What is the minimum fall for a 110 mm drainage pipe?

For a 110 mm foul drain serving a single dwelling, the minimum gradient is 1:40 (25 mm fall per metre) if serving 1 WC, or 1:80 (12.5 mm per metre) if serving 5 or more WCs. Building Regulations Approved Document H recommends 1:80 as the practical minimum for most domestic foul drains. For surface water drains, 1:100 (10 mm per metre) is acceptable for 110 mm pipes.

What pipe size do I need for a foul drain?

The minimum pipe size for a foul drain carrying WC discharge is 100 mm (110 mm OD). This is the standard for most domestic drainage. For drains serving more than 10 dwellings or commercial buildings with heavy usage, 150 mm (160 mm OD) may be required. Branch connections from individual appliances use smaller sizes: 32 mm for basins, 40 mm for baths/showers, and 110 mm for WCs.

What is the difference between 110 mm and 160 mm drainage pipe?

110 mm is the standard domestic foul and surface water drainage pipe. 160 mm is used for larger systems: combined drains serving multiple properties, commercial buildings, or situations where the pipe run is very long with minimal gradient. The larger bore reduces the risk of blockages and provides greater flow capacity. Most domestic installations use 110 mm exclusively.

What bedding type do I need for drainage pipes?

Building Regulations and BS EN 1610 specify bedding classes. Class D (granular bedding with selected backfill) is the most common for domestic drainage. The pipe sits on a minimum 100 mm bed of 10 mm pea gravel or sharp sand, with the same material surrounding the pipe to 300 mm above the crown. Class S (concrete surround) is used for shallow pipes (less than 600 mm cover) or under driveways.

What is the maximum depth for a domestic drain?

There is no absolute maximum depth, but practical considerations apply. Most domestic drains are laid at 600 mm to 1.5 m depth (invert level). The minimum cover for a drain under a garden is 600 mm, under a driveway is 900 mm. Deeper drains require more expensive access chambers and may need concrete surround bedding. If connecting to a public sewer, the depth is dictated by the sewer connection level.

Drainage Pipe Sizes Reference — Free Trade Calculator

Drainage Pipe Sizes and Minimum Fall Gradients

UK drainage pipe sizes for foul and surface water systems, minimum fall gradients per Building Regulations Part H, and pipe material specifications.

Underground Drainage Pipe Sizes

Nominal OD (mm)	Internal Dia (mm)	Material	Common Use
110	~104	PVC-U (BS EN 1401)	Standard domestic foul and surface water
110	~100	Vitrified clay	Traditional foul drainage, roots-resistant
160	~152	PVC-U	Larger domestic, multi-property, commercial
225	~213	PVC-U or concrete	Main sewers, large commercial
300	~284	Concrete or vitrified clay	Public sewers, highway drainage
80 (land drain)	~76	Perforated PVC or clay	French drains, land drainage, soakaways

Minimum Fall Gradients (Part H)

Drain Type	Pipe Size	Min Gradient	Fall per Metre	Condition
Foul (1 WC)	110 mm	1:40	25 mm/m	Minimum for self-cleansing
Foul (5+ WCs)	110 mm	1:80	12.5 mm/m	Higher flow rate self-cleanses
Foul (general)	150/160 mm	1:150	6.7 mm/m	Larger bore allows flatter gradient
Surface water	110 mm	1:100	10 mm/m	Rainwater only, no solids
Surface water	160 mm	1:150	6.7 mm/m	Larger areas, car parks
Combined	110 mm	1:80	12.5 mm/m	Mixed foul and surface water

Minimum Cover Depths

Location	Min Cover (mm)	Bedding Required
Under garden/soft ground	600	Class D (granular)
Under driveway	900	Class D or Class S (concrete)
Under road	1200	Class S (concrete surround)
Under building (in duct)	300 min in duct	Rocker pipe joints, access

Related Calculators

Use the Drainage Fall Calculator for pipe gradients, or the Soakaway Calculator for surface water disposal. The Manhole Depth Calculator helps with inspection chamber sizing.

How We Calculate This

Frequently Asked Questions

Last updated: March 2026

All calculations are estimates. Verify with your supplier.

Drainage Pipe Sizes and Minimum Fall Gradients

Underground Drainage Pipe Sizes

Minimum Fall Gradients (Part H)

Minimum Cover Depths

Related Calculators

How We Calculate This

Frequently Asked Questions

Related Calculators

Concrete Mix Ratios

Insulation U-Values

UK Brick Dimensions

UK Cable Sizes

UK Plasterboard Sizes