Question 1

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

Accepted Answer

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).

Question 2

What gradient does surface water drainage need?

Accepted Answer

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.

Question 3

How far apart should inspection chambers be?

Accepted Answer

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.

Question 4

What pipe bedding class should I use?

Accepted Answer

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.

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

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

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

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

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

Related Calculators

Frequently Asked Questions

Related Calculators

Drainage Pipe Sizes and Falls

Part G Hot Water Standards

Part M Door Widths

Part M Ramp Gradients

Was this calculator helpful?