Level 3 Sanitation System Planning and Design: the Unit 334 Guide

Above-ground discharge pipes are sized by adding the discharge units of the appliances they serve and reading the pipe size from the tables; discharge units are the design load figure in BS EN 12056-2.

Approved Document H 2.18 requires the drainage system to be ventilated by a flow of air: a ventilating pipe at or near the head of each main drain, and an open ventilating pipe (no air admittance valve) on any drain fitted with an intercepting trap or subject to surcharge. Paragraph 1.33 adds that air admittance valves must not reduce that below-ground ventilation. The old rules of one open stack in five and "head and midpoint for 11 to 20 dwellings" came from BS 5572, withdrawn and replaced by BS EN 12056-2, which has no dwelling count.

A vent pipe open to outside air has to finish at least 900 mm above any opening into the building that is within 3 m of it, so a pipe stopping level with a window only complies by being outside that 3 m radius. Approved Document H paragraph 1.31. 0.9 m is the height above an opening, not the distance away from it.

Approved Document H paragraph 1.9 allows a ground floor WC branch to discharge directly to the drain only where the depth from floor level to the drain is 1.3 m or less, so that is as far below the floor as the connection may go. Any deeper and the vertical drop drags air down with the water and the trap seals suffer. 1.8 m is well past the limit.

The termination rule is 900 mm above any opening into the building within 3 m of the pipe, so a stack finishing level with an openable window rather than above it has to clear that 3 m zone horizontally. Approved Document H paragraph 1.31. 2.5 m is still inside the zone, so the 900 mm height requirement would still bite.

Approved Document H Table 2 allows an unventilated 100 mm branch serving up to eight WCs to run 15 m at a fall of 18 to 90 mm per metre, and BS EN 12056-2 Table 6 gives the same 15 m for a group of up to eight WCs and no limit at all for a single one. A 75 mm branch to a single WC is also allowed 15 m. The familiar 6 m is trade and textbook practice, carried in the City and Guilds branch length table and originally from the withdrawn BS 5572. It is a sensible working maximum, but it is not the limit either current document sets.

Table 2 allows an unventilated 100 mm branch serving up to eight WCs to run 15 m at a fall of 18 to 90 mm per metre, so at 8 m the branch may discharge to a primary ventilated stack with no branch ventilating pipe at all. The 6 m working figure is trade and textbook practice from the withdrawn BS 5572, not a limit in the Approved Document. A stub stack is limited by height above the drain invert rather than by branch length, and a combined system is a below ground arrangement, nothing to do with a branch. Where a branch ventilating pipe is used, paragraph 1.22 puts its connection within 750 mm of the trap.

A siphonic WC pan has a double or restricted trapway that fills on flushing and starts a siphon which pulls the contents out; a wash-down pan is emptied only by the force of the flush water. Urinals do not siphon.

Acrylic baths scratch and dull under abrasive or solvent cleaners, so the customer is told at handover to use only the cleaners the manufacturer recommends.

Foul waste carries bacteria and viruses, and the way in is broken skin, so contact between contaminated material and an open wound is the significant hazard. Cover every cut with a waterproof dressing, wear gloves and wash before eating; employers expect hepatitis A vaccination for this work. Electric shock is a real risk on a macerator, but that comes from the supply, not from the waste.