Drainage, Soakaways and Why Water Decides Your Build

Where the rain goes is one of the first questions a build has to answer — and the ground under South Buckinghamshire doesn’t always give the answer you’d like.

Every house sheds two kinds of water, and the rules never let them meet. Foul water is everything from inside — toilets, basins, showers, the washing machine. It has to reach a sewer and, eventually, a treatment works. Surface water is rain: off the roof, the drive, the patio. It should soak back into the ground on your own plot if the ground will take it, or run to a watercourse or a separate surface water sewer if it won’t.

Mix the two and something goes wrong at one end or the other. Rainwater in the foul sewer swamps the treatment works every storm; foul water in a surface drain puts sewage into a stream. Putting a misconnection right after the landscaping is finished means digging it all up again — so the first question of any drainage plan is simple: where does the foul go, where does the rain go, and are they separate the whole way?

Soakaways, and the test that decides them

A soakaway is not a hole full of rubble — or shouldn’t be. Done properly it is a buried chamber, these days usually plastic crates wrapped in a permeable membrane, sized to swallow a storm and release it into the surrounding ground over a day or so. Whether one will work depends entirely on what your ground is made of, which is why nobody should design or price one without a percolation test.

On the clays under much of South Buckinghamshire the honest answer is often that it won’t work — water sits in a clay pit like tea in a cup. On the chalk it can be a different story, and the two can sit within a few hundred metres of each other. There is more in our guide to South Bucks ground.

SuDS, in plain English

SuDS stands for sustainable drainage systems, and the idea under the acronym fits on a beer mat: rain should leave your plot no faster than it did when the plot was a field. Slow it down, hold it, let it soak where it can.

In practice that means permeable paving — blocks or gravel that let rain fall through into a stone bed beneath — and attenuation crates, buried tanks that hold a storm and let it out at a trickle. Most authorities apply a pecking order: soak it away on your own land first; a watercourse second; a surface water sewer only as a last resort, and usually only with the water company’s agreement.

One rule catches people out at the front of the house. As a rule of thumb, laying more than about five square metres of impermeable surface in a front garden needs planning permission, unless the water drains within the plot — permeable paving, say, or a fall to a border. The detail varies, so check with your local planning authority before the drive goes down.

Why drains fail

Underground drainage is gravity work. A foul pipe needs enough fall for the water to carry solids with it, and not so much that the water outruns them — the old rule of thumb for a 100mm pipe is around 1 in 40 to 1 in 80, though a drainage engineer designs to the case, not the proverb. Get the falls wrong and the pipe silts up slowly, invisibly, until one day it doesn’t drain at all.

Bedding matters just as much. A drain should be laid on and surrounded by pea shingle, supported evenly along its whole length. Laid on rough backfill — brick ends, clay lumps, whatever came out of the trench — it settles unevenly, bellies form and joints open. A drive that sinks along one straight line usually has a trench under it that was backfilled with the wrong material. The surface is just reporting what happened below.

Water never argues. It reads the pipe as laid, not as drawn, and it finds every low spot you leave it.

The other regulars are roots — willows and poplars especially — finding the weep at a joint and filling the pipe over a few seasons, and debris from the build itself: mortar and plaster rinsed into gullies while the trades wash their buckets. A CCTV survey before backfilling, and again at completion, is cheap insurance against the lot.

Connecting, and who owns what

A new connection to the public sewer is not something you simply dig down and make. It needs the water company’s approval — an application, a fee, an inspection — and your contractor or engineer will normally handle the paperwork. Building close to a public sewer, within about three metres as a rule of thumb, usually needs a formal build-over agreement as well, so it is worth finding out early where the public runs actually sit. They are not always where the old plans say.

Ownership follows a general pattern in England: the drain serving only your house, on your own land, is yours to maintain; once it joins a shared run or crosses your boundary it usually belongs to the water company. That changed in the early 2010s and still surprises people — check with your water company before paying to repair a pipe that may not be yours.