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.
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