The size and depth of the rain garden the infiltration rate of the soil the size of the uphill drainage area the amount of impervious or semi impervious surfaces rooftops driveways sidewalks and mowed turf in the uphill drainage area the slope of the site and the amount intensity and duration of rainfall all influence rain garden effectiveness in managing rainwater runoff.
Roof to rain garden size.
For example 600 square feet of my 1 200 square foot roof.
First identify the size of the roof driveway or other surface that will divert water into the rain garden.
Determine rain garden size for 100 runoff control 1.
Water is absorbed fastest in sandy soils and slowest in clay like soils.
Residential rain gardens are often sized to hold one inch of runoff in above ground storage from the contributing impervious surface.
Most rain gardens are from 100 to 300 square feet.
The kind of soil you have will help determine the size of your rain garden.
In this area of the country the majority around 90 of storms are one inch or less in size.
For this exercise i ve used square feet as a measurement.
Identify only the area of roof that deposits water into the downspout you ve targeted for the rain garden.
If you are placing your rain garden more than 30 feet from a downspout take the yard space into account when you figure the drainage area.
Measure the distance from the house and the width of the drainage area and add it to your roof figure and find the soil type for your garden.
If necessary you may want to amend the soil in your planting area.
Sizing your rain garden.
This gives you the size to build the rain garden in square feet.
Natural looking and kidney bean shapes are popular.