How a single stack protects trap seals, and how pressure, disuse or a wick can break them.
In the single-stack detail shown, the basin discharges through a water-sealed trap. The water blocks drain air and smells from entering the room.
The diagram shows the basin branch joining a stack that vents at the top and carries waste to the foul drain below. That ventilation lets air move when an appliance discharges and helps keep pressure changes from emptying nearby traps.
The seal is kept when the stack is properly vented, the branch pipe is within its length and gradient limits, and each use refills the trap. The seal can be lost through self-siphonage (a long or steep branch), induced siphonage (flow elsewhere on the stack), back-pressure, evaporation in an unused trap, or a blocked or missing vent.
Approved Document H Table 1 gives a 75 mm starting seal for the washbasin shown. A WC pan, not shown in this detail, has a 50 mm starting seal. Under working and test conditions, each trap must still hold at least 25 mm.
Capillary action: another way a seal is lost
A trap seal can also be drawn away without any siphonage or pressure change at all. If hair, lint or a rag lies draped over the weir, it acts as a wick: capillary action pulls the water out along the fibres, slowly lowering the seal until it fails. Keeping the trap free of debris prevents it.