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.
Each connection needs a suitable barrier against foul air. Conventional traps retain water; they can be tubular, bottle-shaped or integral to an appliance such as a WC. An approved membrane waste valve provides an alternative for its permitted applications. A standard basin or sink trap starts with a 75 mm water seal, while baths, showers and WC pans normally use 50 mm. Check the stated exceptions and arrangement before selecting the fitting.
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, limiting pressure changes that could siphon a seal out or blow water back into an appliance.
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.
Approved Document H sets the scope for foul, waste and rainwater drainage. Use this overview to understand why pipes are sized, sloped and vented as they are, and confirm specific limits against Part H.