Drainage and Sewerage
Domain 10 covers drainage systems, gradients, venting, inspection chambers and manholes, and the maintenance that keeps a drain self-cleansing.
Systems
- One-pipe: soil and waste discharge to one stack, with a separate vent. Two-pipe: separate soil and waste stacks. Single stack: one pipe with a vented stack.
- House drain runs to a gully trap, inspection chamber, manhole, then the house sewer.
- Storm drainage is separate or combined; where there is no sewer, a septic tank with soak pit is used.
Rules
- Self-cleansing velocity is about 0.6 m/s; small drains commonly fall between 1 in 40 and 1 in 80.
- An inspection chamber goes at every junction and change of direction; use 45° bends, never 90°.
- The vent pipe runs to roof level with a cowl clear of windows, protecting trap seals.
- Manholes need airtight covers and steps; entry only with confined-space precautions.
Worked example
A 110 mm drain runs 15 m at a gradient of 1 in 60. Reasoning: fall = 15 ÷ 60 = 0.25 m, so the downstream invert sits 250 mm lower. If only 100 mm of fall were available, the pipe would be too flat; raise the upstream level, shorten the run, or check whether a larger pipe is needed, rather than laying a flat drain that silts.
Traps
Laying drains "level"; a 90° bend at the base of a stack; no vent pipe; connecting storm water into the foul sewer without approval.