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  1. Lay out the shape of the rain garden with a rope or flags. Rain gardens should be laid out on the contour – that is across the slope. Long and narrow rain gardens are recommended, so make the long sides lay across the slope and have the narrow ends running up and down the slope.
  2. Place stakes at the upper edge of the rain garden and stakes at perpendicular angles on the lower edge of the rain garden. Tie a rope at the base of the upper stake. Then tie the rope to the lower stake at an elevation that is level with the ground at the upper stake. Use a carpenter’s level to make sure the rope is level.
  3. Now measure the distance from the ground at the lower stake to the rope. This tells you how much the slope has dropped from the upper stake to the lower stake. To get a level surface in the rain garden, you’ll have to excavate to that depth at the upper stake.
  4. It is important that the rain garden be level from side to side and end to end so that water infiltrates uniformly across the bottom of the rain garden.
  5. Excavate subsoil and use it as fill material to create a berm on the lower edge of the rain garden.
  6. Stomp the fill down in 2 inch lifts to make sure it’s compacted (you want the berm compacted, but this is the only place in the yard you’ll want compaction).
  7. Make sure the berm is constructed level across the top. Use a carpenter’s level and a long 2 x 4 board to make sure the top of the berm is level.
  8. If you want to impound 6 inches of water, you’ll have to build a berm 8 inches high on the lower edge of the rain garden. Leave one end or both ends of the rain garden 2 inches below the berm to serve as an overflow outlet.
  9. The cut slope on the upper edge of the rain garden should be sloped back to a stable slope.
  10. Rain gardens must be level side to side, end to end, and the berm must be level. Note the low spot in the berm.

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