Drainage Preparation in Surrey

Most jobs have two kinds of drainage. Foul water comes from toilets, baths, showers, sinks, dishwashers and washing machines. Surface water is rain from roofs, drives and paved areas. They run in separate pipes, and the drawing shows where each one goes.

Approved Document H, the government guidance behind the drainage part of the Building Regulations, puts surface water into a soakaway or other infiltration system wherever that is practicable. A watercourse comes next, and that can need consent from the Environment Agency. A sewer is the last choice.

Falls matter as much as the route. The same guidance sets a minimum fall of 1:80 for a 100mm foul drain serving at least one WC, and a minimum of 1:100 for 75mm and 100mm rainwater drains. We set out each run from the levels on the drawing and check the fall along the trench as we dig, so nothing ends up flat or back falling.

A drain is only as good as the bed it lies on. We trim the trench bottom to line and level, lay the pipe on granular bedding, surround it as the design requires and backfill in layers. Plastic pipes flex under load, so the bedding and backfill are what hold their shape.

Where the pipe cannot have the cover the guidance recommends, for example under a shallow drive, it is protected with a reinforced concrete slab over a layer of granular material. A drain passing under a building needs at least 100mm of granular or other flexible fill all round it.

Trenches near a house need care. A drain trench should not go deeper than a nearby foundation unless it is filled with concrete to the right level, and pipes passing through a wall or footing are built in with short rocker pipes or through an opening with clearance all round. After laying, new drains are tested for water tightness with an air test or a water test.

Every drain has to be clearable. Approved Document H asks for an access point at or near the head of each run, at a bend, at a change of gradient, at a change of pipe size and at most junctions. That access can be a rodding eye, an access fitting, an inspection chamber or, on deep runs, a manhole.

We set the chambers as the pipework goes in, bedded and aligned with the runs. When a new drive or patio raises or lowers the finished ground, existing covers are reset to the new level at the same time, so they sit flush with the surface rather than becoming a trip or a puddle.

• Rodding eyes at the head of short runs
• Inspection chambers at bends and junctions
• Existing covers raised or lowered to new levels
• Gullies and channels for drives and yards

A soakaway lets rainwater back into the ground, but only where the ground will take it. Approved Document H says an infiltration device should not be built within 5m of a building or road, in unstable land, or where the water table reaches its base at any time of year.

For roofs and paving over 25 square metres, soakaways are sized to BRE Digest 365. The test behind that design is a trial pit dug where the soakaway will go, filled with water and timed as it drains from three quarters full to a quarter full, three times over, with the slowest result used for the sizing.

We can dig the trial pit and fill it for the test, then build the soakaway the designer specifies, whether a rubble filled pit, a pit lined with perforated ring units or another design. Heavy clay can show that a soakaway will not work at all, and it is far better to learn that before the extension is built than after.

New runs usually join an existing drain or a public sewer. Thames Water, for example, asks for its permission when you connect directly to a public sewer, inspects that connection, and expects the work finished and the connection point open on the day it visits. Connecting through a private drain also needs the drain owner's consent.

Sewers often run under back gardens, which matters for extensions. Thames Water and Southern Water both ask for a build over application before you build within 3 metres of a public sewer, and Thames Water also within 1 metre of a public lateral drain. Planning permission does not replace that agreement, and it should be in place before work starts.

Before any of that is fixed, we find out what is really in the ground. Buried cables and pipes are located before we dig, following the HSE guidance HSG47, and we open trial holes at the connection point so the depth and direction of the existing run are known and the new falls will work.

Some gardens stay wet all winter, particularly on clay. A land drain is a perforated pipe laid in a gravel filled trench, set to a gentle fall across the wettest ground and taken to somewhere the water can go, such as a soakaway or a ditch. Discharging into a watercourse can need Environment Agency consent.

Land drains are best laid before a new lawn, patio or path, while the digger is already there and the levels are being set. We plan the runs around trees, services and the finished design, so the drain does its job without being dug up again later.