Drains and ducts are the only part of a build that gets buried the day it is finished and then judged for the next fifty years. Almost everything that decides whether they work is settled before the trench is filled in.
Two lots of work go under a house and neither ever shows. Drainage takes water away. Foul from inside, rain off the roof and the paving. Utilities bring things towards it: water, power and data. Both go in at the same stage, so what goes wrong is usually sequence rather than skill.
The ground has opinions. Chalk, gravel, clay-with-flints and London Clay turn up within a few streets of each other across Buckinghamshire, so a soakaway that works one side of Beaconsfield is a wasted hole a mile away in Gerrards Cross.
We also take on insurance repair work: a collapsed or root-damaged drain, a failed soakaway, or a run that needs digging out and relaying after a claim. If your insurer or loss adjuster needs a quote, send us the details and we will price it the same way as any other job.
Two systems that must never meet
Foul water is everything from inside the house, and it has to be collected and treated. Surface water is rain, and it belongs in the ground on your own plot, or a watercourse, or a surface-water sewer. The two stay apart the whole way. Rain in a foul sewer swamps the treatment works every storm. Foul in a surface drain puts sewage into a stream. A misconnection is usually found years later, by a camera, during a sale, under a finished drive.
Where the rain goes
A soakaway done properly is a buried chamber, plastic crates in a permeable membrane, that swallows a storm and lets it out into the ground slowly. Not a rubble pit. Whether the ground will take it comes from a percolation test: a pit dug to the depth the soakaway will sit at, soaked first, then refilled and timed as the water drops. On heavier clay the answer is often no, and the design turns to attenuation. Our drainage guide has the test and the sizing.
SuDS, or sustainable drainage systems, means rain should leave your plot no faster than it did when the plot was a field: permeable paving, crates that hold a downpour and let it out at a trickle. In England it runs through planning, so check with your local planning authority.
Where the foul goes
Connecting to a public sewer is paperwork before pipework. The right to connect carries a duty to give notice first: an application under section 106 of the Water Industry Act 1991, after which the undertaker has a window, commonly quoted as 21 days, to approve or refuse. Connecting without consent is an offence. Building over or near a public sewer needs a separate agreement. The trigger is often roughly three metres, though it varies. Find out where the public runs actually sit before the drawings are fixed. A sewer found at foundation stage moves more than the drainage, as our guide to foundations explains.
Off mains, a septic tank settles the solids and holds them for emptying. What leaves it is only part-treated and goes on to a drainage field of perforated pipe in stone-filled trenches. A package treatment plant aerates the sewage and does more of the work itself, so the effluent can in the right circumstances go to a watercourse. The Environment Agency’s general binding rules in England set out when a small system may run without a permit. One line in them is absolute: since January 2020 a septic tank must not discharge to a surface water such as a ditch or stream. Systems that did had to be replaced with a treatment plant or redirected to a drainage field, and the requirement is triggered by selling the property too, so it is worth establishing early. As always, check the current position with the Environment Agency, because these rules do move. They are stricter than they were about septic tanks discharging to surface water.
Falls, bedding and the test before backfill
The pipe is the simple part, and where careless work shows first. A foul drain wants enough fall to carry solids along with the water, and not so much that the water outruns them. For a 100mm pipe the old rule of thumb is between about 1 in 40 and 1 in 80. Your engineer sets the real gradients.
Lay it to the level
Levels come off the drawing and a laser, invert by invert. A fall set by eye looks convincing in an open trench and leaves a belly nobody can see once the ground closes over it.
Bed it and surround it
The pipe sits on and is surrounded by clean granular material, pea shingle usually, supported evenly along its length. Dropped onto whatever came out of the hole, it settles unevenly and joints open. A drive that sinks along one line is reporting the trench beneath it.
Test it while you can
Before backfill the run is plugged, tested with air or water, and the gauge watched. A drop means a leak, and a leak is twenty minutes’ work while the trench is open. Building control will want to see drainage before it is covered.
Inspection chambers are access you hope never to need. As a rule of thumb they go in at junctions, at changes of direction and gradient, and at intervals along straight runs. Approved Document H sets out the spacings. A shallow plastic chamber has a depth limit of about a metre and a bit, past which chamber and cover step up a grade.
Ducts, depths and warning tape
Utilities go in to the providers’ specifications, not to convenience, and the providers disagree with each other. A service trench is coordination as much as digging.
- Colour is the label. Blue for water, black for low-voltage electricity, red for high voltage, yellow for gas, grey or green for data.
- Depth follows the service, not the trench. Street Works UK guidance, once NJUG, puts typical minimum cover at roughly 450mm for a low-voltage cable in a footway and 750mm for water. Data sits shallower. Each provider’s own spec is what gets signed off.
- Warning tape goes above the service, a couple of hundred millimetres up in the backfill, so a bucket finds tape before it finds cable.
- Every empty duct gets a draw cord. Without one it has to be dug up.
Drain runs and ducts want the same ground and cannot share it. Drainage is fixed by gravity, so it claims the levels first and the ducts route round it, which only works if both are on one sheet before anyone digs. Set out against finished levels, too: a duct at the right cover against a level that later drops 200mm is now too shallow.
The cheapest metre of trench you will ever dig is the one that is already open.
Sequence, and what it costs to get wrong
Drainage goes in while the foundation trenches are open and the ground is already broken. Ducts go in before the sub-base, the kerbs and the surface, because at that point a service trench is a scrape across an open site. Afterwards it is a saw cut through a finished drive, which is why a spare duct for a charger or an outbuilding is worth laying now. Our driveway guide covers the build-up on top.
WHAT MOVES THE PRICE
- Depth and distance. How deep the connection sits, how far from the house.
- Whether the ground percolates. A failed test turns a soakaway into attenuation, or a drainage field into a treatment plant.
- Gravity or pumps. A pumped system brings a chamber, a rising main, a standby pump and servicing.
ASK ANY CONTRACTOR
- Where does the foul go, and where does the rain go? Two separate answers.
- Has a percolation test been done, or is the soakaway priced on an assumption?
- Has the sewer connection been applied for, and what is the lead time?
- What are the pipes bedded on, and who tests the runs before backfill?
- How many ducts, what size, and set out against finished or existing levels?
None of this photographs well. It is also the part that decides whether the rest works: a drive that stays flat, a garden that drains, no trench cut across the front for a cable somebody forgot.
Where the water goes is a local question. There is more on the ground in each of these.
Planning something on your own plot?
Request a Quote