How much fall does a drainage pipe need is one of those questions where the honest answer is that it depends, and the variables are specific: what the pipe carries, its diameter, and the flow it has to handle.
In England those figures are set out in Approved Document H of the Building Regulations. This guide gives the actual numbers for both foul and surface water drainage, explains how a fall is expressed and set, and covers what happens when the levels simply will not allow gravity drainage, which is far more common than people expect.
Quick answer:
For foul drainage, the flattest gradients in Approved Document H are 1:40 for 75mm and 100mm pipes at low flow, 1:80 for a 100mm drain carrying at least one WC, and 1:150 for a 150mm drain serving at least five WCs.
For surface water, 75mm and 100mm rainwater drains should be laid at not less than 1:100, 150mm at 1:150 and 225mm at 1:225.
Where levels make gravity drainage impracticable, the regulations expect a pumping installation instead.
What does fall actually mean?
Fall is the vertical drop of a pipe over a horizontal distance, written as a ratio.
A gradient of 1:40 means the pipe drops one unit for every forty it travels, so over a 10 metre run that is 250mm of drop.
At 1:80 the same run drops 125mm, and at 1:150 it drops just 67mm. The point that catches people out is that a larger second number means a flatter pipe, so 1:150 is considerably shallower than 1:40.
The figures in the regulations are minimums, in other words the flattest a pipe should be laid, and pipes should be laid to even gradients throughout, with any change of gradient combined with an access point so the run can still be rodded.
What happens if a drain is laid too flat?
The gradients in the regulations are minimums, meaning the flattest a pipe should be laid. Below them the flow may not be sufficient to carry solids reliably along the pipe, which increases the risk of deposits building up and causing blockages. Pipes should also be laid to even gradients, with any change of gradient combined with an access point so the run can be cleared.
Minimum gradients for foul drainage
These are the flattest gradients at which foul drains should be laid, together with the capacity they then have. Which row applies depends on the peak flow, and for design purposes a typical single dwelling is taken as around 2.5 litres per second.
| Peak flow | Pipe size | Minimum gradient | Maximum capacity |
|---|---|---|---|
| Less than 1 l/s | 75mm | 1:40 | 4.1 l/s |
| Less than 1 l/s | 100mm | 1:40 | 9.2 l/s |
| More than 1 l/s | 75mm | 1:80 | 2.8 l/s |
| More than 1 l/s | 100mm | 1:80, minimum of 1 WC | 6.3 l/s |
| More than 1 l/s | 150mm | 1:150, minimum of 5 WCs | 15.0 l/s |
Alongside the gradient, the diameter matters. A drain carrying foul water should be at least 75mm internal diameter, and a drain carrying the effluent from a WC or trade effluent should be at least 100mm. Where a drain serves more than one property it becomes a sewer, and should normally be at least 100mm when serving up to ten dwellings, or at least 150mm above that.
Minimum gradients for surface water drainage
Rainwater and surface water drains carry no solids, so they can be laid flatter than foul drains. The minimum gradients are set by pipe size alone.
| Pipe size | Minimum gradient | Notes |
|---|---|---|
| 75mm | 1:100 | Smallest permitted surface water drain |
| 100mm | 1:100 | Minimum size for a surface water sewer serving more than one building |
| 150mm | 1:150 | Applies to both drains and sewers at this size |
| 225mm | 1:225 | For larger pipes, refer to BS EN 752-4 |
Falls on paved areas and paths
Surfaces have their own requirements. An impervious paved surface should be laid to at least 1 in 60 so water drains quickly, while the gradient across a path should not exceed 1 in 40. Levels should direct water away from the building, and where the levels would otherwise carry water towards a wall, a reverse gradient should be formed for at least 500mm from that wall. For normal situations a design rainfall intensity of 0.014 litres per second per square metre may be assumed.
Where the water then goes is set by a clear order of priority: an adequate soakaway or other infiltration system first, then a watercourse, and only then a sewer. Infiltration devices come with their own siting rules, including that they should not be built within 5 metres of a building or road, or in ground where the water table reaches the bottom of the device at any time of year.
When gravity will not work: foul drainage
This is the situation the tables above cannot solve, and the regulations address it directly. Where levels do not permit drainage by gravity, a pumping installation should be provided. The same applies where protection is needed against flooding caused by surcharge in the sewer downstream, and for a basement containing sanitary appliances where the sewerage undertaker considers the surcharge risk high, the basement drainage should be pumped.
It is worth being clear that this is not a workaround, it is the recognised solution.
You can see the range of sewage pumps we supply for exactly this purpose, and our ultimate guide to sewage pumps explains how the pump types differ.
For how the regulations treat pumped foul drainage in more depth, including how the receiving chamber is sized around the number of people it serves, see our guide to sewage pumps and Building Regulations. Where a chamber needs to be sized and quoted around your own site, we handle that through our below ground pump stations and tanks page.
When gravity will not work: surface water
The same principle applies on the surface water side. Where the levels in the building or on the site make a gravity connection impracticable, surface water pumping equipment will be needed. In practice that means collecting the water at the low point and pumping it up to a drain, ditch or soakaway that can actually accept it.
For clearing and moving surface water, our surface water pumps cover most situations.
Where a permanent station is required, that is either an off the shelf unit from our packaged pump stations range, or a bespoke chamber designed around the site, which again is handled through our below ground pump stations and tanks page.
If you are not sure whether the water you are dealing with is foul or surface water in the first place, our guide to sewage pump vs sump pump sets out the difference.
Do you need Building Regulations approval for garden drainage?
This comes up often, and the answer is more nuanced than either extreme suggests.
Installing a French drain, land drain or perforated pipe purely to relieve waterlogging within an existing private garden would not normally require a separate Building Regulations application.
Approved Document H3 is concerned with rainwater from the roof of the building, and with the drainage of specified paved areas, namely those providing access to the building, access to refuse storage, or common passages shared between buildings.
It does not set out to control every lawn, flowerbed or privately used area of garden, and its provisions for paved areas apply only where those surfaces are paved.
So the useful distinction is this: garden land drainage on its own generally does not need a Building Regulations application, whereas a designed drainage or pumping system that serves or protects the building may well involve Building Control. Either way, none of that removes the need to give the water a lawful and suitable destination.
Where this guidance applies
Approved Document H applies in England. Wales has its own Approved Documents, Scotland works to the Technical Handbooks and Northern Ireland to the Technical Booklets, so the figures above should not be assumed to transfer. The document is also revised from time to time, so check the current edition on GOV.UK before relying on any figure for design. This guide is a summary written to help you plan, not a substitute for the document itself or for advice from your Building Control body or a drainage engineer.
