Drain, Waste and Vent for Roofers, Framers and HVAC Techs
Why this matters
The pipe sticking out of the roof looks like the least important part of the plumbing. It carries no water, it makes no noise, and a homeowner will happily ask a roofer to shingle over one that "nobody uses." It is the reason every sink, tub and toilet in the house drains quietly and keeps its water seal. Cover it, cap it, bend it into a soffit or cut it in a wall, and the house starts gurgling, draining slowly and smelling of sewer gas, often weeks after the trade that did it has been paid.
What the system is
Drain, waste and vent (DWV) is the gravity side of plumbing. Three jobs, one pipe network:
- Drains and waste lines carry water and waste down and out to the sewer or septic, sloped so gravity moves it.
- Traps are the U-shaped bends under every fixture (built into a toilet, visible under a sink, hidden under a tub). Each holds a small plug of water, the trap seal, that blocks sewer gas from coming up into the room. The plumbing code keeps that seal between 2 and 4 inches deep (IRC P3201.2 in the 2021 edition, as adopted).
- Vents connect the drain system to outside air, usually through the roof, so moving water does not pull a vacuum or push a pressure wave through the pipes.
HOUSE SECTION (simplified)
open vent pipe through roof, above the shingles
|
=====roof======|========== boot / flashing
| <- VENT STACK: carries air only
+---------+--------+
| vent vent |
| | |
[LAV] [TUB] [TOILET] fixtures
U trap U trap trap built in
| | |
---+-----------+--------+--- drain, sloped down
|
| <- stack: drains below, vents above
|
======floor====|=====================
+-------------> building drain
to sewer or septic
Stack is the vertical pipe that carries drainage below the highest fixture and continues up as vent above it. Plumbers distinguish a stack vent from a vent stack; for a non-plumber the point is that both carry air, and the roof termination is where that air comes in. See related: What a Stack Vent and a Vent Stack Each Do.
How a covered vent shows up inside
When a toilet flushes, a slug of water runs down the drain like a piston. Behind it, air has to rush in or the drain pulls a vacuum. With an open vent, the air comes from the roof. With the vent covered or blocked, the air comes from the nearest path it can find, which is through another fixture's trap. The trap seal gets pulled down the drain (siphoned), or air bubbles back up through it.
The symptoms, in the order a homeowner notices them:
- Gurgling in a tub or sink when a toilet flushes or a washing machine drains.
- Slow drains that are not clogged; the water runs off once air finds a way in.
- Sewer smell, from traps that have lost their seal. Sewer gas contains hydrogen sulfide and methane; a strong, persistent sewer smell indoors is a reason to ventilate and call the plumber, not to wait.
- Toilet water level bobbing when another fixture drains.
None of these require a blockage in the drain. That is why a plumber called for "slow drains after the new roof" will go up the ladder before snaking anything. See related: The Roofer Who Shingled Over the Plumbing Vent.
The roof termination: what roofers need to know
It must stay open. No cap, no plastic bag, no shingle, no underlayment over it. Flashing it is the roofer's job; covering it is not. Some cold-climate jurisdictions allow or require frost protection, which is a plumbing or code decision, not a roofing one.
It must stay above the roof. The 2021 IRC (P3103.1, as adopted) leaves the minimum height above the roof for each jurisdiction to fill in; 6 inches is the common figure, more where snow piles up, and 7 feet where the roof is used for something other than weather protection. Check the number your jurisdiction adopted. A new, thicker roof can bring a short vent below its required height; that is a finding for the plumber, not a reason to cut a coupling and glue an extension on yourself.
It must not be moved into a soffit or next to a window. The same section of the 2021 IRC (P3103, as adopted) keeps vent terminals away from operable windows, doors and air intakes, and does not allow a vent that exits through a wall to end under an overhang with soffit vents, because sewer gas would be drawn straight back into the attic or the house. "Can we just run it out the eave so there's no pipe on the front of the roof" is a plumbing redesign, not a roofing detail.
It is not the only pipe up there. Tell the penetrations apart before you flash or move anything:
| Penetration | Usually looks like | Owner |
|---|---|---|
| Plumbing vent | Open PVC, ABS, cast iron or old galvanized pipe, no cap | Plumber |
| Gas appliance vent | Round metal pipe with a cap; label reads Type B | HVAC or gas |
| Condensing appliance vent | Two PVC pipes close together, or one with a fitting that looks like a tee or a split cap | HVAC |
| Bath or kitchen exhaust | Hood or louvered roof jack | Electrician or HVAC |
| Radon mitigation | PVC, often with a label or a fan in the attic below | Radon contractor |
The second-to-last row trips up roofers most: a condensing furnace's PVC exhaust looks like a plumbing vent but carries flue gas, and the manufacturer sets its termination height and spacing.
