What is a soil vent pipe?
A soil vent pipe — in Dutch ontspanningsleiding — is the pipe that keeps the internal drainage system in open connection with the outside air. It works in both directions: air can enter and leave the system, so pressure differences are relieved.
When a toilet flushes or a bath empties, the slug of water pushes air ahead of it and draws air in behind it. Without an open connection to the outside, this creates negative pressure, which siphons the water out of the traps serving nearby fittings. The result is a dry trap and drain odour in the bathroom. The vent pipe is, quite literally, the breathing organ of the house.
How it is built
- The vent pipe is usually the continuation of the soil stack, carried up through the roof and finished with a roof terminal.
- It has the same diameter as the stack; in housing, 110 mm is standard. For the open connection to outside air, a minimum of around 75 mm applies.
- The diameter must not reduce along the way — a restriction makes the venting unreliable.
- The position of the roof terminal must be chosen so that no odour nuisance occurs, for example near openable windows, roof lights or ventilation inlets.
- In long or branching systems, secondary vent pipes or air admittance valves are added.
- Exact requirements and sizing come from the applicable drainage standard; in the Netherlands this is NEN 3215.
Common mistakes
- Omitting or capping the vent pipe during a refurbishment — the problem often only shows up months later as recurring drain smell.
- Treating an air admittance valve in the loft as a full replacement; it lets air in but not out, and does not simply replace a pipe carried through the roof.
- Placing the roof terminal too close to a roof light or an air intake.
Related terms
- Soil stack
- Roof penetration
- Water trap
- Rodding eye chamber
- Rainwater downpipe
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