What is a soil stack, and why do drains need vents?

The soil stack is the large vertical pipe carrying waste from upstairs down to the drain — and the part people miss is that it also has to carry air, which is why it continues up past the highest fitting and out through the roof.

Why air matters. Every fitting — sink, bath, basin, toilet, washing machine — has a trap: a U-bend holding a small plug of water that seals the drain and stops sewer gas entering the room. That water seal is the only barrier, and it is easily removed by pressure changes.

What happens without ventilation:

A slug of water going down the stack pushes air ahead of it and drags a partial vacuum behind it. That suction can siphon the water out of a nearby trap, breaking the seal.

Positive pressure further down can push water the other way, producing bubbling in a WC or a gurgle in a bath.

Either way the result is the same: a dry trap and a smell. This is why a bathroom smells after a toilet upstairs is flushed, and why the smell is intermittent rather than constant.

How it is solved:

The stack is left open at the top, extending above the roof and above any window — a distance is specified in the regulations, so foul air disperses rather than entering the building.

Branch pipes have limits on length, gradient and number of bends, precisely because long or steeply falling branches self-siphon their own traps.

An air admittance valve (AAV) — sometimes called a Durgo valve — is the alternative where a stack cannot be taken through the roof. It opens to admit air when negative pressure occurs and closes otherwise, so air can enter but nothing escapes. It cannot replace all venting on a system, and regulations require at least one open stack ventilating the drainage system.

Common causes of drain smells: a dried-out trap in an unused fitting, a failed AAV, a blocked stack vent (frequently a bird's nest), a washing machine hose pushed too far into a standpipe, or a missing trap on a new appliance.