A rain garden is a shallow, planted depression that collects runoff from a roof, driveway or lawn and lets it soak into the ground instead of rushing to the storm drain. It reduces flooding and erosion, filters pollutants and makes a home for pollinators. Size is the key design decision.
How much water?
The runoff from a storm is:
runoff = draining area × rain depth × runoff coefficient
The coefficient is the share of rain that runs off: about 0.9 for roofs and pavement, 0.3 for lawn. Many guides size rain gardens for a 1 in storm, which covers most rain events in a typical year.
A 1,000 sq ft roof in a 1 in rain sheds 1,000 × 1/12 × 0.9 = 75 cu ft, or about 561 gallons.
How big a garden?
The garden holds the water in its ponding depth, usually 4 to 8 in:
garden area = runoff volume ÷ ponding depth
At 6 in deep: 75 ÷ 0.5 = 150 sq ft, or 15% of the roof area. Many extension guides recommend 10 to 30% of the draining area depending on soil and depth; this calculation lands in that range.
Check the drainage
A rain garden should empty within 24 to 48 hours, so plants do not drown and mosquitoes cannot breed (they need several days of standing water).
drain time = ponding depth ÷ infiltration rate
Sandy soils drain about 1 in per hour or more, loam about 0.5 in, clay as little as 0.1 in. At 6 in deep in loam: 12 hours. In heavy clay, make the garden shallower and larger, or replace the top 12 to 18 in with a mix of sand, topsoil and compost.
Test your soil by digging a hole 12 in deep, filling it with water, and measuring how fast it drops.
Placement and planting
- Place it at least 10 ft from the house foundation, downhill, and away from septic systems and the well.
- Avoid spots where water already stands for days.
- Bring roof water to it with a downspout extension, a swale or a buried pipe.
- Plant native, deep-rooted perennials, grasses and shrubs that tolerate both wet spells and drought; the center gets the wettest.
- Mulch with shredded hardwood to hold soil and suppress weeds.
Call 811 before digging. Some municipalities offer rebates or credits on stormwater fees for rain gardens.