Pipe Bedding Calculator — Granular Bed, Haunch & Surround Volumes

Pipe bedding is the material placed in the trench to support the drainage pipe. Proper bedding distributes loads evenly, prevents point loading that could crack the pipe, and provides a stable foundation. Without correct bedding, pipes can deflect, crack or develop leaking joints.

The bedding classes are defined by their geometry and bedding factor, and Approved Document H (2015) Diagram 10 draws four of them for rigid pipes with their factors. Class D (factor 1.1): pipe laid directly on a hand-trimmed natural trench bottom, no imported bedding at all, and the diagram allows it only where accurate hand trimming by shovel is possible. Class N (factor 1.1): a compacted 100mm granular bed only, the alternative where that trimming is not possible. Class F (factor 1.5): a flat 100mm granular bed shaped so the pipe beds into it. Class B (factor 1.9, the most common domestic class): a 100mm granular bed with granular fill carried up the sides to half the depth of the pipe, dimensioned OD/2 on the diagram. That is why Class B is called bed and haunch: the granular stops at the pipe springline and does not go over the crown. Class S (factor 2.2): a full granular surround carried to 300mm above the crown, often used for flexible plastic pipes; it is not drawn in Diagram 10 and comes from National Highways CD 533 Table 3.21N2. Class A (factor 2.6): a concrete cradle or surround for heavy loading or very shallow cover. The 150mm band shown above the crown on the AD H details is Diagram 10 Key 1 selected fill, which is screened site-won backfill, not imported granular, so it is not bedding material and this calculator does not charge you for it.

Approved Document H (2015) Diagram 10 Key 2 specifies it: for rigid pipes the granular material should conform to BS EN 1610 Annex B Table B.15 (that table reference is Approved Document H’s own, and the edition its reference list names is BS EN 1610:1998, so look for the table there rather than in the current edition) and should be single size material or graded material from 5mm up to a maximum size of 10mm for 100mm pipes, 14mm for 150mm pipes, 20mm for pipes from 150mm up to 600mm diameter and 40mm for pipes more than 600mm diameter, with a compaction fraction of at most 0.3 for class N or B and 0.15 for class F. In practice that means 10mm single-size pea gravel or 10mm clean aggregate on a domestic 110mm run, which flows easily around the pipe. It is used for classes N, F, B and S. Class A bedding uses C20/25 concrete (BS 8500 designated mix GEN3) for the cradle or surround. The BS 8500 general-purpose GEN family runs GEN0 (C8/10), GEN1 (C12/15 — the simple drain-haunching grade), GEN2 (C16/20) and GEN3 (C20/25 — the structural pipe-surround grade); there is no GEN4. Sharp sand can be used as an alternative to pea gravel but must be compacted in layers. Pea gravel weighs roughly 1.6 tonnes per cubic metre; concrete is roughly 2.4 tonnes per cubic metre.

Two different figures are in circulation and it is worth knowing which is which. UK building-drainage workmanship guidance allows about 150mm each side of the pipe barrel for jointing and for compacting the sidefill, so the pipe outside diameter plus 300mm: 410mm for a 110mm pipe, typically dug at 450mm, and 460mm for a 160mm pipe. That is the Clay Pipe Development Association wording, referenced to BS 8000 Part 14, and the same sentence appears in the BS 8301 lineage as "not less than the outside diameter of the pipe plus sufficient extra width (usually about 150mm) on each side". BS EN 1610, the construction standard for drains and sewers, asks for more: its Table 1 gives a minimum trench width of the horizontal outside diameter plus 0.40m for DN up to 225, which is 200mm of working space each side, rising to plus 0.50m for DN over 225 up to 350. So a 110mm drain to BS EN 1610 wants a 510mm trench, not 410mm. BS EN 1610 clause 6.3.3 does permit a narrower trench where nobody has to enter it, in constricted situations, or with self-compacting fill, and National Highways CD 533 clause 3.20 sets no trench-width restriction at all for pipes below 375mm internal diameter. This calculator does not refuse a narrow trench: it reports the working space each side beside the result and leaves the choice with you and your building control officer. Wider trenches are needed for deeper installations or where concrete bedding is used. See our Foul Drainage Calculator or Surface Water Drainage Calculator for full pipe run designs.

Pea gravel weighs approximately 1.6 tonnes per cubic metre. A standard bulk bag contains about 0.5m³ or 0.8 tonnes. Calculate your net volume in m³, multiply by 1.6 to get tonnes, then divide by 0.8 for bulk bags. This calculator already adds a waste and compaction allowance (10% by default, adjustable under Advanced Options), so the bulk-bag figure shown includes that allowance — no need to add it again.