Drainage Pipe Sizes and Minimum Fall Gradients

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.

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.

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.

BS 9295:2020+A1:2025 defines the bedding classes and BS EN 1610 covers the installation. Watch the class letters, because two of them are easy to swap. Class B is the common domestic class: the pipe sits on a 100 mm bed of 10 mm pea gravel, with granular fill haunched up the sides to half the depth of the pipe, dimensioned OD/2 on Approved Document H (2015) Diagram 10 and captioned there "Granular fill to half depth of pipe". The granular stops at the springline, so Class B is a bed and haunch, not a surround. The 150 mm band shown above the crown on that diagram is Key 1 selected fill, which is screened site-won backfill rather than imported granular. Class S is a full GRANULAR surround carried to 300 mm above the crown, often used with flexible plastic pipe. Class D is not granular bedding at all: it is the pipe laid directly on a hand-trimmed natural trench bottom with no imported bedding, and only suits uniform stone-free soil. Concrete is Class A, a concrete bed, cradle or surround for shallow cover (less than 600 mm), heavy loading or drains under driveways and roads. Ordering concrete because a drawing said Class S is the expensive version of this mistake.

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 or other light road it depends on the pipe material: 900 mm for thermoplastic (PVC-U) pipe, which Approved Document H Table 10 gives as 0.9 m to 7 m for 100 mm to 300 mm SN4 pipe, but 1.2 m for clay or concrete, which Tables 8 and 9 set “irrespective of pipe strength”. Most domestic runs are 110 mm PVC-U and take the 900 mm figure; a clay run under the same driveway needs the extra 300 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.

Vitrified clay drainage pipe is made in a fixed set of nominal sizes to BS EN 295-1: 100 mm, 150 mm, 225 mm and 300 mm, with larger sizes available in some jointless and channel ranges. These are nominal bore sizes rather than outside diameters, so a 100 mm clay pipe measures larger than 100 mm across the outside, unlike PVC-U pipe which is sized by outside diameter.

The Defra National Build Standards for new gravity foul sewers and lateral drains (Annex B, December 2011, made under section 106B of the Water Industry Act 1991) set a minimum of 100 mm nominal diameter for a sewer or lateral drain serving 10 properties or fewer, rising to 150 mm where it serves more than 10 properties. Water companies apply this on top of, not instead of, the Approved Document H gradient and cover rules. Check with the local water company before finalising pipe size on a shared or estate drain.

In trade use, yes: 110 mm outside diameter PVC-U drainage pipe is commonly sold and referred to as "4 inch" pipe, and 160 mm as "6 inch" pipe. These are trade equivalents rather than exact conversions, since true 4 inches is 101.6 mm and true 6 inches is 152.4 mm. The 100 mm nominal clay drainage pipe size is actually the closer metric match to a true 4 inch pipe; 110 mm PVC-U is one size up.

Drainage pipe usually means underground pipework carrying foul or surface water away from a building to a sewer or soakaway, which is what this page covers. Soil pipe usually means the above-ground stack and branch pipework on or inside a building that carries waste down to the underground drain. Both commonly use 110 mm and 160 mm diameters, but the fittings, gradients and support requirements differ; see the UK soil pipe sizes reference for the above-ground figures.