How deep should the rain garden be?

The slope of your yard will play a large role in determining the depth of your rain garden. Most rain gardens are between four and eight inches deep. Any shallower and it will need to be very large or it may not be able to provide enough capacity to handle most rainfalls. Any deeper than eight inches and it might take too long to drain and result in standing water.

Why does the slope of your yard matter?

No matter the depth of your rain garden, it is important that the bottom of the garden be level. Installing a very shallow garden on a steeply sloped yard will require lots of extra soil. On the other hand, a deep garden on a shallow slope will result in much more digging.

Determining your yard's slope.

  1. Pound a stake in at the uphill end of your rain garden site and another at the downhill end. The stakes should be about 12 to 15 feet apart. Make sure the downhill stake is tall enough so that a level string can be run between the stakes.
  2. Tie a string at the bottom of the uphill stake and extend it to the downhill stake.
  3. Using a string level (inexpensive and available at most hardware stores) or a carpenter's level, level the string and tie it to the downhill stake. If using a carpenter's level, use the longest one you have—short ones are more difficult to use over long distances.
  4. Measure the width (in inches) between the two stakes.
  5. Now measure the height (in inches) on the downhill stake between the ground and string.
  6. Divide the height by the width and multiply the result by 100 to find the lawn's percent slope. If the slope is more than 12%, you may need to find another location or consult with a professional landscaper or engineer.

Example: If in the above illustration, the stakes are 15 feet (180 inches) apart and the string is 11 inches above the ground on the downhill stake, the slope of the yard is 11/180 times 100, which is about 6%. From the guidelines below, we see that our garden will be 6 inches in depth.

Garden depth guidelines

  • If the slope is less than 4%, it is easiest to build a 4-inch deep rain garden.
  • If the slope is between 5 and 7%, it is easiest to build one 6 inches deep.
  • If the slope is between 8 and 12%, it is easiest to build one about 8 inches deep.
  • If greater than 12%, find a different location or consider other storm water management techniques.