Soil stacks, roof vents and air admittance valves

Most UK houses have one big pipe that takes the toilets and the other wastes down to the drain. It is called the soil stack, or the soil and vent pipe. In many homes it runs up an outside wall and out above the roof. In others it is boxed in inside, and may end in the loft with a valve on top instead of an open pipe. Both arrangements do the same job: they let the drain breathe.

This page explains what that means and what goes wrong. We do not give a method for opening a soil stack or working on a roof, and we do not suggest swapping a valve in a loft void or high on a wall yourself.

Why the stack needs air

When a toilet flushes, a slug of water drops down the stack. It pushes air ahead of it and leaves a gap behind it. If air cannot get in to fill that gap, the falling water pulls on the nearest available air, which is the water sitting in the traps of your sinks, basins, bath and shower. Pull hard enough and the seal is sucked out. Once a trap is empty, drain air can come into the room.

The top of the stack provides the air. An open vent above the roof lets air in and lets drain gases out, high enough to disperse. The trap seals at each fitting stop those gases entering the house. How a trap holds its water is covered in basin wastes and bottle traps.

Air admittance valves

An air admittance valve, often shortened to AAV, is a one-way valve that sits on top of a stack or on a branch. When the pipe needs air, it opens and lets air in. When the pipe is at rest, or under slight positive pressure, it closes so drain gas does not escape. Plumbers sometimes call them by a brand name. The common short form on the trade counter is "Durgo-style valve", though many makes exist.

AAVs are used when running a vent through the roof is awkward, for example on an extension, a downstairs cloakroom or a stack that ends in the loft. They let air in but do not let gas out. A drainage system normally still needs an open vent somewhere, usually at the head of the drain run, so pressure from the sewer side can escape. Whether a particular house needs one, and where, is a design question. In England the guidance on sanitary pipework and drainage is in Approved Document H. We have not reproduced its figures here; a plumber or building control will work from it.

Signs of a vent problem

The usual symptoms are noises and smells, not floods:

• A gurgle or gulp from a basin or bath when a toilet upstairs flushes.
• A sink or shower trap that is empty soon after another fitting drains.
• A drain smell that comes and goes with use, rather than one that is always there.
• Toilets that flush sluggishly, or a bowl whose water level drops after a flush elsewhere.

These can also point to a partial blockage further down. A vent that cannot let air in and a drain that cannot let water out look similar from the bathroom. Work through drain smells first, which covers dry traps and biofilm. A toilet that will not clear is in blocked toilet, and an outside gully or drain run in outside drains.

What can go wrong with an AAV

An AAV is a moving part. Over time it can stick shut, so it no longer admits air, or stick open, so drain smells escape where it sits. Dust and fluff in a loft, insects, or a valve that has been knocked can all play a part. A valve fitted where it can freeze, or below the level of the fittings it is meant to serve, may not work as intended. An AAV boxed in with no air around it cannot do its job either; boxing usually needs a grille.

Open roof vents have their own problems: birds' nests, leaves, or a cap that has been fitted the wrong way. You cannot safely check those from the ground beyond a look with binoculars.

When to call a plumber

If you can see an AAV in a cupboard at floor level, you can note its position, whether it has a ventilated box around it, and whether it smells. Leave anything else to a plumber when:

• the valve is in a loft void, high on a wall, or inside a boxed stack;
• the stack goes through the roof and the vent may be blocked;
• several fittings gurgle or lose water at once;
• there is any sign of sewage backing up;
• you suspect a cracked stack or a leaking joint on it.

Opening a soil stack lets sewage and gas out, and roof access is a fall risk. A plumber can check the vent, test the valve and see whether a blockage is causing the trouble. If the WC connection to the stack is leaking, see WC pan connectors. Hiring a plumber covers what to ask.

What is an air admittance valve for?

It is a one-way valve on a soil stack or waste branch that lets air into the pipe when water runs down it, so trap seals are not sucked dry. It closes again when the pipe is at rest so drain gas stays in.

Why does my sink gurgle when the toilet flushes?

Often because the stack is short of air and is pulling it through the sink trap. A stuck air admittance valve, a blocked roof vent or a partial blockage can all cause it. A plumber can tell which.

Can I replace an air admittance valve myself?

If it is in a loft void, high up or boxed in, have a plumber do it. They can also check whether the valve is the real cause and whether the system has the open vent it needs.