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.
Each appliance connection needs a suitable barrier against foul air. Conventional traps use a retained water seal; an approved waterless waste valve uses a self-closing membrane for its permitted applications. The distinction matters when selecting a fitting that may be left unused.
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 trap must fit the space while retaining the required seal depth; 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.
How the WC flush restores its own seal
A WC flush does two jobs in one action: it washes the pan's contents through the pan's own integral trap into the branch, and the water it leaves behind re-forms that trap's seal. An adequate flush volume and pattern matters for both - too little water can fail to clear the pan or fail to leave enough behind to reseal it.
Trap patterns: P-traps, S-traps and waterless valves
P-traps discharge horizontally at the outlet, while S-traps turn downwards. The names describe those outlet directions; the pipework beyond them may follow different routes. That vertical drop can run full-bore as the appliance empties, siphoning the trap's own seal out behind it, which is why an S-trap is more prone to self-siphonage than a P-trap. A bottle trap offers a compact removable cup for cleaning. Select it by the manufacturer’s appliance suitability and rated flow, rather than assuming all bottle traps have one flow capacity. Suitable models also serve urinals. Where a conventional water seal would dry out from infrequent use, a self-sealing membrane waste valve is an alternative: it opens under flow and closes afterwards, blocking drain odours without relying on standing water that could evaporate. Follow its permitted applications, orientation and maintenance instructions.