In England and Wales, below-ground drainage is governed by Approved Document H of the Building Regulations, supported by the harmonised European standard BS EN 752 (drain and sewer systems outside buildings). Getting the fall (gradient) right is one of the most important parts of any drainage design: too flat and the pipe silts up; too steep and a foul drain can leave solids stranded. This guide explains what Part H requires, the minimum gradients you should design to, and how to set those falls accurately on site.
What Approved Document H covers
Part H is divided into several sections, the most relevant of which for falls and gradients are:
- H1 — Foul water drainage: carrying soil and waste water from WCs, basins, baths, kitchens and appliances to a public sewer, septic tank or treatment plant.
- H2 — Wastewater treatment systems and cesspools.
- H3 — Rainwater drainage: carrying surface water run-off from roofs and paved areas.
- H4 — Building over sewers.
- H5 — Separate systems of drainage.
- H6 — Solid waste storage.
For gravity pipework, the central design questions are the pipe diameter, the gradient, and the discharge (flow) it must carry. These three are linked: Part H sets out gradient/diameter combinations that are deemed to satisfy the requirement for a given peak flow, and BS EN 752 underpins the velocity and capacity logic behind them.
Minimum gradients for foul drainage (H1)
Foul drains must be laid steeply enough to maintain a self-cleansing velocity that carries solids along, but not so steeply that water outruns the solids. Approved Document H gives the following well-established guidance for foul drains:
- A 110 mm foul drain carrying the flow from at least one WC may be laid at a minimum gradient of 1:80 (12.5 mm of fall per metre), provided peak flow is adequate.
- A 150 mm foul drain may be laid as flat as 1:150 (about 6.7 mm/m), again subject to carrying a sufficient peak flow.
The “at least one WC” condition matters: the surge from a flushing WC is what mobilises solids in a small-diameter pipe. A 110 mm drain serving only basins and sinks, with no WC, may need a steeper gradient to stay self-cleansing because it lacks that periodic flush.
Minimum gradients for surface water drainage (H3)
Surface water (rainwater) carries no solids, so the gradient only has to move clean water and keep grit moving. Shallower falls are acceptable:
- Surface water drains are commonly laid at a minimum of around 1:100 (10 mm/m).
- Larger surface water pipes (150 mm and above) are sometimes laid flatter, in the 1:100 to 1:150 range, where capacity allows.
The governing constraint for surface water is usually capacity — the pipe must pass the design storm without surcharging — rather than self-cleansing. Sizing the pipe for the design rainfall event normally fixes a gradient that comfortably exceeds the self-cleansing minimum.
Self-cleansing velocity
The reason these gradients exist is to achieve a minimum flow velocity that keeps the pipe clear. As a rule of thumb derived from BS EN 752 practice:
- A self-cleansing velocity of roughly 0.7 to 1.0 m/s at peak (or design) flow is the target for foul drains.
- Velocities are typically calculated with the Manning equation, relating gradient, pipe diameter and roughness to flow.
The standard minimum gradients above are simply the slopes that deliver this velocity for the common pipe sizes when running at their design flow. If your pipe runs only part-full at low flow (as foul drains usually do), the periodic WC flush is what tops up the velocity to scour the invert.
Foul vs surface water — keep them separate
Part H (H5) and modern practice require separate systems wherever practicable: one set of pipes for foul, another for surface water. The two have different design priorities:
- Foul needs a steeper, self-cleansing gradient (e.g. 1:80 for 110 mm) and connects to the foul sewer or treatment system.
- Surface water can be laid flatter (e.g. 1:100) and should discharge to a soakaway, watercourse or surface water sewer — increasingly via SuDS (sustainable drainage systems) to control run-off rates.
Cross-connecting the two (foul into a surface water sewer, or surface water into the foul system) is a common and serious defect: it overloads treatment works or pollutes watercourses.
