Soakaway Size Calculator

Key Takeaways

• A soakaway is sized from the area of roof or paving that drains into it. Outdoor Aggregates' soakaway size calculator turns that area into a pit size, a weight of gravel and an area of membrane.
• A soakaway stores about 26L of water for every 1m² of roof or paving that drains into it, at 20mm of rain with the usual allowances added.
• A gravel filled soakaway is only about 30% empty space, so a 1m³ pit holds roughly 300L of water and the other 700L is stone.
• A 20m² roof or patio needs a gravel filled pit of at least 1.76m³. Rounded up to a size you can mark out, that is a hole 1.25m long, wide and deep.
• Filling a soakaway pit 1.25m each way takes about 3.2 tonnes of 20mm gravel, which is four 850kg bulk bags once 10% wastage is added.
• Soakaway crates are about 95% empty space, so crates hold roughly three times the water of gravel in the same size of hole.
• A soakaway only works in ground that drains. A test hole still holding water the next day means no size of pit will fix it.

Most of the failed soakaways I was called out to had been sized by eye. Somebody dug a hole about the size of a wheelie bin, tipped a bulk bag of stone in, and wondered why the patio still flooded in November. The hole looked big. It held about as much water as a bath. That is the bit people miss. You are not sizing a hole. You are sizing the gaps between the stones.

How Do I Work Out What Size Soakaway I Need?

The size of a soakaway comes from three numbers: the area draining into it, the depth of rain to allow for, and how much empty space the fill has.

1. Measure the area that drains in, as seen from above. A 5m by 4m patio is 20m².
2. Multiply by the rainfall. The calculator starts on 20mm, which is what most sizing tables for houses and gardens work to.
3. Add 20% for heavier rain in years to come, then a 10% safety margin.
4. Divide by the empty space in the fill. Gravel is about 30%. Crates are about 95%.

For that 20m² patio the water to store comes to 528L. Divide by 30% and the pit needs to be at least 1.76m³. The calculator rounds each side up to the nearest 50mm, never down, so the pit is 1.25m each way, which is 1.95m³. If you've already done the calculation and you're ready to build, check my guide on how to build a soakaway.

How Do I Measure a Roof for a Soakaway?

You measure a roof as it looks from above, like a plan drawing. You do not measure up the slope.

1. Find the downpipes that will run into the soakaway. Only the parts of the roof that drain into those downpipes count.
2. Measure the length of the gutter along that side of the roof.
3. Measure across on the ground, from the wall under the gutter to the point under the ridge. For a lean-to, measure to the wall it leans against. Take in the overhang too.
4. Multiply the two. That is the area the roof covers from above.
5. Add extra for the slope, using the table below.

Why add extra? Rain rarely falls straight down. Wind drives it into the face of a sloping roof, so a steep roof catches more water than the ground under it.

The calculator works from a length and a width. So multiply the length by the figure in the table before you type it in. A 45 degree roof 6m long and 5m across goes in as 900cm by 500cm. The figures are the ones building control works to, taken from Horsham District Council's soakaway guidance. If you are not sure of your roof pitch, the house plans or a roofer will tell you. Patios, drives and flat roofs need nothing added. If more than one area drains into the same soakaway, add the areas together.

The table shows what that looks like for common sizes, with a gravel filled pit and the calculator on its starting settings.

How Much Gravel Do I Need to Fill a Soakaway?

A gravel filled soakaway takes about 1,640kg of gravel for every 1m³ of pit, which includes 10% for wastage. The sum is pit volume, times 1,490kg per m³, plus the wastage. That 1,490 figure is from our own bag at Outdoor Aggregates: an 850kg bulk bag fills about 0.57m³. Plenty of calculators work at 1,650kg or more, which has you ordering a bag you do not need on a bigger job.

20mm gravel is the fill I recommend for the body of the pit. Round the pipe I would rather use 10mm gravel, because small rounded stone beds a pipe evenly along its whole length. Whatever goes in, it has to be clean. Soil and dust in the stone silt the pit up from the inside. Wrap every face in a non-woven geotextile too, the top included. A weed control fabric is a different sheet for a different job, and it is the wrong one here. The calculator adds 20% to the membrane for overlaps.

Will a Soakaway Work in Clay Soil?

