What is the minimum gradient for a 110mm foul drain?

For a 110 mm foul drain serving a single dwelling, the minimum gradient is 1:40 (25 mm fall per metre). For drains serving 5 or fewer dwellings, a gradient of 1:80 may be acceptable provided the drain receives at least one WC flush. For larger drains (150 mm+), 1:150 may be acceptable. These are set out in AD-H Section H1, Diagram 5.

What gradient does surface water drainage need?

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.

How far apart should inspection chambers be?

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.

What pipe bedding class should I use?

Class D bedding (granular bedding with no concrete surround) is suitable for most domestic drainage in normal ground conditions, supporting rigid pipes to at least 150 mm below and 150 mm above the crown. Class S (concrete surround) or Class B (granular with compacted fill) may be needed for deeper drains, poor ground, or drains under roads. Flexible pipes (HDPE, PVC) use different support specifications per the manufacturer.

Part H Drainage Gradients & Pipe Falls

Foul Drainage Minimum Gradients (AD-H Section H1)

Pipe Diameter	Application	Min. Gradient	Fall per Metre
110 mm	Foul drain (single dwelling, 1+ WC)	1:40	25 mm
110 mm	Foul drain (≤ 5 dwellings, 1+ WC per dwelling)	1:80	12.5 mm
150 / 160 mm	Foul drain (larger developments, 150mm clay or 160mm plastic equivalent)	1:150	6.7 mm
225 mm+	Foul sewer	1:150 to 1:200	5-6.7 mm

Surface Water Drainage Gradients (AD-H Section H3)

Pipe Diameter	Min. Gradient	Fall per Metre
110 mm	1:100	10 mm
150 mm	1:150	6.7 mm
225 mm+	1:200	5 mm

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 EN 1295-1)

Bedding Class	Description	Typical Use
Class D	Granular bedding, 150 mm below & 150 mm above crown	Standard domestic in good ground
Class N	Natural ground (only suitable for certain pipes)	Selected soils, flat trench bottom
Class B	Granular with compacted fill to 300 mm above crown	Deeper drains, moderate traffic loading
Class S	Concrete surround (180° or 360°)	Under roads, poor ground, heavy loading

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.

Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.

Part H Drainage Gradients & Pipe Falls

Foul Drainage Minimum Gradients (AD-H Section H1)

Surface Water Drainage Gradients (AD-H Section H3)

Inspection Chamber and Access Spacing

Pipe Bedding Classes (BS EN 1295-1)

Scotland & Northern Ireland

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