Drainage below ground

5.3.15Laying pipework

Pipework shall be laid to the designed lines and gradients. Issues to be taken into account include:

  1. bedding
  2. sidefill and backfill.


Pipes should be firmly supported throughout their length and bedded as specified in the design to resist loads from overlying fill and traffic.

Where pipework is installed under a suspended floor and is supported on ground or fill where movement is likely to occur, additional provisions may be required. See Clause 5.3.8.

Bricks, blocks or other hard material should not be used as temporary supports to achieve the correct gradients, as they may create hard spots which can distort the completed pipe run.

Pipes should be either:

  • bedded on granular material, minimum 100mm deep, or
  • laid directly on the trench bottom, where the trench bottom can be accurately hand trimmed with a shovel but is not so soft that it puddles when walked on.

For 150mm diameter and 100mm diameter drains, a bed and surround pea gravel in accordance with Table 8 (to a thickness of 100mm all round the drain) will be acceptable for drains under gardens, paths and drives.

Proprietary systems should be assessed in accordance with Technical Requirement R3 and supported in accordance with the manufacturer’s recommendations. Some proprietary systems permit a minimum of 50mm depth of bedding in certain circumstances.

Depressions should be formed where necessary in the trench bottom to accommodate pipe joints.

Pipe bedding, including the bedding material, should be in accordance with:

  • BS EN 13242
  • BS EN 1610
  • BS EN 752.

Bedding material and specification should be in accordance with Table 8. Backfill and bedding that includes recycled or secondary materials should conform to the appropriate regulatory requirements for waste, as defined in the Waste Framework Directive 2008.

Table 8: Bedding size

Nominal pipe sizeBedding material complying with BS EN 13242
110mm flexible pipes
100mm rigid pipes
4/10mm pipe bedding gravel
160mm flexible pipes
150mm rigid pipes
2/14mm pipe bedding gravel

Sidefill and backfill

Sidefill and backfill should be placed as soon as the pipes have been bedded, jointed and inspected.

Sidefill should be either granular material or selected backfill material from the trench excavation, free from:

  • stones larger than 40mm
  • clay lumps larger than 100mm
  • timber
  • frozen material
  • vegetable matter.

Backfill should be well compacted and placed in layers no deeper than 300mm. Mechanical compacting should only be used when compacted backfill is over 450mm above the crown of the pipe.