How traps, branch pipes, the soil stack and ventilation work together to carry foul water away and block smells.
Foul (above-ground) drainage takes used water and waste from appliances to the drain while keeping the smells of the drain out of the building. A few connected ideas explain how it works.
Every basin, bath, sink and WC has a trap: a U-shaped section that holds a small amount of water called the water seal. That seal is a plug of water that stops foul air from the pipework coming back into the room.
Waste from each appliance runs through a branch pipe to the main vertical soil-and-vent pipe, the stack. Branch pipes are laid to a gentle, even gradient so water flows away without either running too slowly (which lets solids settle) or too fast (which can pull the seal out of the trap behind it).
The stack is ventilated, usually by continuing it up above the roof to open air. This ventilation lets air move as water discharges, so the pressure changes caused by a flush do not siphon a seal out or blow it back.
If a trap seal is lost, by siphonage, back-pressure or simply drying out, the barrier against drain air is gone and the room smells, which is why seal protection drives so much of the design.
Above-ground foul drainage is guided by Approved Document H. Use this overview to understand why pipes are sized, sloped and vented as they are, and confirm specific limits against Part H.
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