Key Indicators:

The first step when planning to fix additional drainage points to an existing system is to identify the type of system: is it a dual system with separate foul and surface water pipelines, or is it a combined system, where foul and surface water are mixed together and carried in the same pipes?

There are a few basic pointers that can be used to aid identification.

1 - Age of property

Most modern properties (1970 or later) tend to have DUAL systems, but this must be verified.

adjacent manholes
Adjacent manholes or access cover on the road or on a driveway usually suggests a DUAL system

2 - Number and layout of manhole covers

DUAL systems tend to have more manhole covers (often a source of irritation to householders) and there are sometimes two covers side-by-side.

3 - Presence of sewage in chamber

If there is obvious sewage in a chamber when the cover is lifted, then it must be a FOUL or a COMBINED system. Toilet tissue is often visible stuck to the benching or the channels within inspection chambers and manholes.

4 - Depth to invert

In DUAL systems, the foul should always be deeper than the surface water. This is a safety precaution in case of a breakage to the foul system, which, if it were to be the shallower of the two, could percolate down and contaminate the surface system.

Checking the system:

While the pointers listed above can aid in preliminary identification of a sewer system, the actual classification of a drain must be verified as outlined below before installing any additional drainage.....

1 - Lift cover and check layout of drainage within chamber

2 - try to identify fittings that are likely to be connected to the chamber under inspection. These must be upstream of the chamber, and may be rain water pick-ups, kitchen waste gullies, toilet stacks, linear drains etc.

3 - put a garden hose into the fittings and see if the water appears in the chamber. For indoor fittings, such as baths, wash-basins or toilets, try turning on a tap or flushing the cistern. If in doubt, a drain-tracing dye (available for a couple of quid at most Builders' Merchants) can be added, turning the water a distinctive colour that will be readily identified if it passes through the chamber under inspection.

  • In a COMBINED system, the test water is observed in the chamber from both foul and surface sources.
  • In a SURFACE system, the test water will only be observed when added to a surface drainage point
  • In a FOUL system, the test water will be observed when the toilet is flushed
  • If in doubt, use trapped fittings.

Surface Sources

  • - Rainwater pickups (Downspouts)
  • - Yard or Road Gullies
  • - Linear drains

Foul Sources

  • - Kitchen waste
  • - Toilet/Soil and Vent Pipe
  • - Bath/Shower/Bidet
  • - Any industrial process


Some properties may have the surface water drained to a soakaway , the older of which are notorious for silting-up and causing water-logging problems. If a large area of paving (50m² or more) is to be drained into a soakaway, its efficacy should be tested first by putting a garden hose on medium flow into one of the feeder gullies for 2-4 hours. If the soakaway cannot cope with the inflow, it may need to be rebuilt, or a new soakaway may need to be constructed at a distance of at least 20m from the existing one.

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