Drain Fall and Gradient: How Much Fall Does a Drain Need? (UK)
A drain must slope steadily so that the water carries the solids along. For a 100 mm foul drain serving at least one WC, Approved Document H gives a gradient of 1 in 80, which is 12.5 mm of fall in each metre. A drain of 15 m falls 188 mm. The drain fall calculator gives the fall, the depth at the lower end and the bed for your drain.
Fall by length and gradient
| Length | 1 in 40 | 1 in 60 | 1 in 80 | 1 in 100 | 1 in 150 |
|---|---|---|---|---|---|
| 5 m | 125 mm | 83 mm | 63 mm | 50 mm | 33 mm |
| 10 m | 250 mm | 167 mm | 125 mm | 100 mm | 67 mm |
| 15 m | 375 mm | 250 mm | 188 mm | 150 mm | 100 mm |
| 20 m | 500 mm | 333 mm | 250 mm | 200 mm | 133 mm |
| 30 m | 750 mm | 500 mm | 375 mm | 300 mm | 200 mm |
| 50 m | 1,250 mm | 833 mm | 625 mm | 500 mm | 333 mm |
The usual gradients
- 100 mm foul drain with at least one WC: 1 in 80 or steeper.
- 150 mm foul drain with at least five WCs: 1 in 150 or steeper.
- Steepest usual fall: about 1 in 40, since a steeper pipe can leave the solids behind.
- Surface water drains: often laid at about 1 in 100, because the flow is only water.
These come from Approved Document H: ask building control which applies to your drain.
Bedding the pipe
The pipe sits on a bed of 10 mm pea shingle 100 mm deep, is surrounded by it, and has another 100 mm over the top. The shingle for 15 m, and for each metre of drain:
| Pipe | Trench | Per metre of drain | For 15 m |
|---|---|---|---|
| 100 mm pipe | 450 mm trench | 0.22 tonnes | 3.28 tonnes |
| 100 mm pipe | 600 mm trench | 0.30 tonnes | 4.45 tonnes |
| 150 mm pipe | 450 mm trench | 0.24 tonnes | 3.58 tonnes |
| 150 mm pipe | 600 mm trench | 0.33 tonnes | 4.94 tonnes |
Laying the drain
- Set the levels. Mark the depth at the top and the lower end, using a string line or a laser level, and dig the trench to the fall.
- Bed. Put 100 mm of pea shingle in the bottom and shape it for the pipe.
- Lay. Start at the lower end with the sockets facing uphill, and push each pipe home to the mark.
- Access. Fit a rodding point or an inspection chamber at each bend and at the head of the drain.
- Test. Test with air or water before you backfill.
- Cover. Put another 100 mm of shingle over the pipe, then backfill with soil in layers.
See also the gravel calculator for stone, and the concrete calculator for the surround of a chamber. These are estimates: check the levels on site and the rules for your drain with building control.
Frequently asked questions
What fall does a drain need?
For a 100 mm foul drain serving a WC, 1 in 80, which is 12.5 mm for each metre: 188 mm over 15 m. A 150 mm drain at 1 in 150 falls 100 mm over the same length.
What is the maximum fall for a drain?
About 1 in 40 is the usual limit. A steeper drain can let the water run ahead of the solids and leave them behind.
How do I measure a fall of 1 in 80?
Divide the length by 80: 15 m is 188 mm, so the pipe is 188 mm lower at the far end.
How much pea shingle do I need for a drain?
For a 100 mm pipe in a 450 mm trench, about 0.22 tonnes per metre of drain, for a bed 100 mm deep that covers the pipe by 100 mm.
Do I need permission for a new drain?
A new drain connected to a sewer needs approval from the water company, and building work needs building control. Ask before you dig.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.