Build a rain garden by digging a flat-bottomed depression 15-30 cm deep and 3-5 square metres in area, filling it with fast-draining soil, planting water-tolerant species and routing a roof downspout or paved-area runoff into it. Size it by dividing the runoff volume by the ponding depth: a 40 square metre roof in a 25 mm storm sheds about 1 cubic metre, and 1 / 0.20 gives 5 square metres at a 20 cm depth. It must empty within a day or two.
Part of Flood Control and Drainage
A rain garden is a planted basin that collects runoff from a roof or paved area and lets it soak into the ground, handling moderate rainfall entirely by absorption.
Size The Garden To The Roof, Not To The Space You Have
A rain garden that is too small overflows in every storm and teaches nobody anything. The area depends on how much water it must hold and how deep it is allowed to pond. Start from the roof.
What You Need
- A site where the soil drains — see Soil Drainage Capacity.
- Fast-draining soil — to fill the bowl. Sandy loam works better than clay.
- Stones — for the inlet splash apron and the overflow.
- Water-tolerant plants — species that survive both wet feet and dry spells.
- Mulch. See Mulching.
Method: Build The Rain Garden
Step 1: Choose the site. Pick a spot downslope of the roof or paved area, away from the foundation so the water soaks in beside the house, not against it.
Step 2: Work out the size. Runoff volume = roof area x storm depth x runoff coefficient. Area = volume / ponding depth.
Step 3: Dig the bowl. Dig 15-30 cm deep with a flat bottom. Keep the bottom level so water spreads evenly instead of running to one corner.
Step 4: Loosen, do not compact, the bottom. Fork up the subsoil. Do not trample it or drive carts across it, since compaction cuts the soaking rate.
Step 5: Fill with fast-draining soil. Replace the dug soil with a fast-draining mix, so the garden empties soon after the storm.
Step 6: Build the berm and the overflow. Pile spoil along the low side and cut a lower, stone-lined notch where overflow may leave safely.
Step 7: Protect the inlet. Lay flat stones where the downspout or channel enters so the water does not dig a hole in the soil.
Step 8: Plant and mulch. Plant water-tolerant species and cover the soil with mulch to keep it open.
Worked Example: Size And Drain-Down
Size. A roof of 40 m2, a 25 mm storm, a runoff coefficient of 1 (all the rain runs off the roof) and a 20 cm ponding depth.
- Volume: 40 m2 x 0.025 m = 1.0 m3.
- Area: 1.0 m3 / 0.20 m = 5 m2.
That matches the parent article’s 3-5 square metres.
Drain-down. Standing water 20 cm deep should be gone in a day or two; the working figure here is 24 hours.
- Required absorption: 200 mm / 24 h = 8.3 mm/h.
- In 48 hours: 200 / 48 = 4.2 mm/h.
A 30 cm hole that takes 4 hours to empty is absorbing about 75 mm/h, nine times what this garden needs (75 / 8.3), so soil rated slow by the hole test can still suit a rain garden. Only a hole that takes more than about 36 hours (300 / 8.3) fails the requirement. The hole test overstates the rate a little because water also leaves through the sides, so leave margin, and heavy clay that holds water for days does not qualify.
Common Mistakes
- Sizing to the available space — the garden overflows every time.
- Compacting the bottom — the soaking rate falls and water stands.
- Digging too close to a wall — the water ends up at the foundation.
- No inlet protection — the downspout digs a pit in the soil.
- No overflow — a storm larger than the design cuts through the low side.
What’s Next
- Learn next: Swale Design — the long, thin version
- Combine with: Cisterns and Rainwater Storage — store roof water first, soak the overflow