Plain definitions of the above-ground drainage terms a Level 2 learner keeps meeting.
Above-ground drainage has its own vocabulary, and the terms explain why pipes are sized, sloped and vented as they are. These plain definitions are for revision; confirm figures against Approved Document H.
- Trap: the shaped pipe section under an appliance that holds the water seal.
- Water seal: the plug of water held in a trap that stops foul air entering the room; its depth is measured in millimetres.
- Branch pipe: the waste pipe carrying discharge from an appliance to the stack or drain.
- Stack: the main vertical pipe; a soil-and-vent stack carries discharge down and is open at the top to ventilate the system.
- Self-siphonage: loss of a trap seal caused by the discharge of the appliance itself pulling the water out behind it, common on long or steep basin wastes.
- Induced siphonage: loss of a seal caused by another appliance discharging in the stack and lowering the pressure past this trap.
- Back-pressure (in drainage): positive pressure low in the stack that can push a seal out, for example at the base of a tall stack.
- Air admittance valve: a one-way valve that lets air into the system to protect seals where an open vent is not provided, used where permitted.
- Gradient (fall): the slope of a pipe, written as a ratio such as 1 in 40, that lets it drain by gravity.
- Rodding or access point: a fitting that lets the pipe be cleared if it blocks.
Keep these definitions handy when working through drainage problems, and confirm seal depths, branch limits and gradients against Part H.
More terms: pipe names and appliance types
- Sanitation means the hygienic removal of foul water and waste from appliances such as WCs, basins, baths and sinks.
- A waste pipe carries basin, bath, shower and sink water; it never carries WC discharge.
- A soil pipe carries that same waste plus WC discharge, which is why it needs a bigger bore.
- A WC branch is sized larger than a basin waste because it carries far more volume, plus solids.