There will be no more water expelled versus what normally would have fallen on that area of the roof to begin with. In fact, the EarthTainer will actually retain some of that water which would have fallen in that same footprint.
In a heavy rain, any type of outer tray would likely fill from the downpour on its own. As I indicated, the EarthTainer would act as a "buffer" to trap rain water into its own water reservoir under most conditions other than a Hurricane.
Raybo
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