TradeCalculator.co.uk
Part H Drainage Gradients & Pipe Falls
Minimum drainage gradients by pipe size, inspection chamber spacing, and bedding requirements, referencing Approved Document H: Drainage and Waste Disposal (2015 edition).
Foul Drainage Minimum Gradients (AD-H Section H1)
| Pipe Diameter (mm) | Application | Min. Gradient | Fall per Metre (mm) |
|---|---|---|---|
| 100/110 | Foul drain (1+ WC discharge) | 1:80 min | 12.5 |
| 100/110 | Foul drain (no WC, low flow only) | 1:40 min | 25 |
| 150 / 160 | Foul drain (larger developments, 150mm clay or 160mm plastic equivalent) | 1:150 | 6.7 |
| 225+ | Foul sewer | 1:150 to 1:200 | 5-6.7 |
Surface Water Drainage Gradients (AD-H Section H3)
| Pipe Diameter (mm) | Min. Gradient | Fall per Metre (mm) |
|---|---|---|
| 110 | 1:100 | 10 |
| 150 | 1:150 | 6.7 |
| 225+ | 1:200 | 5 |
Inspection Chamber and Access Spacing
| Access Type | Max. Spacing | Depth Range |
|---|---|---|
| Rodding eye | At head of drain run, within 12 m | Any |
| Inspection chamber (IC) | 45 m on straight runs (≤ 150 mm pipe) | Up to 600 mm invert depth |
| Shallow manhole | 45 m on straight runs | 600 mm to 1.0 m invert depth |
| Deep manhole | 90 m for 225 mm+ pipes | Over 1.0 m invert depth |
Pipe Bedding Classes (BS 9295:2020+A1:2025)
Class letters are geometry, not a ranking, and two of them are easy to swap by mistake. Class D is the pipe laid straight onto a hand-trimmed natural trench bottom with no imported bedding at all. Class S is a full granular surround. Concrete is Class A. Bedding factors below are from National Highways CD 533 Table 3.21N2, which cites BS 9295.
| Bedding Class | Description | Bedding Factor | Typical Use |
|---|---|---|---|
| Class D | Pipe laid directly on a hand-trimmed natural trench bottom. No imported bedding | 1.1 | Uniform, stone-free soil only |
| Class N | Granular or sandy bed under the pipe (about 100 mm), no haunch | 1.1 | Selected soils, shallow domestic runs |
| Class F | Flat granular bed, shaped to allow for settlement | 1.5 | Normal ground, rigid pipes |
| Class B | Granular bed and haunch, granular fill to half the depth of the pipe (OD/2), stopping at the springline | 1.9 | The common domestic class, moderate loading |
| Class S | Full granular surround, carried to 300 mm above the crown | 2.2 | Flexible plastic pipe, deeper drains, poor ground |
| Class A | Concrete bed, cradle or surround (C20/25, BS 8500 GEN3) | 2.6 | Under roads, heavy loading, very shallow cover |
Work out the volumes with the Pipe Bedding Calculator, which uses these same class geometries.
Scotland & Northern Ireland
Scotland follows Technical Handbook Section 3.5 (Drainage and waste disposal) with similar gradient requirements. Northern Ireland uses Technical Booklet N. In all cases, local water authority requirements (e.g. for adoptable sewers) may impose stricter standards than building regulations.
Related Calculators
Use the Drainage Fall Calculator for gradient calculations, or the Drainage Pipe Sizes reference for pipe specifications. See also the Part H Drainage Calculator.
Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.
Next up: related pages
Frequently Asked Questions
For a 100/110 mm foul drain carrying at least one WC discharge, the minimum gradient is 1:80 (12.5 mm fall per metre) per AD-H Section H1 Diagram 5. A steeper 1:40 is permitted where the drain does not receive WC flow. For 150 mm drains carrying WC flow from multiple dwellings, 1:150 is acceptable. Gradient must also not exceed about 1:10 (very steep) because solids may separate from water (scouring).
Surface water drains typically require a minimum gradient of 1:100 (10 mm per metre) for 110 mm pipes. For larger diameter pipes (150 mm+), the gradient can be reduced to 1:150. These gradients ensure self-cleansing velocity is achieved during normal rainfall. Refer to AD-H Section H3 and BS EN 752 for design flows.
Inspection chambers (or manholes) should be provided at changes of direction, changes of gradient, junctions, and at maximum intervals of 45 m on straight runs for drains up to 150 mm diameter. For 225 mm+ drains, the maximum spacing is 90 m. An access point (rodding eye, inspection chamber, or manhole) is also required within 12 m of the head of any drain run.
For most domestic drainage in normal ground the answer is Class B, bedding factor 1.9: a 100 mm granular bed with granular fill carried up the sides to half the depth of the pipe, dimensioned OD/2 and captioned "Granular fill to half depth of pipe" on Approved Document H (2015) Diagram 10. The granular stops at the pipe springline, which is why the class is called bed and haunch rather than a surround. The 150 mm band drawn above the crown on that diagram is Key 1 selected fill, screened site-won backfill rather than imported granular, so do not order granular for it. Class D is not that. Class D is the pipe laid directly on a hand-trimmed natural trench bottom with no imported bedding at all, and it only works in uniform stone-free soil. Class S is a full GRANULAR surround to 300 mm above the crown, commonly used with flexible plastic pipe. Concrete is Class A, a concrete bed, cradle or surround for drains under roads, heavy loading or very shallow cover. Getting S and A the wrong way round is the expensive mistake: it puts concrete in a trench that was designed for pea gravel.
Was this calculator helpful?
Last updated: August 2026
Verified against UK standards · estimates only, confirm with your supplier.