Vent Pipe Sizing — Individual, Branch, and Stack Vents
The vent system does one job — keep air behind the water so traps do not siphon. Undersized or overlong vents pull trap seals and let sewer gas into the building.
Why vents exist
As waste falls down a stack it drags air with it, creating suction behind it. Without a vent path, that suction pulls water out of nearby fixture traps. Vents admit make-up air so the trap seal holds.
Size by drain served and length
A vent is sized from the DFU load of the drain it protects and the developed length from the vent connection to the point it reaches outside air (or joins a vent stack).
| Drain served | DFU | Vent size (short runs) |
|---|---|---|
| 32–40 mm branch | ≤ 8 | 32 mm |
| 50 mm branch | ≤ 24 | 40 mm |
| 75 mm branch | ≤ 42 | 50 mm |
| 100 mm stack | ≤ 500 | 50–75 mm |
Long developed lengths force the next size up — the tables cut the allowable length roughly in half for each size step down.
The key rules
- A vent must be at least half the diameter of the drain it serves, and never smaller than 32 mm.
- The vent stack or stack vent must be at least one-half the diameter of the soil stack.
- Individual vents connect above the trap weir; horizontal vent runs stay at least 150 mm above the flood-level rim of the highest fixture served.
- Where a full vent system is impractical, air admittance valves (where the code permits them) vent individual fixtures without a roof penetration.
Common configurations
Individual vent — one fixture. Common vent — two fixtures on the same or adjacent levels. Wet vent — a drain that also carries vent air for limited fixtures. Circuit vent — one vent for a battery of up to eight floor-mounted fixtures.
EngEst Pro sizes vents alongside the drainage system — every fixture you add contributes to the vent load, and the Plumbing Calculator returns vent pipe sizes and quantities in the same BOM as the supply and drainage pipe.
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