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 round here 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 be collected and treated: for most houses that means a public sewer and, eventually, a treatment works, but plenty of plots round here have no sewer to reach and treat their own on site, which is a section of its own further down. 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 the ground round here 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 Buckinghamshire 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, through 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, whether brick ends, clay lumps or 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, which means an application, a fee and 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 2011, when a great many private sewers and lateral drains transferred to the water companies, and it still surprises people. Check with your water company before paying to repair a pipe that may not be yours.

When there is no sewer to connect to

Not every house has a sewer to reach. Down a long drive, out at the edge of a village, on a plot that sits well below the lane. Off-mains is ordinary round here, not exotic, and it changes the whole shape of a drainage plan. Instead of a connection and a bill, you have a treatment system on your own land, and it is yours to run for as long as you own the house.

Two arrangements cover most plots. A septic tank settles the solids and holds them. What leaves it is only part-treated, so it has to go on to a drainage field, perforated pipe in stone-filled trenches, where the ground finishes the job. A package treatment plant does more of the work itself, aerating the sewage and keeping a bacterial population alive to break it down, so what comes out is clean enough that in the right circumstances it can go to a watercourse.

In England, the Environment Agency’s general binding rules set out when a small domestic system can run without an environmental permit. In outline, as the rules stand: a discharge to the ground is capped at two cubic metres a day and one to surface water at five; since the start of 2020 a septic tank must not discharge straight into a ditch, stream or river, so that arrangement needs upgrading to a treatment plant or redirecting to a drainage field; and if there is a public foul sewer within 30 metres of your boundary you are generally expected to connect to it rather than treat on site. Sensitive ground, such as a groundwater source protection zone or a protected site nearby, can take you out of the rules and into a permit application. Wales and Scotland run their own versions. Treat that as the shape of it, not the last word, and get the current position confirmed by the Environment Agency or your installer before you buy a system.

A drainage field asks the same question as a soakaway. Will this ground take water? It is answered by a similar test, though to its own standard and its own sizing. Clay that fails a soakaway test will usually disappoint a drainage field too, which is one reason an off-mains plot on heavy ground often ends up with a treatment plant and an outfall rather than a tank and a field. Either way the test comes first: the system follows the ground, not the brochure.

When gravity will not do it

Drainage wants to run downhill and the best system is always the one that does. Sometimes it can’t. A basement WC sits below the drain it has to reach; a plot falls away from the lane the sewer runs under; a new annexe finishes lower than everything it needs to join. Then the water has to be lifted, and a pump joins the list of things in the house that can fail.

For foul water that usually means a packaged pumping station. A sealed chamber, a pump or a pair of them, level controls, and a small-bore rising main carrying the flow up to a chamber where gravity takes over again. Two things are worth asking about. First, what happens when a pump fails: many installations run a duty and standby pair so one can drop out without the house stopping. Second, how much storage sits in the chamber. Guidance in Approved Document H points at a receiving chamber able to hold roughly a day’s flow, reckoned for domestic purposes at about 150 litres per person per day, so a failure or a power cut buys you hours rather than minutes. Your drainage engineer will confirm the sizing and the standards the unit itself has to meet.

A basement waterproofing sump is a different animal and the two should never be muddled. That pump deals with groundwater gathered in the drained cavity behind the walls and under the floor. Clean water in principle, and a lot of it after a wet week. It wants its own chamber, a second pump beside the first, battery backup for power cuts, a high-water alarm somebody will actually hear, and a service interval written down rather than remembered. Where that water is allowed to go matters too: as a rule water companies will not accept groundwater into a foul sewer, so the discharge route wants agreeing early instead of being assumed on site.

Pumps carry an ownership question as well. A station serving your house alone, inside your own boundary, is normally yours to maintain and eventually to replace. Shared arrangements may sit with the water company. Ask before you budget, and keep the maintenance paperwork with the house. A pumped system nobody has serviced since the day it was commissioned is an expensive way to find out how a basement floods.