Drainage

Underground drainage is the part of the build that disappears under gravel and backfill and stays there for the lifetime of the house. Get it right and nobody will think about it again; get it wrong and you are digging up the patio in a few years to chase a smell or fix a flood. Building control inspects the new drainage before the trench is backfilled and expects it to have been tested. A fault that takes half a day to fix with the trench open can take far longer once the slab is down.

These guides cover the products used to build the buried drainage on an extension: 110mm pipe and push-fit fittings to connect into the existing run, plastic inspection chambers for access at junctions and changes of direction, bottle gullies under rainwater downpipes, soakaway crates and geotextile membrane to return roof water to the ground, and channel drains to catch surface water at flush thresholds next to bifold or sliding doors.

Where to start

Read underground drainage pipe and fittings first. It sets out the Part H rules on pipe size, gradient, push-fit joints and testing before backfill that the inspector will check. Then read plastic inspection chambers, and if your rainwater is going to a soakaway, soakaway crates, because the ground needs a percolation test before any crates are ordered.

The guides, grouped by job

Pipework and access. Pipe and fittings, inspection chambers, and the bottle gully that takes a downpipe into the ground. The pea gravel that beds and surrounds the pipe is covered with the aggregates.

Rainwater into the ground. Soakaway crates, which Part H expects you to consider before a watercourse or sewer, wrapped in geotextile membrane to keep silt out of the void.

Thresholds. Channel drains along a level threshold, so rain on the patio does not run into the kitchen.

Where these fit in the build

Find existing runs first with locating underground services, and check whether you need a build over agreement with the water company. The new runs go in at drainage and are signed off at the drainage and oversite inspection. Threshold drainage is planned with the bifold and sliding doors, and gullies and channels are often finished during garden and external works.