Sanitary Appliances and Traps
Above-Ground Sanitation and Drainage · Plumbing
Every appliance that takes water away needs a trap, and every trap has to keep its water. This topic covers the appliances in a home, the traps under them and the ways a seal goes missing.
Plumbers split used water into soil water from WCs and urinals and waste water from everything else. Both count as foul water and normally meet in the soil stack. Pipe size follows the appliance, from a small basin waste up to the large branch a WC needs.
A trap's water stops drain air reaching the room. In England, Approved Document H guidance gives deeper seals to basins and sinks than to baths and showers, and expects a minimum depth to survive normal use. P, S and bottle traps suit different routes out of the room.
Seals are lost when flowing water pulls them out, when an unused trap dries up, or when air squeezed at the bottom of a stack pushes them back. A pumped macerator lets a toilet go where gravity cannot, and in England the guidance expects a gravity WC to remain available too.
A trap is only as good as the water it keeps
Updated
How to reason about the system
Work out what each appliance discharges, which trap suits its route and how deep a seal it starts with. Then ask what could pull that seal out: its own flow, another appliance's flow, or a long spell with nobody using it.
Questions to take further
- Why does my sink smell of drains?
- Often the trap has lost its water, through siphonage, evaporation from an outlet nobody uses, or a slow leak. Running the tap refills it; if the smell keeps coming back, the venting or the pipe layout may need looking at.
Sources
- GOV.UK: Approved Document H — Guidance for England on above-ground drainage, traps, venting, access and rainwater disposal; guidance is not itself the legislation.
Free interactive sample
Choose an option, check it and retry here. No account, payment or energy needed. These public samples are separate from your learning path; progress is not saved.
Original plumbing learning material. For revision only; do not use samples to guide work on site.
Learning content, not a qualification: it won't make you competent, certified or registered. Follow the regulations that apply and a qualified plumber before acting. About our content
Lessons on the plumbing path
- Appliances and where they drain
- Traps and the water seal
- How a trap loses its seal
- WCs, flushing and macerators
Terms to know
Before you start: some plumbing work is regulated or notifiable
Some water fittings work needs notice to, and consent from, the water supplier before it starts. Check the rules where you live.
Gas work is for competent workers covered by a Gas Safe registered business in Great Britain. Northern Ireland has separate gas regulations. Do not work on gas pipes or appliances yourself.
In England, installing an unvented hot-water system over 15 litres is notifiable building work: use a competent installer registered with a competent-person scheme, or tell building control before work starts. Leave this work to a suitably skilled person.
Wales, Scotland and Northern Ireland have their own water and building rules; check which apply before work starts. Electrical connections belong with an electrician.
This guide does not qualify you or make you competent, approved or registered for plumbing, gas or unvented work.