View Full Version : Drywell calculator

08-11-2012, 12:03 PM
A customer would like to tie his gutters into a drywell. How do I measure how big of a pit I need to dig? The soil is sandy so it drains well. I was going to do a 4'x4 pit filled with 1"-2" gravel. I'm sure it has to do with roof sq ft and amount of rain but the only formulas I found had to do with soil analysis. Looking for a quick real world answer. Is one of the basins they sell at supply yards that much better than a gravel filled pit?

08-11-2012, 12:50 PM
That's a LOT of water for a drywell to handle, especially a 4x4 drywell. One downspout is easily capable of putting out 10 gallons per minute in heavy rains. If you had two tied into it, you could quickly put 100-200 gallons in it in 10 minutes during steady rain.

08-11-2012, 12:58 PM
Install a rainwater harvesting system. HO would have extra FREE water available for irrigation.

DVS Hardscaper
08-11-2012, 01:56 PM
An engineer can calculate it.

But I never had a dry well engineered.

We usually do 6x6x6
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08-11-2012, 02:40 PM
Thanks. It's not the whole house only three gutters which is about half the roof.

08-11-2012, 03:40 PM
If you know the area (sq/ft) of the portion of the roof involved and find your local rainfall rate (seasonal), you can easily determine the required capacity of a drywell or a reservoir for a rain harvesting system.