NHBC Standards 2006

5.3 Drainage below ground

5.3 - Appendix 5.3A
   Minimum dimensions for access fittings and chambers
   Minimum dimensions for manholes
5.3 - Appendix 5.3B
   Percolation test procedures for septic tank installations
5.3 - Appendix 5.3C
   Septic tank field drain design
   Chart 1Field drains trench area
   Chart 2Field pipe length
5.3 - Appendix 5.3D
   Cross section of typical underdrain
5.3 - Appendix 5.3E
   Percolation tests and design method for surface water soakaways

Appendix 5.3-A

Minimum dimensions for access fittings and chambers
Type Depth to invert
from cover level (m)
Internal sizes Cover sizes
Length x Width
(mm x mm)
Length x Width
(mm x mm)
Rodding eye

As drain but min 100

Same size as

Access fitting
small 150 diam
150 x 100
0.6 or less,
except where
situated in a
150 x 100 150 150 x 1001 Same size as
access fitting
large 225 x 100 225 x 100 225 225 x 1001

Inspection chamber
shallow 0.6 or less
1.2 or less
225 x 100
450 x 450
190 2
Min 430 x 430
190 1
deep greater than 1.2 450 x 450 450 max 300 x 300 3 Access restricted
to max 3503


1 The clear opening may be reduced by 20mm in order to provide proper support for the cover and frame.

2 Drains up to 150mm.

3 A larger clear opening cover may be used in conjunction with a restricted access. The size is restricted for health and safety reasons to deter entry.

Minimum dimensions for manholes
Type Size of largest pipe (DN) (mm) Min internal dimensions1

  Min clear opening size1

Rectangular length and width (mm) Circular diameter (mm) Rectangular length and width (mm) Circular diameter(mm)
less than 1.5m deep to soffitequal to or less than 150
greater than 300
750 x 675 7
1200 x 675
1200 x 750
1800 x (DN+450)
The larger of 1800
or (DN+450)
750 x 675 2
1200 x 675 2
na 3
greater than 1.5m
deep to soffit
equal to or less than 225
375 - 450
greater than 450
1200 x 1000
1200 x 1075
1350 x 1225
1800 x (DN+775)

The larger of 1800
or (DN+775)
600 x 600600
Manhole shaft 4
greater than 3.0m deep to soffit pipeSteps 51050 x 8001050600 x 600600

Laddder 51200 x 8001200

Winch 6900 x 800900600 x 600600

1 Larger sizes may be required for manholes on bends or where there are junctions.

2 May be reduced to 600 by 600 where required by highway loading considerations, subject to a safe system of work being specified.

3 Not applicable due to working space needed.

4 Minimum height of chamber in shafted manhole 2m from benching to underside of reducing slab.

5 Min clear space between ladder or steps and the opposite face of the shaft should be approximately 900mm.

6 Winch only - no steps or ladders, permanent or removable.

7The minimum size of any manhole serving a sewer (i.e any drain serving more than one property) should be 1200mm x 675 mm rectangular or 1200mm diameter.

Reproduced from Tables 11 and 12 of Approved Document H to the Building Regulations by permission of HMSO.

Appendix 5.3-B

Percolation test procedure for septic tank installations

  1. Excavate a hole 300mm square x 250mm deep below the proposed invert level of the land drain.
  2. Fill the hole with water to a depth of 250mm and allow to drain away over night.
  3. Refill to a depth of at least 250mm and note the time taken (in seconds) to drain away completely.
  4. Repeat the exercise two more times and calculate the average of the three results, as follows:

percolation value (s) = time to drain away (seconds)/depth of water (mm)


percolation valuesuitability
up to 100use Appendix 5.3-C Chart 1 to determine field drain area
100 to 140use Appendix 5.3-C Chart 1 but with underdrains*
over 140field drains unsuitable

* Where underdrains are necessary, drainage trenches should be constructed not less than 600mm deeper than the pipe level specified in the design, and the lower part filled with pea gravel (see Appendix 5.3-D)

A second system of drainage pipes should be laid on the bottom of the trenches to convey surplus drainage to an outfall in a surface ditch or watercourse.

Underdrains are costly, and a secondary treatment system able to produce an effluent suitable for surface discharge may be preferable.

Appendix 5.3-C

Septic tank field drain design

Capacity based on Potential Occupancy
Minimum capacity (litres)Number of persons/bed spaces
2700< 4

Chart 1 field drains trench area. Chart 2 field pipe length

Appendix 5.3-D

Cross section of typical underdrain

Cross section of typical underdrain

Appendix 5.3-E

Percolation tests and design method for surface water soakaways


The rate at which water will disperse into the ground depends on the permeability of the ground, which varies with the soil type.

The test will give a fairly accurate assessment of how the ground drains. As the test hole can be used as part of a soakaway, it should be dug in a place that would be suitable for a soakaway, at least 5m from the foundations of a building.

A summary of the test procedure is given below:


A trial hole in a similar location and to the same depth as the proposed soakaway or septic tank land drain will give a measured rate of percolation.

The procedure is as follows:  

Bore a hole 150mm in diameter with an auger to a depth of one metre.

Fill with water to depth of 300mm above the bottom. As an aid, mark a stick 300mm from one end, place in the hole and fill up to the mark on the stick (it takes approximately 5.5 litres to fill a 150mm diameter hole to a depth of 300mm).

Observe the time taken in minutes for the water to soak away (this may take several hours, in some cases need to be left overnight).

If possible the test should be repeated and the average time used.

A second group of tests are carried out after the hole has been bored out to a depth of 2 metres, still using 300mm of water above the bottom of the hole.

If the soil appears to become more permeable with depth, it may be useful to deepen and retest the bore in one metre stages.




The relationship between the diameter or effective depth required for a soakaway to suit a given area of roof and/or paved area, and the average time (t) given by the test outlined on the previous page, is shown on the graph below. The diameter and effective depth below invert level are assumed to be the same dimension (D).

Relationship between the diameter or effective depth for a soakaway and the average percolation test time


Test time (t)= 900 minutes
Plan area to drain= 150m2

Therefore, using the graph, the diameter and effective depth of the soakaway (D) are both 2.8m.


  • If the ground is of low permeability, dig separate soakaways to drain smaller but distinct parts, for example:
    • one side of a roof to one soakaway
    • the other side to a second soakaway
    • the driveway or yard to a third soakaway.
  • Where the permeability of the ground increases with depth, tests in the deepened trial holes will give shorter percolation times, so it may be cheaper to build a smaller soakaway at a greater depth below the surface.
  • Where possible, soakaways should be built on land lower than, or sloping away from, buildings. They should also be kept a "safe" distance away from buildings. This should be at least 5m from building foundations. In chalk and other soil and fill material subject to modification or instability, the advice of a specialist geotechnologist should be sought regarding the siting and advisability of soakaways.