The common soil and waste appliances, the traps that serve them and why each is shaped as it is.
Sanitary appliances are the fittings people actually use, from basins to WCs, and each is paired with a trap and waste suited to its job. This lesson gives the overview.
They divide broadly into two groups. Soil appliances, chiefly the WC, deal with foul waste and connect to larger soil pipework. Waste appliances, such as wash basins, sinks, baths, showers and bidets, deal with used water and connect to smaller waste pipes.
Every appliance has a trap: a shaped section of pipe that holds a small plug of water called the water seal. That seal is the barrier that stops foul air from the drainage coming back up into the room, so it is the single most important feature.
Trap shapes suit the appliance. A basin or sink often uses a tubular or bottle trap that is easy to fit and clean under the appliance; a bath uses a shallow trap to fit the tight space beneath it; a WC has its trap built into the pan.
Waste sizes suit the flow. A wash basin uses a smaller waste than a sink or bath, and a WC uses much larger soil pipework, because each carries a different amount and type of discharge.
The themes that follow are seal depth, branch length and ventilation, all aimed at keeping the seal in place so the appliance stays smell-free and drains properly. Confirm the correct trap, seal depth and waste size for each appliance against Approved Document H.
Something wrong, unclear or out of date? Report an issue with this note.