Sewer Connection Calculator — Pipe Run, Chambers & Bedding

Under Approved Document H 2015 (para 2.49), access points should be provided at or near the head of each drain run, at every bend and change of gradient, at any change of pipe size, and at junctions (unless each run can be cleared from another access point). Table 13 sets the maximum spacing: inspection chamber to inspection chamber 45m, and start of drain to a small access fitting 12m. This calculator adds a chamber at the head and tail of the run, one per bend and junction, plus one for every additional 45m of straight run. All chambers must be accessible for rodding. Use our Inspection Chamber Calculator to determine chamber sizes and components.

Under Approved Document H 2015 the limits of cover are measured to the top of the pipe barrel (the crown), not the invert, and they depend on the pipe material. For flexible thermoplastic pipes (uPVC/PP, Table 10) the minimum cover is 600mm in fields and 900mm under both light and main roads. For rigid clay (Table 8) or concrete (Table 9) pipes it is 600mm in fields and 1200mm under light or main roads. Where the minimum cover can't be achieved, the pipe should be protected, for example with a reinforced concrete cover slab.

Bedding classes are geometry, not a ranking, and Class D, Class N and Class S are the three most often swapped for each other. Approved Document H 2015 Diagram 10 draws four of them for rigid pipes, with their bedding factors. Class D (factor 1.1) is the pipe laid straight onto a hand-trimmed natural trench bottom with no imported bedding at all, and the diagram permits it only where accurate hand trimming by shovel is possible. Class N (factor 1.1) is a granular or sandy bed under the pipe, dimensioned 100mm, offered as the alternative where that accurate trimming is not possible. Class F (factor 1.5) is a flat 100mm granular bed shaped so the pipe beds into it. Class B (factor 1.9) is the usual domestic choice: a 100mm granular bed with granular fill carried up the sides to half the depth of the pipe, dimensioned OD/2 on the diagram, which is why it is called bed and haunch. Class S is not drawn in Diagram 10; National Highways CD 533 Table 3.21N2 lists it as a granular surround with a bedding factor of 2.2, carried to 300mm above the crown, and it is granular, not concrete. Concrete is Class A (factor 2.6), a concrete bed, cradle or surround; it is not offered here because this calculator reports one granular volume, so use our Pipe Bedding Calculator for concrete quantities. One point that catches people out: the 150mm shown above the crown on the D, N, F and B details is selected fill (Diagram 10 Key 1, screened site-won material free from stones larger than 40mm), not imported granular, so this calculator counts it as backfill rather than bedding. Sewers offered for adoption follow the water company construction detail under the Sewerage Sector Guidance and its Design and Construction Guidance, which commonly calls for Class S and can specify a thicker bed than the 100mm in AD H, so confirm the detail with the undertaker before ordering. Full class table in our Part H drainage gradients reference.

You need a Section 106 sewer connection agreement from your local water company. They will specify the connection point, method and supervise the connection. A build-over agreement is needed if building within 3 metres of a public sewer. The connection is usually made via a saddle connection or new manhole.

Most domestic foul drainage uses 110mm (4-inch) uPVC pipe. Note the regulatory minimum-diameter rule: Approved Document H 2015 (para 2.30) requires a sewer (a drain serving more than one property) to be at least 100mm where it serves no more than 10 dwellings, and at least 150mm where it serves more than 10 dwellings — so 110mm is suitable for up to 10 dwellings on a single branch, not 20, even though it can hydraulically carry more. A single foul drain must be at least 75mm, or 100mm if it carries WC or trade effluent (para 2.33). Surface-water drains also typically use 110mm, though 160mm may be needed for larger catchment areas, and the public sewer main is usually 150mm or larger.