether the two numbers match.
References
- IRC Section P3103, vent terminals, as adopted by your jurisdiction; check the current edition and local snow amendments
- 29 CFR 1926.501, fall protection in construction
- The shingle manufacturer's installation instructions for pipe flashing
- See related: Drain, Waste and Vent for Roofers, Framers and HVAC Techs
- See related: The Roof Assembly for Trades Who Penetrate It