Drainage
Soakaway installation in Southport
Rainwater from your roof or drive, taken somewhere it can soak safely into the ground. We test your ground first with a percolation test, size the soakaway to suit, and dig and build it with our own machines.
Test first, then dig
The most common soakaway mistake is guessing the size. Too small and it fills up and floods the garden in the first heavy storm.
A percolation test tells us how fast your ground takes water. On sandy ground near the coast it can be quick. On clay it can be very slow. The test decides the size, and sometimes whether a soakaway will work at all.
Building an extension?
Building control will want to know where the rainwater from the new roof goes. A percolation test and a properly sized soakaway answers that. We can do it alongside the foundations.
Soakaway size calculator
Work out roughly how big a soakaway you need, using the method building control expects: BRE Digest 365, from the Building Research Establishment.
It sizes a square soakaway for a 10 year storm using North West rainfall. It is an estimate: we confirm the size with a percolation test on site before we dig.
Fill in the form and press Work it out to see an estimated size.
How the sum works
BRE Digest 365 balances water in against water out: storage needed = rain in − water soaking away.
Rain in = area × the 10 year storm rainfall. The rainfall comes from the Digest's Tables 1 and 2, starting from 20 mm in an hour. We work through storms from 5 minutes to 24 hours and every North West rainfall ratio from 0.21 to 0.36, and keep the one that needs the biggest soakaway.
Water soaking away = the sides of the soakaway up to half its depth × how fast your ground drains × the length of the storm. The base is left out, because it silts up over time.
The Digest also says a soakaway must drain from full to half empty within 24 hours, ready for the next storm. We check that too.
The percolation test sum is the Digest's: the water lost between 75% and 25% full ÷ (the pit's sides to half depth plus its base × the time taken).
Types of soakaway
Soakaway crates
Plastic boxes that clip together into a big underground tank, wrapped in a membrane. Hold far more water than stone for the same hole, so the hole can be smaller.
Stone-filled pit
The traditional soakaway: a pit filled with clean broken stone. Simple, but it holds less water and can silt up over the years.
Ring chamber
Concrete rings with holes in them, stacked in a pit with stone round the outside. Has a lid, so it can be checked and emptied of silt.
How we put in a soakaway
- 1
Percolation test
We dig a test hole where the soakaway will go, fill it with water and time how fast it drains away. This tells us if a soakaway will work and how big it needs to be.
- 2
Size and position
We work out the size ourselves from the test result and the roof or drive it serves, at least 5 metres from the house and clear of foundations, boundaries and services.
- 3
Dig
With our own diggers. The spoil goes on our grab lorries.
- 4
Build the soakaway
Crates, stone or rings, wrapped in membrane to keep the soil out, with the pipe from the house laid to a fall.
- 5
Backfill and make good
Covered, topsoiled and turfed, or finished under a drive if the crates are rated for it.
Questions people ask
What is a soakaway?
An underground pit or tank that collects rainwater from your roof or drive and lets it soak slowly into the ground. It takes rainwater only, never toilet or sink waste.
Do I need one?
Often, for a new extension, a new drive, or downpipes that currently pour onto the ground. The Building Regulations want rainwater to go to a soakaway first if the ground allows it, before it is allowed into a drain.
What is a percolation test?
A test of how fast water drains into your ground. We dig a hole, fill it with water and time how long it takes to drain. Fast means a soakaway will work and can be smaller. Slow means it needs to be bigger, or a soakaway is not the answer.
Will a soakaway work in clay?
Sometimes, but clay drains slowly and often fails the test. The options are a larger, shallower soakaway, a better spot in the garden, or, with the water company's permission, a connection to a surface water sewer. The test tells us which.
How do I work out what size soakaway I need?
Use the calculator on this page for an estimate. It follows BRE Digest 365, the method building control expects: the rain from a 10 year storm on your roof and drive, minus what soaks away into the ground during the storm. The answer depends heavily on how fast your ground drains, so we confirm it with a percolation test before we dig.
How many soakaway crates do I need?
It depends on the area draining to it and your ground. As a rough idea, a 60 square metre roof on average ground needs a soakaway of a few cubic metres. Put your own figures into the calculator above for a crate count.
How close to the house can it go?
At least 5 metres from the house or any building, so the water does not soften the ground under the foundations. Further on clay or sloping ground.
Is this the same as a septic tank soakaway?
No. A septic tank needs a drainage field, a set of perforated pipes that spread the treated waste water through the ground. That has its own rules and its own percolation test. See off-grid drainage.
Why is my old soakaway not working?
Usually it has silted up, been crushed, or was too small to start with. Older stone soakaways often clog after decades. The fix is usually a new one, sized properly, in fresh ground.
Also worth a look
Tell us what the soakaway is for
Phone lines open Monday to Friday 8am to 5pm, Saturday 8am to 1pm. WhatsApp a photo of the job any time.