Framers and HVAC techs: the vent in the wall
Most vent piping lives inside walls and attics, not on the roof. Three things go wrong:
Cutting into it. A 2 inch vent in a wall cavity is exactly where a new return duct or a pocket door wants to go. A vent may not be cut, capped, shrunk to a smaller size or rerouted by anyone but a plumber. A cut vent left open in a wall vents sewer gas into the cavity.
Notching the stud around it. Where a vent runs through studs, the plumber may already have notched or bored them and covered the spot with a steel shield plate (IRC P2603.2.1 in the 2021 edition, as adopted, wants that protection where the pipe is within 1-1/4 inches of the framing edge). A framer who adds a stud or sister beside it, or shaves a notch deeper to fit blocking, can remove the plate or expose the pipe to the next fastener. Structural limits on notching are their own rule; check the framing code before cutting any notch deeper.
Sagging or flattening it. Vent piping is sloped so condensation and any water that gets in drains back to the drain. A vent pushed into a belly by new framing or by a duct pressing on it can trap water and block air.
Air admittance valves. Some fixtures, especially islands and remodels, vent through a mechanical valve that opens to let air in and closes to keep gas out. The code (IRC P3114 in the 2021 edition, as adopted) wants it accessible and in a ventilated space. If your drywall or cabinet would seal one in, stop and call the plumber.
Worked example: a reroof with four pipes and one customer request
A roofing crew is tearing off and replacing an asphalt shingle roof. On the tear-off the lead lists four items through the roof, five pipes in all:
- A 3 inch PVC pipe near the ridge, open top, cracked rubber boot.
- A 1-1/2 inch PVC pipe over the kitchen, open top, boot in fair shape.
- Two 2 inch PVC pipes 8 inches apart over the utility room, one with a tee at the top.
- A 3 inch round metal pipe with a cap, labeled Type B.
Identify. Items 1 and 2 are plumbing vents. Item 3 is a condensing furnace's intake and exhaust. Item 4 is a gas appliance vent, probably the water heater.
The customer's request. The homeowner asks the crew to remove the 1-1/2 inch vent over the kitchen because "the kitchen sink drains fine and that pipe is ugly from the street." The lead declines: removing it means the kitchen sink's trap has no air source, and that is a plumbing change. He tells the customer it would need a plumber and probably an air admittance valve under the sink, if local code allows one there.
Flashing. The crew replaces the boots on items 1 and 2 with new pipe boots, which is roofing work. On the old boot from item 2, the lead finds the flashing collar underneath is lead, not rubber, from an earlier roof. Lead flashing is handled with gloves, not cut, sanded or burned, kept out of the dumpster's general debris if local rules require separate disposal, and hands are washed before eating or smoking.
Height. The new roof adds about 1/2 inch of thickness. Item 2 stood 7 inches above the old shingles; it now stands about 6-1/2 inches, still above the 6 inch minimum this jurisdiction adopted. Item 1 stands 11 inches. Both pass. If item 2 had started at 6 inches, the new roof would have left it at 5-1/2, and the lead would have called the plumber rather than gluing on a coupling.
Items 3 and 4. The crew flashes around them using the furnace and vent manufacturer's flashing details, does not cut or shorten either, and keeps shingles and underlayment clear of both openings.
The tally. Four items, five pipes. Two plumbing vents re-flashed and left open; one removal request declined and routed to a plumber; one pair of furnace pipes and one gas vent flashed around without alteration. The job closes with no plumbing call needed and one written note to the customer about the kitchen vent.
What changes the answer
The vent is cast iron. Old cast iron stacks are heavy, held by their supports and their joints, and can crack. Do not pry against one, cut its supports or use it as an anchor.
A homeowner already capped it. You find a vent with a cap or a screen glued on. Report it; do not remove a fitting that a plumber may have installed for a reason, but do not shingle over it either.
Septic homes. The vent also relieves the septic system. See related: Private Wells and Septic for Non-Specialists.
How to verify you got this right
- Every plumbing vent is open, flashed, and at least as tall above the roof as the code you are under requires.
- No vent was moved, shortened, extended, capped or covered by your crew.
- Every penetration was identified by type before it was flashed.
- If the customer reports gurgling after the job, someone goes on the roof before anyone snakes a drain.
References
- International Residential Code, 2021 edition, Sections P2603.2.1, P3103, P3114 and P3201.2, as adopted by your jurisdiction.
- Furnace and gas vent manufacturers' installation instructions for termination height and roof flashing.
- See related: What a Stack Vent and a Vent Stack Each Do
- See related: The Roofer Who Shingled Over the Plumbing Vent