Rain garden and dry well calculator
How big should a rain garden or dry well be for your roof or driveway? Size it from the drainage area, rainfall and how fast your soil drains.
A rain garden is a shallow planted dip that catches runoff from a roof or driveway and lets it soak in. A dry well does the same job underground, in a pit filled with gravel. Both depend on how fast your soil drains, so do a drainage test first.
Building a rain garden
- Location: a low spot at least 10 ft from the house, where water already heads, but not where it stands for days.
- Shape: longer than it is wide, across the slope, with a level bottom so water spreads evenly.
- Inlet and overflow: line the inlet with stone so it doesn’t erode, and give the overflow a safe path for storms bigger than the design.
- Plants: wet-tolerant species in the bottom, drought-tolerant ones on the rim. Native plants with deep roots keep the soil open.
- Soil: if your soil drains poorly, mix compost into the bottom, but don’t create a “bathtub” of loose soil in dense clay. An underdrain is the fix for very slow soils.