What is the plumbing vent on my roof for?
Short answer. The plumbing vent is the open pipe, usually 1.5 to 4 inch PVC or cast iron, that lets sewer gas escape above the house and lets air into the drain lines so sinks and toilets drain smoothly without gurgling. It is supposed to stay open, and the rubber or metal boot around its base is one of the parts of a roof most likely to leak.
How it works
Every drain in the house connects to a vent stack that runs up through the attic and out the roof. When water rushes down a drain, air has to replace it. Without the vent, the water would pull air through the nearest trap and siphon out the water that blocks sewer gas. Plumbing codes require vents to end above the roof and away from windows, doors and air intakes so the gas disperses.
Problems homeowners notice
- Sewer smell outside: often normal near the stack on still days, but a smell inside points to a dry trap or a cracked pipe in the attic.
- Gurgling drains: a blocked vent, sometimes leaves, a nest or a ball. A plumber clears it from the roof.
- Ceiling stains below the stack: almost always the pipe boot. The rubber collar cracks in the sun, often years before the shingles wear out.
- Chewed lead boots: squirrels gnaw soft lead flashing, a common Georgia complaint.
Things not to do
- Do not cap or cover the pipe to stop smells or rain. It is designed to take a little rain.
- Do not rely on caulk around the base as a permanent fix.
- Do not reroute it into the attic. Vents must reach the outdoors.
During a roof replacement
Every plumbing vent should get a new boot, flashed under the shingles above it and over the shingles below. On cast iron pipes, ask about lead or silicone boots, since some rubbers break down against iron. Learn the warning signs in pipe boot leaks and other suspects in common causes of roof leaks.