Invert levels and setting falls on site
A drainage design is realised on site through invert levels. The invert level is the level of the inside bottom of the pipe — the surface the water actually runs along. Falls are set by fixing the invert at each end of a run so the drop matches the design gradient.
The basic relationship is:
Total fall (m) = pipe length (m) ÷ gradient ratio
So a 12 m run at 1:80 falls 12 ÷ 80 = 0.15 m = 150 mm. If the start invert is 24.500 m AOD, the end invert is 24.500 − 0.150 = 24.350 m.
To transfer these levels into the trench accurately, use one of:
- Laser level set to the design gradient, with a receiver on a staff at the pipe invert.
- Dumpy or automatic level and staff, reading invert levels off temporary benchmarks.
- Boning rods and sight rails (travellers) — the traditional method, where a sight rail is set at each manhole and the boning rod confirms the invert as the trench progresses.
Always work from a fixed, recorded benchmark so levels are consistent across the whole site, and double-check the connection invert at the receiving manhole or sewer before backfilling.
UK minimum gradient reference
| Pipe size | Type | Typical minimum gradient | Fall per metre |
|---|---|---|---|
| 110 mm | Foul | 1:80 | 12.5 mm/m |
| 150 mm | Foul | 1:150 | 6.7 mm/m |
| 110 mm | Surface water | 1:100 | 10 mm/m |
| 150 mm | Surface water | 1:100 – 1:150 | 6.7 – 10 mm/m |
| 225 mm | Foul / sewer | ~1:200 | 5 mm/m |
Worked example
A 15 m run of 110 mm foul drain serving a downstairs WC, laid at the Part H minimum of 1:80:
- Total fall = length ÷ ratio = 15 ÷ 80 = 0.1875 m = 187.5 mm
- Fall per metre = 1000 ÷ 80 = 12.5 mm/m
- Percentage grade = 100 ÷ 80 = 1.25%
- If the start invert is 25.000 m AOD, the finish invert is 25.000 − 0.1875 = 24.8125 m AOD
Because the run includes a WC, the 1:80 gradient is acceptable. If this were a 110 mm drain with no WC, you would steepen it (or check the velocity with the Manning equation) to keep it self-cleansing. You can run any of these conversions instantly with the Drainage Fall Calculator.
Common mistakes
- Laying foul drains too steep “to be safe.” Steeper is not always better for foul — solids get stranded above 1:40 or so.
- Ignoring the “at least one WC” condition and using 1:80 on a 110 mm waste-only drain that lacks the flushing surge to stay self-cleansing.
- Setting falls off the trench bottom instead of the invert. Always level to the inside bottom of the pipe, allowing for bedding.
- Cross-connecting foul and surface water systems, overloading treatment works or polluting watercourses.
- Accumulated levelling errors over a long run — re-check against a fixed benchmark at each manhole.
- Forgetting capacity for surface water. The design storm, not just the minimum gradient, governs the pipe size.
When to involve building control
Below-ground foul and surface water drainage is notifiable work under the Building Regulations. In practice that means:
- Submit a building control application (full plans or a building notice) before you start, or use an approved inspector.
- Building control will want to see drainage layouts, pipe sizes, gradients and connection details, and will normally witness an air or water test on the completed drains before backfill is signed off.
- Any connection to a public sewer, or building over or near a public sewer, also needs the agreement of the sewerage undertaker (water company), separate from building control.
If you need to depart from the deemed-to-satisfy figures in Approved Document H — for example, an unusually flat gradient justified by a hydraulic calculation — be ready to demonstrate compliance with the functional requirement (often via a Manning-equation velocity check) rather than relying on the standard tables alone.
Related tools and reading
- Drainage Fall Calculator — convert between gradient ratio (1:X), total fall in mm and fall per metre, with a Part H check.
- Manning’s Equation Explained — the velocity/capacity formula behind self-cleansing gradients.
- Reading a Drainage Plan — how invert levels and gradients are shown on drawings.