What size roof vent is needed for plumbing?
Short answer. Plumbing vent pipes are sized by the plumbing code from the drains they serve, generally at least half the diameter of the drain and never less than 1 1/4 inches, which is why most houses have a 3 inch main stack plus some 1 1/2 or 2 inch vents through the roof. Sizing the vent is the plumber's job; the roofer's job is matching the flashing boot to each pipe that comes through.
How plumbing vents are sized
Every drain needs air behind it so water flows freely and traps are not siphoned dry, which would let sewer gas into the house. The residential code sizes vents from the drains they serve, using drainage fixture units, and requires every building to have at least one vent that reaches the outdoors. A typical house ends up with:
- A main stack, often 3 inch, serving toilets and the main drain.
- Branch or individual vents, often 1 1/2 or 2 inch, for sinks, tubs and laundry, sometimes tied together in the attic before passing through the roof.
- Air admittance valves under some sinks where codes allow them, which reduce the number of roof penetrations.
What happens at the roof
- The pipe must extend above the roof, generally at least 6 inches under the plumbing code.
- Each pipe gets a boot sized to its outside diameter: about 1.9 inches for 1 1/2 inch pipe, 2.375 for 2 inch, 3.5 for 3 inch and 4.5 for 4 inch.
- Pipes ideally sit a few feet away from valleys and walls so the flashing lies flat and sheds water.
Things not to do during roof work
Never cut a vent pipe off below the roof line, cap it, or end it in the attic to avoid a boot. That breaks the plumbing code and can send sewer gas into the attic and the house. Do not shrink a pipe's size where it passes through the roof. If a vent pipe sits in a bad spot, such as in a valley, have a plumber relocate it before or during the reroof. See pipe boot leaks and roof flashing explained.