Outside Drain Overflowing

What's happening?

An outside drain that has filled up is telling you the blockage is downstream of it. Drainage runs by gravity through a chain of chambers, and each one is a window into the pipe below: a chamber standing full means the run leaving it is blocked, while an empty chamber means the water is not getting that far. Working from the house outwards and lifting each cover in turn is how a drainage engineer finds the fault in minutes, and it is also how you can tell whether the problem is yours or the water company's. The last chamber before the boundary is usually the dividing line. Full at the boundary chamber and beyond means the shared lateral or the public sewer, which is your sewerage company's to clear.

What usually causes it?

• Fat, oil and grease congealed in the run and bound together with wipes
• Wipes and sanitary items forming a mass at a bend, junction or change of gradient
• Tree roots that have entered through a joint or a crack and grown into a mesh
• A collapsed or displaced section of clay pipe, common in older properties
• Silt, grit and leaves washed in through an uncovered or damaged gully
• Building rubble, mortar or paint flushed into the drain during works
• A blockage in the shared lateral drain or the public sewer beyond the boundary

When should you call a drainage engineer?

Call a drainage engineer when the blockage is within your boundary and rodding has not cleared it, or when you would rather not handle sewage at all. Call your sewerage company first, free, if the boundary chamber is full, if the smell and the overflow appeared at the same time as a neighbour's, or if sewage is escaping onto a road or a shared area. Treat it as urgent in either case once sewage is on the surface.

What will the drainage engineer actually do?

• Lift and read every chamber on the run to locate the blockage precisely
• Clear it by rodding, mechanical machine or high-pressure jetting according to the material and pipe condition
• Run a CCTV survey to identify roots, displaced joints, fractures or a collapse behind the blockage
• Produce a survey report and a plan of the run where the pipe needs repair
• Confirm the boundary and advise which lengths are private and which are the sewerage company's
• Clean down the affected surfaces and prove free flow from every fitting in the house

Reading the chambers

1. Start at the chamber nearest the house and work outwards.
2. Lift each cover and note whether it is empty, part full, or standing full.
3. The last empty chamber and the first full one have the blockage in the pipe between them.
4. If every chamber including the boundary one is full, the fault is downstream — ring the sewerage company.
5. Stop using water in the house until it is cleared.