Fall per metre for common gradients
| Gradient | Fall per metre | As a percentage | As an angle |
|---|---|---|---|
| 1 in 20 | 50 mm | 5% | 2.86° |
| 1 in 40 | 25 mm | 2.50% | 1.43° |
| 1 in 60 | 16.7 mm | 1.67% | 0.95° |
| 1 in 80 | 12.5 mm | 1.25% | 0.72° |
| 1 in 100 | 10 mm | 1% | 0.57° |
| 1 in 150 | 6.7 mm | 0.67% | 0.38° |
| 1 in 200 | 5 mm | 0.50% | 0.29° |
The bigger the second number, the flatter the pipe. To turn a percentage into "1 in X", divide 100 by it: 1.25% is 100 ÷ 1.25 = 1 in 80. Pipe lasers are usually set as a percentage, so 1 in 40 is 2.5% and 1 in 80 is 1.25%.
Fall over a whole run
| Run length | 1 in 40 | 1 in 60 | 1 in 80 | 1 in 150 |
|---|---|---|---|---|
| 1 m | 25 mm | 17 mm | 12 mm | 7 mm |
| 2 m | 50 mm | 33 mm | 25 mm | 13 mm |
| 3 m | 75 mm | 50 mm | 38 mm | 20 mm |
| 5 m | 125 mm | 83 mm | 62 mm | 33 mm |
| 10 m | 250 mm | 167 mm | 125 mm | 67 mm |
| 15 m | 375 mm | 250 mm | 188 mm | 100 mm |
| 20 m | 500 mm | 333 mm | 250 mm | 133 mm |
| 30 m | 750 mm | 500 mm | 375 mm | 200 mm |
Fall over a run = length in mm ÷ X. For anything not in the table, the free Drainage Fall Calculator chains runs together from a starting invert level and gives the level at every manhole.
Working out the gradient from two levels
Gradient (1 in X) = run length ÷ difference in invert levels, both in the same units. Use the inverts (the inside bottom of the pipe), not the cover levels.
| Value | |
|---|---|
| Invert at MH1 | 98.450 m |
| Invert at MH2 | 98.210 m |
| Difference | 0.240 m |
| Run length | 18 m |
| Gradient | 18 ÷ 0.24 = 1 in 75 |
1 in 75 is steeper than 1 in 80, so this run meets the minimum for a 100 mm drain serving a WC.
UK minimum falls (Approved Document H)
Approved Document H (England) gives these minimum gradients for foul drains, depending on the peak flow and pipe size:
| Peak flow | Pipe size | Minimum gradient | Condition |
|---|---|---|---|
| Less than 1 litre/second | 75 mm | 1 in 40 | |
| Less than 1 litre/second | 100 mm | 1 in 40 | |
| More than 1 litre/second | 75 mm | 1 in 80 | No WCs |
| More than 1 litre/second | 100 mm | 1 in 80 | At least one WC |
| More than 1 litre/second | 150 mm | 1 in 150 | At least five WCs |
- Plastic 110 mm drain pipe is the 100 mm size in these tables.
- In practice that means 1 in 40 for a 100 mm drain with no WC, such as a run taking only a sink or shower, and 1 in 80 once a WC discharges into it.
- Above-ground waste pipes from basins, baths and showers have their own falls, usually somewhere between 18 and 90 mm per metre depending on the pipe size and length; Table 2 of Approved Document H gives them.
- Scotland and Northern Ireland have their own building regulations with similar guidance. Check with building control for anything unusual.
Setting out the fall on site
- Pipe laser: set the grade as a percentage (2.5% for 1 in 40, 1.25% for 1 in 80) and check it at the start of each run.
- Level and straight edge: on a 1 m level, 1 in 40 is 25 mm of packing under the low end, and 1 in 80 is 12.5 mm.
- Boning rods or a string line: set the two ends at the right inverts, then sight or string between them to check the bedding in between.
- Check every few metres for dips. A low spot (a back fall) holds water and collects solids, even when the overall fall is right.