A soakaway in heavy clay usually fails, because the water has nowhere to go once the pit is full. A bigger pit only delays the flood. So test before you order anything. The test takes a day and costs nothing.

How Do I Test My Ground for a Soakaway?

The test is called a percolation test. It shows how fast water soaks into your ground. The answer is a number called Vp: the seconds the water takes to drop 1mm. You need a spade, a tape measure, water and a watch. To get the best result from your plan:

1. Dig a hole 300mm square where the soakaway will go. Take it 300mm deeper than the pipe that will feed the soakaway. If the hole has to be deep, dig it wider at the top, but keep the bottom 300mm at 300mm square.
2. Fill the bottom 300mm with water and leave it to soak away overnight. This wets the ground, as it would be after a wet spell.
3. Next day, fill it to 300mm deep again.
4. Time how long the water takes to drop from 225mm deep to 75mm deep. That is a drop of 150mm. Marks at 225mm and 75mm on a garden cane make it easy to watch.
5. Divide the time in seconds by 150. That is your Vp.
6. Do the test three times, in at least two holes, and take the average.

This is the method building control uses. You can check it for yourself on Stratford-on-Avon District Council's percolation test page. For example, water that takes 60 minutes to drop the 150mm is 3,600 seconds. Divide by 150 and Vp is 24. That is good ground for a soakaway. Type your Vp into the calculator and it tells you what it means:

• Under 12 is very fast.
• 12 to 60 is good.
• 60 to 100 is marginal.
• Over 100 is not suitable.

Do not test in heavy rain, hard frost or a drought, because the answer will be wrong. If water is still standing in the hole the next morning, a soakaway will not work there. If the hole fills with water on its own, stop there. That is the water table, and a soakaway sitting in it is a buried pond. My guide to what a soakaway is and when it works covers what to do on clay.

How Deep Should a Soakaway Be?

A soakaway's depth is measured from the pipe that feeds it down to the base, because only the stone below that pipe stores water. That catches some people out. The pit size the calculator gives sits under the pipe. The dig is deeper by however far down the pipe arrives, and that pipe wants at least 300mm of cover under a lawn or a patio.

The top of a gravel filled pit has no set depth. Fold the membrane over the stone so soil cannot wash down into it, then backfill to the surface. Over crates I would rather have 500mm of cover, and more under a drive, where the crate maker's own figure is the one to follow.

The base has to stay above the water table, because a soakaway sitting in groundwater has nowhere to empty to. For a garden job I treated about 1m³ of digging as the working minimum. If you have already dug, type your pit size into the calculator. It tells you how much water that pit holds and whether it is enough.

How Far Should a Soakaway Be From Your House?

A rainwater soakaway sits at least 5m from any building and 2.5m from a boundary under Approved Document H of the Building Regulations. "Any building" includes your neighbour's garage and extension. Scotland works to its own building standards, so check locally north of the border. Building control can ask for a formal test and a bigger design storm, which is why the rainfall setting goes up to 40mm.

Next Steps

Run your own area through the calculator, then read how to build a soakaway before the spade goes in. The wrap and the fall on the pipe decide whether it lasts thirty years or three. If the water is crossing the garden instead of sitting in one spot, a French drain feeding the soakaway is the pairing I would use. We stock the 10mm and 20mm gravel, delivered in bulk bags.

If you would rather talk your soakaway gravel through with a person, call Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever built a soak away before? How did it go and what would you do differently next time if you had to do it again? Tell us in the comments.

FAQs

What are the rules for soakaways in the UK?

Rainwater soakaways in England and Wales come under Part H of the Building Regulations: 5m from any building, 2.5m from a boundary, and rainwater only. A garden soakaway rarely needs planning permission.

Can all my gutters feed one soakaway?

One soakaway can take every downpipe on the house, provided it is sized for the whole roof. Add the areas together in the calculator.

Are soakaway crates better than gravel?

Soakaway crates hold about three times the water of gravel in the same hole, so they suit small gardens and drives. Gravel is cheaper and simpler for a lawn or a small patio.

Do you fill soakaway crates with gravel?

Soakaway crates stay empty inside, because the empty space is the storage. Gravel goes under them, round them and over the top, outside the membrane wrap.

Why is my soakaway still flooding?

A soakaway that still floods is nearly always in ground that will not drain, or it has silted up because it was never wrapped. Making it bigger fixes neither - it may be time to call a professional.