Above-Ground Drainage, Soil, Waste and Vent Systems
Domain focus
This domain tests above-ground drainage: soil, waste and vent systems, stacks, branches, traps, gradients, supports and testing. It includes both design checks and installation faults.
Core rules
- Keep soil (WC) and waste (grey water) connections correctly sized; use the specified stack and branch sizes.
- Grade horizontal branches continuously; typical falls are about 1:60 for 100 mm pipe, but follow the project specification.
- Fit traps with the required water seal and vent them to prevent siphonage.
- Support stacks and branches; allow expansion at the stack base and at long vertical runs.
- Terminate vents to open air, clear of openings; use air admittance valves only where permitted.
- Provide rodding eyes and access at changes of direction.
Worked example
A 6 m branch at 1:60 has a fall of 6000 ÷ 60 = 100 mm. With an upstream invert of 2.500 m, the downstream invert is 2.500 − 0.100 = 2.400 m. Check the invert at the fitting, not the top of the pipe.
Traps
No vent causing gurgling; back-fall in a branch; reducing a 100 mm branch to 50 mm; connecting a vent incorrectly; ignoring expansion; burying a joint without access.