A drain works by gravity, so the angle it is laid at matters as much as the pipe itself. Lay it too flat and solids settle and block the pipe. Lay it too steep and the water runs ahead and leaves the solids behind. This guide explains the gradients used for household drains in England, how to work out the fall, and what to do if you think a drain at your home has the wrong fall.
Need a drainage engineer? Call Drain 247 on 0208 387 5591. Answered 24/7, no call-out fee, and the price is agreed before we start. If a run has lost its fall or cracked, see drain repair and lining.
What Is a Drainage Gradient?
The gradient (or fall) is the drop in height over the length of the pipe. It is usually written as a ratio. A gradient of 1:80 means the pipe drops 1 unit for every 80 units of length. That is 12.5mm per metre, or 1m over 80m.
What Gradient Do Household Drains Need?
In England, the guidance is set out in Approved Document H of the Building Regulations, which covers drainage. For foul drains, its minimum gradients include 1:80 for a 100mm drain serving at least one WC, and 1:150 for a 150mm drain serving at least five WCs. For rainwater, it says 75mm and 100mm rainwater drains should be laid at no less than 1:100.
In practice, drainage engineers often aim for a fall between about 1:40 and 1:80 on a typical house drain. That keeps the flow fast enough to carry solids, without being so steep that the water outruns them. Steeper runs, for example down a slope, can be broken up with a backdrop manhole.
| Gradient | Fall per metre | Fall over 10 metres |
|---|---|---|
| 1:40 | 25mm | 250mm |
| 1:60 | about 17mm | about 167mm |
| 1:80 | 12.5mm | 125mm |
| 1:100 | 10mm | 100mm |
| 1:150 | about 7mm | about 67mm |
How to Work Out the Fall on a Drain
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Measure the depth from the cover level to the bottom of the channel (the invert) in two manholes on the same run.
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Allow for any difference in ground level between the two covers.
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Divide the length of the run between the manholes by the difference in invert levels. For example, 8m of pipe with a 100mm drop is 8,000 ÷ 100 = 1:80.
This gives the average fall between the chambers. It will not show a dip in the middle of the run. That needs a camera.
Signs a Drain Has the Wrong Fall
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The same drain blocks repeatedly, often with toilet paper or fat, even after jetting.
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Standing water in the pipe or in the channel of a manhole after the water has stopped running.
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Slow drainage from fittings at the far end of a long run.
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Smells from a manhole that is never fully empty.
Common causes are poor original installation, ground settlement, tree roots lifting a pipe, or a dip (sometimes called a “belly”) where a section has sunk. A CCTV drain survey shows standing water and dips along the pipe. Our domestic surveys are from £150. If you need a scaled plan with levels, see drain mapping, or read mapping drains vs surveying drains for the difference.
Can a Drain with the Wrong Fall Be Fixed?
A short dip can sometimes be managed with regular drain jetting. Lining will not correct the fall, because the liner follows the existing pipe. Where the gradient is causing repeat blockages, the usual fix is to excavate and relay that section at the correct fall. See our drain repair and drain excavation and replacement services.
Planning an Extension or New Drain?
New drainage must meet the Building Regulations and is inspected by building control. Before you build, check where the existing drains run and how deep they are, and whether a public sewer crosses the site. Our guides to extension drain rules in Hertfordshire and build-over agreements explain what to check.
Frequently Asked Questions
What Is the Minimum Fall for a 100mm Drain?
For a 100mm foul drain carrying at least one WC, Approved Document H gives a minimum of 1:80. Rainwater drains of 75mm and 100mm should be no flatter than 1:100.
Can a Drain Be Too Steep?
Yes. On very steep runs the water can run ahead of the solids and leave them behind. Steep falls are often handled with a backdrop manhole.
How Do I Know the Fall of My Existing Drain?
Measure the invert depth at two manholes and divide the run length by the difference. For a full picture, including dips, book a CCTV survey.
Need a Drain Checked?
Drain 247 surveys, maps and repairs household drains across Watford, St Albans, Harrow and the rest of our areas served. Call 0208 387 5591 or get in touch.
