How do I find out where my drains run?

You need to know where the drains are before you build an extension, lay a drive, dig a soakaway, set a fence post or plant a tree. The information is largely there to be read: drainage runs from inlets to chambers to the boundary, in straight lines with a fall, and the visible fittings tell you most of the route. Where certainty matters, it can be traced properly for a modest cost.

Safety

What you'll need

• Manhole lifting keys
• Torch
• Tape measure and a notepad
• Waterproof gloves
• Water company's sewer records request (optional)
• Drainage survey with a camera and sonde (optional)

Step by step

1. Map the inlets first, from above ground.Walk round the building and mark on a sketch every gully, every downpipe, every soil and vent pipe, and every waste that discharges outside. Each is a point where the underground system starts.
2. Find every chamber cover.Look in the obvious places — near the back of the house, at changes of direction, at the boundary — and also under gravel, in borders and under decking and paving, where covers get buried. A metal detector finds cast iron covers under a lawn quickly.
3. Lift each cover and read the chamber.The base has a main channel running through it and shaped benching either side. Note which direction the channel flows, and note every branch pipe entering, its size and the direction it comes from. That tells you what is connected upstream.
4. Join chambers with straight lines.Drains run in straight lines between access points, with a consistent fall. Two chambers give you the line between them, and the branch directions tell you what feeds each one. Most domestic layouts resolve quickly this way.
5. Measure and record everything.For each chamber, take two measurements from fixed points — a house corner, a boundary — so it can be relocated exactly. Note the invert depth, the pipe size and the direction of each branch. Photograph looking down into each one.
6. Request the water company's sewer map.Water companies hold records of public sewers and lateral drains and will provide a map, usually for a small fee or free online. It shows public sewers on and near your property, which is the information that matters for building over or near them.
7. Get a survey where the answer really matters.A drainage contractor with a camera and a sonde can trace a pipe from inside and mark its exact line and depth on the ground. For an extension, a large excavation or a suspected defect, this is a modest cost against the alternative of finding out with a digger.
8. Check for a build-over agreement before you build.Building over or within a set distance of a public sewer requires the water company's agreement, and building control will ask about it. Applying is straightforward; discovering after the fact that you have built over a public sewer is not.
9. Write it all down and keep it with the house papers.A dated sketch with measurements is one of the most useful documents a house can have, and it takes an afternoon to produce.

Tips

• Foul and surface water systems are separate in most modern properties, and combined in many older ones. Note which each chamber is — you can tell from what discharges into it — because the distinction governs what you may lawfully connect.
• A dye tablet or food colouring flushed down a specific toilet or poured into a specific gully, with someone watching the chambers, resolves any ambiguity about which branch is which in a few minutes.
• Look at the neighbours' layouts. Estates were built to a pattern and their chamber positions are usually informative about yours.
• Old properties frequently have abandoned drain runs, disused cesspits and unrecorded connections. Absence from a map is not proof of absence in the ground.
• If you find a chamber you cannot account for, photograph and record it anyway. Unexplained access points are often the key to a later problem.

Common mistakes

• Assuming the sewer map shows everything — Water company records cover public sewers and lateral drains. Private drainage within your boundary is generally not on them, and neither are historic or abandoned runs.
• Digging a foundation or a soakaway before checking — Striking a drain with an excavator is expensive, contaminating and, if it is a public sewer, a serious matter. The checks above cost an afternoon.
• Paving over a chamber cover — It is done constantly and it removes the access the whole system depends on. If a cover must be within a paved area, use a recessed tray cover that can be infilled to match and still lifted.
• Relying on where the drain 'must' run — Drain layouts follow the original build sequence, later extensions and whatever was easiest at the time. Straight lines between chambers are reliable; assumptions about routes between them are not.

Questions people ask