How many crates do I need? Crates for your soakaway's size
Size of the soakaway, in metres
Not sure of the size? It's usually on your drawings or building control plans.
Want the whole job designed?
We'll design it in full for free, with the membrane, tape, catchpits and flow controls, and send you a priced schedule. Send your drawings if you have them, or just the size and where it's going.
A guide only, not a design or a guarantee. It counts crates for the size you enter, but it can't tell you that size suits your ground.
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Hydrocell 62 Tonne Soakaway Crate - 1m x 0.5m x 0.4m (190 litres/0.2m3 Per Crate)
Code HydrocellCrate
Regular price £35.95 ex VAT £43.14 inc VATSale price £35.95 ex VAT £43.14 inc VAT -
Best seller
4.5m x 10m Non-Woven Geotextile Handy Pack
Code GEOHP-DL
Regular price £79.95 ex VAT £95.94 inc VATSale price £79.95 ex VAT £95.94 inc VAT -
75mm x 30m Single Sided Lap Tape for DPM
Code DPMTSS7533
Regular price £6.49 ex VAT £7.79 inc VATSale price £6.49 ex VAT £7.79 inc VAT -
StormMaster Heavy Duty Soakaway Crate 1m x 1m x 0.4m 40 tonne (0.4m3)
Code StormMasterCrate
Regular price £66.00 ex VAT £79.20 inc VATSale price £66.00 ex VAT £79.20 inc VAT -
Top hat 110mm Sewer EN1401
Code TH110F
Regular price £11.55 ex VAT £13.86 inc VATSale price £11.55 ex VAT £13.86 inc VAT -
Single Rainwater Diffuser Unit
Code DIFFSINGLE
Regular price £125.00 ex VAT £150.00 inc VATSale price £125.00 ex VAT £150.00 inc VAT -
DPM Damp Proof Membrane 500mu 2000 Gauge (4m x 12.5m)
Code DPM2000G
Regular price £49.99 ex VAT £59.99 inc VATSale price £49.99 ex VAT £59.99 inc VAT -
AQUAVOID Home Soakaway Crate 20 Tonne - 1m (L) x 0.5m (W) x 0.4m (H)
Code Aquavoid
Regular price £37.25 ex VAT £44.70 inc VATSale price £37.25 ex VAT £44.70 inc VAT -
DPM Damp Proof Membrane 300mu 1200 Gauge (4m x 25m)
Code DPM1200G
Regular price £45.00 ex VAT £54.00 inc VATSale price £45.00 ex VAT £54.00 inc VAT -
10x AQUAVOID Attenuation Bundle 1m (L) x 0.5m (W) x 0.4m (H)
Code SOAKBUN
Regular price £294.88 ex VAT £353.85 inc VATSale price £294.88 ex VAT £353.85 inc VAT -
Top Hat 160mm Sewer EN1401
Code TH160F
Regular price £33.33 ex VAT £39.99 inc VATSale price £33.33 ex VAT £39.99 inc VAT -
Top Hat 150mm Twin Wall
Code TH150TW
Regular price £38.00 ex VAT £45.60 inc VATSale price £38.00 ex VAT £45.60 inc VAT -
Top Hat 225mm Twin Wall
Code TH225TWW
Regular price £65.00 ex VAT £78.00 inc VATSale price £65.00 ex VAT £78.00 inc VAT
How to choose soakaway crates
1. Soakaway or attenuation?
Soakaway: the water soaks into the ground. Wrap the crates in non-woven geotextile, which lets water out but keeps soil and silt out of the crates. You need free-draining ground with the water table below the base. Test first: an infiltration test to BRE Digest 365 shows whether the ground drains and how big the soakaway must be. Keep soakaways at least 5m from buildings and roads.
Attenuation: the water is stored and let out slowly to a sewer or watercourse. Seal the crates in an impermeable membrane to the designer's spec and fit a flow control on the outlet. Use attenuation where the ground won't drain, or where the approval limits the discharge rate.
2. Match the crate to the load
| Crate | Size | Rating | Use under |
|---|---|---|---|
| AquaVoid Home | 1m x 0.5m x 0.4m | 20 tonne | Gardens, landscaping and patios. No vehicles. |
| StormMaster | 1m x 1m x 0.4m | 40 tonne | Driveways, car parks and other trafficked areas |
| Hydrocell | 1m x 0.5m x 0.4m | 62 tonne | Driveways, car parks and other trafficked areas |
Ratings only hold when the crates are bedded, backfilled and covered as the maker's data sheet says.
3. Work out how many
Your drainage design gives the storage volume. Each 1m x 0.5m x 0.4m crate is 0.2 m³ and each 1m x 1m x 0.4m crate is 0.4 m³, before allowing for the void ratio on the data sheet. So 2 m³ of storage needs about 10 small crates or 5 large ones.
4. Wrap, seal and keep silt out
- Geotextile: the 4.5m x 10m non-woven handy pack wraps a small soakaway.
- DPM: 300 and 500 micron sheet as a protective base or side layer, with 75mm lap tape for the joints.
- Top hats: seal pipes where they pass through the membrane. 150mm and 225mm suit twinwall; 110mm and 160mm suit EN 1401 sewer pipe.
- Silt: fit a catchpit or silt trap before the crates. Silt is what blocks a soakaway.
