Soakwells and Stormwater in Perth: The Rules and the Digging

Visitors from the eastern states are puzzled by Perth roofs: the downpipes go into the ground and stop. Because most of the city sits on deep, free-draining sand, stormwater is disposed of on site in soakwells rather than piped to a street drain. It is cheaper, it recharges the aquifer, and it means every house, extension and shed comes with a digging job.

What councils require

Every local government has a stormwater policy, and they differ, but the pattern is:

• All roof and paved-area runoff must be retained on the lot, usually for a design storm of a 1-in-10 or 1-in-20 year event.
• Soakwells must be a set distance from boundaries and buildings, commonly 1.8 m from a building footing and 1.5 m from a boundary, so water does not undermine anything.
• A stormwater plan showing soakwell sizes and positions is part of the building permit.

Check your council's policy or ask your builder; the numbers above are typical, not universal.

Sizing

Soakwells are sized by the area draining into them. A common Perth rule of thumb is roughly one cubic metre of soakwell capacity per 60 to 80 square metres of roof, adjusted for the design storm and soil. A 200 square metre roof therefore usually needs two or three 1,200 mm diameter by 1,200 mm deep concrete or polypropylene soakwells. Your council's policy or a drainage plan gives the precise figure.

Ground makes the difference

• Bassendean and Spearwood sand: ideal. Soakwells drain quickly and the excavation is easy, if prone to collapse. See why trenches collapse in Perth sand.
• Sand over limestone: the hole may need to go through caprock to reach free-draining rock. See Tamala limestone.
• Clay and the Hills: soakwells do not work in clay. Councils in the Hills accept alternative arrangements such as swales, larger leach drains or connection to a street drain.
• High water table areas: soakwells sit in water for half the year and do nothing. Councils may require raised or larger systems.

Digging and installing

1. Mark out to the council's setbacks and locate services with Before You Dig Australia.
2. Dig oversize. A 1,200 mm soakwell needs a 1,500 mm hole so you can backfill around it with clean sand or aggregate.
3. Keep the base level and undisturbed. A wide toothless bucket on a 1 to 1.5 tonne machine leaves a flat base.
4. Lower the soakwell with the machine and a sling, never by hand, and never with anyone in the hole.
5. Connect the pipe at the inlet with a fall from the downpipe, then fit the lid.
6. Backfill in layers and compact lightly so paving over the top does not settle.

Machine choice

Soakwell work is where the compact excavators shine: an R10 or R15 fits down the side path, digs to 1.8 m and lifts a polypropylene soakwell easily. Plan the job for autumn or spring on low-lying blocks; see the winter water table.