How branch positions, stub stacks, vents and air admittance valves keep trap seals intact in above-ground sanitary pipework.
What the layout has to protect
For the water-sealed traps discussed here, keep at least 25 mm of seal under working and test conditions. This retained depth is distinct from the trap’s specified initial seal. Discharges create suction (siphonage) and pressure (back-pressure), so the layout controls where branches join, how far they run and where air can enter. Approved Document H’s England guidance gives normal initial seals of 75 mm for basins and sinks, and 50 mm for baths, showers and WCs. Read the table’s stated exceptions with the appliance type. A bath, shower or washing machine draining straight to a gully has a separate minimum of 38 mm.
Where branches join the stack
Falling water compresses air where the stack turns into the drain. In single dwellings of up to three storeys the lowest branch sits at least 450 mm above the invert of the tail of the foot bend. That bend has a centre-line radius of at least 200 mm. For multi-storey buildings up to five storeys, use at least 750 mm or drain ground-floor appliances separately. Above five storeys, those appliances need their own stack or a direct route to a drain or gully. Avoid offsets in the wet part of a stack, and in buildings of up to three storeys keep branches 750 mm clear of any unavoidable offset. To limit crossflow, opposed unswept branches up to 65 mm need a 110 mm offset on a 100 mm stack, or 250 mm on a 150 mm stack. Larger opposed branches need at least 200 mm; the entry arrangement still matters.
Stub stacks and direct connections
A ground-floor WC may connect straight to a drain if the floor-to-drain depth is no more than 1.3 m. A stub stack must join a ventilated stack or a ventilated drain that is not subject to surcharging. No WC floor may be more than 1.3 m, and no other branch centre line more than 2 m, above the invert of the connection.
Venting branches and stacks
Read the permitted length and fall for the chosen bore together. A larger branch within its own limits can be an alternative; a layout exceeding the unventilated limits needs suitable ventilation. A branch vent joins the discharge pipe within 750 mm of the trap, rises continuously and connects above the highest spillover level. For a branch serving one appliance it is at least 25 mm, or 32 mm where the branch is over 15 m long or has more than five bends. Vents open to outside air finish at least 900 mm above any opening within 3 m, under a cage or perforated cover.
Air admittance valves
An AAV admits air when pressure falls; it provides no outlet for positive drain pressure. Approved Document H describes an internal AAV installation with ventilation and maintenance access, protected from dusty conditions. Follow the particular valve’s permitted location and approval: products differ in their siting limits. A drain with an intercepting trap, or one liable to surcharge, still needs the specified open ventilation without an AAV.