Drain Pipe Falls — Minimum Gradients
Drain Pipe Falls — Minimum Gradients
Minimum Falls (Approved Document H)
| Pipe Size | Min Gradient | Notes |
|---|---|---|
| 75mm waste pipe | 1:40 | 1:80 sometimes accepted with higher flow |
| 100mm foul drain | 1:40 (low flow <1 l/s) | Can reduce to 1:80 with higher flow (>1 l/s) |
| 150mm foul drain | 1:150 | Typical minimum for combined/estate drains |
What the Gradients Mean
| 1:40 | 25mm per metre |
| 1:80 | 12.5mm per metre |
| 1:150 | 6.7mm per metre |
Self-cleansing velocity target: 0.75 m/s at ~⅓ full flow
Key Rules
- • Steeper is fine — no maximum gradient in Part H
- • Flatter needs justification — hydraulic calculation
- • Access points required at changes of direction/gradient/pipe size
Common Mistakes
- • Using 1:80 on a 100mm pipe serving a single WC (too flat for low flow)
- • No allowance for settlement on clay or made-up ground
- • Forgetting that waste pipes inside the building also need minimum falls
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