The AquaVoid bundle includes 10 crates, a geotextile handy pack and a DPM sheet. For a whole site, send your drawing for a free drawing take-off and we'll send back a priced list of crates, wrap and fittings.
Testing the ground before you dig
Test how fast the ground drains before you design a soakaway. For rainwater, Building Control usually asks for an infiltration test to BRE Digest 365, which many people call a percolation test. Ask them which test they want before you start, and avoid testing in heavy rain, frost, snow or drought, which all give false readings.
Infiltration test for a rainwater soakaway (BRE Digest 365)
- Dig a trial pit where the soakaway will go, to the depth it will be built, with straight sides.
- Fill it with water to about the depth the soakaway will hold.
- Time how long the water takes to fall from three-quarters full to a quarter full.
- Do this three times, on the same day or on days that follow. Use the slowest result.
The result gives the soil infiltration rate, which your drainage designer uses to size the soakaway. If the pit hardly drains, the ground won't take a soakaway: build an attenuation tank instead.
Percolation test for a septic tank drainage field
Not for soakaway crates. This test and the sizing below are for the outflow from a septic tank or sewage treatment plant. That is foul water: it must go to a drainage field of perforated pipe in gravel trenches, never into soakaway crates. Crates are for rainwater and surface water only. See septic tanks and sewage treatment plants.
A percolation test times how fast water soaks into the soil at drainage field depth. Use the method in Building Regulations Approved Document H for England; Wales, Scotland and Northern Ireland have their own rules.
- Dig holes 300mm square to 300mm below the planned pipe depth, along the line of the field.
- Fill each with at least 300mm of water and let it soak away, to wet the soil.
- Refill to at least 300mm and time, in seconds, how long the water takes to fall from three-quarters full to a quarter full.
- Divide that time by 150 to get the seconds per 1mm drop.
- Repeat at least three times per hole and average the results to get the Vp. If results vary widely, test more holes.
Sizing a drainage field
The Vp sets the trench floor area: the slower the soil, the bigger the field. Multiply the most people the tank is designed to serve by the Vp, then by the factor below.
| Outflow from | Trench floor area in m² |
|---|---|
| Septic tank | people × Vp × 0.25 |
| Sewage treatment plant | people × Vp × 0.20 |
For example, a 6 person tank with a Vp of 30 needs 6 × 30 × 0.25 = 45 m² of trench floor. To size a whole site, send your drawing for a free drawing take-off.
Questions about soakaway crates
What is the difference between a soakaway and an attenuation tank?
A soakaway lets rainwater soak into the surrounding ground, so the crates are wrapped in geotextile that lets water through. An attenuation tank stores the water and releases it slowly to a sewer or watercourse through a flow control, so the crates are sealed in an impermeable membrane. The same crates can do either job; the wrap and the outlet make the difference.
Can soakaway crates go under a driveway?
Yes, if the crate is rated for it. The 40 tonne StormMaster and 62 tonne Hydrocell crates are for driveways, car parks and other trafficked areas. The 20 tonne AquaVoid Home crate is for gardens and landscaping with no vehicles. Whichever you use, follow the maker's data sheet for bedding, backfill and minimum cover, or the rating won't hold.
How many soakaway crates do I need?
Divide the storage volume from your drainage design by the volume of one crate. A 1m x 0.5m x 0.4m crate is 0.2 m³ and a 1m x 1m x 0.4m crate is 0.4 m³. So 2 m³ of storage needs about 10 small or 5 large crates. Check the void ratio on the data sheet, as usable storage is slightly less than the crate size.
Do soakaway crates need wrapping in geotextile?
Yes. For a soakaway, wrap the whole crate structure in non-woven geotextile. It lets water pass into the ground but stops soil and silt getting into the crates. Our 4.5m x 10m Don & Low handy pack suits small soakaways. For an attenuation tank, geotextile is normally used to protect the impermeable membrane, not instead of it.
How far from a house should a soakaway be?
Approved Document H of the Building Regulations says a soakaway should not be built within 5m of a building or road, or in unstable ground. Building control or your drainage designer may ask for more, for example on chalk or where the ground slopes towards the house. Check the distance with them before you dig.
Do soakaway crates need maintenance?
Very little, provided silt is kept out. Fit a catchpit or silt trap before the crates and clear it out regularly. Silt, grit and leaves are what block a soakaway over time, and buried crates are hard to clean, so fit the silt trap from the start.
Will a soakaway work in clay soil?
Often not. Clay drains slowly, so a soakaway can fill up and stay full. Run an infiltration test to BRE Digest 365 before you design it. If the ground won't take the water, build an attenuation tank instead and release the water slowly to a sewer or watercourse, with the relevant approval.
How do I do a percolation test for a soakaway?
Dig a trial pit where the soakaway will go, to its full depth. Fill it with water, then time how long it takes to fall from three-quarters full to a quarter full. Do this three times and use the slowest result. This is the infiltration test in BRE Digest 365, which Building Control usually asks for on a rainwater soakaway. The Vp percolation test in Approved Document H is for septic tank drainage fields, not for soakaway crates.
Can soakaway crates take the outflow from a septic tank?
No. The outflow from a septic tank or treatment plant is foul water. It must go to a drainage field of perforated pipe laid in gravel trenches, built to BS 6297, so the soil can finish treating it. Soakaway crates are for rainwater and surface water only.
Pricing a whole job? Send your drawings for a free take-off, ask for a trade quote or call 0121 351 3230.