Dry Well Sizing Chart

  • The dry well must be at least 10 feet from your house and any other buildings that are level with yours.
  • The dry well must be at least 25 feet from buildings that are downhill from it.
  • Refer to the sizing chart. Decide what size storm you would like to store and infiltrate in your dry well. Find the closest number in Column A. About one-third of storms in the Philadelphia area result in 0.25 inches or less of rainfall. 60% of storms result in 0.5 inches or less, and 85% are 1.0 inch or less.
  • Estimate the roof area draining to the dry well (length [ft.] x width [ft.] = area in square feet). Disregard roof slope. Find the closest value in Column B for the storm depth you have chosen. At this point, you have narrowed your choice down to just one line of the table.
  • Find the area required for your dry well in Column D. When you multiply your dry well length and width, the resulting number (area) needs to be at least as great as the number in Column D. Columns E and F show examples of lengths and widths that will work.
  • Determine whether your yard and budget will allow you to build a dry well of this size with a safe overflow. If not, choose a smaller storm and repeat the steps. Storing a larger storm provides a greater benefit, but also requires more space and costs more. Storing even the smallest storm in the table will provide benefits.
  • The dry well should have a safe overflow, such as an overflow to your yard drain. In larger storms, your dry well will fill up, and you need to make sure that the overflow doesn’t damage your property or your neighbors’ properties. Keep in mind that the yard drain has to be slightly downhill from the dry well.
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