This was a pic I took of the drainage from a Wal-Mart roof into the store parking lot in central Florida soon after a typical Florida afternoon rain shower.
We currently collect rain water from our own personal roof which happens to be flat like many commercial roofs are. It's amazing how much water can be collected. We have 6 of the 50 gallon drums that are at the end of the roof downspout and during a classic Florida afternoon gully washer we easily fill all containers up and usually have spillage. Of course during a hurricane or tropical storm the amount is easily double or triple this amount.
Rainwater catchment or harvesting is certainly a way of life in many island nations who have limited access to water.
But it really makes sense for anyone in the USA with a single family home that lives in an area that receives good rain fall yearly to collect their rain water.
Calculate Your Rainwater Harvesting Potential
Courtesy of the City of Portland Bureau of Planning and Sustainability
If you're interested in rainwater harvesting at your home, here's how to estimate annual water supply:
Collection Area (sq. ft) x Rainfall (in/yr.) / 12 (in/ft) = Cubic Feet of Water/Year
Cubic Feet/Year x 7.43 (Gallons/Cubic Foot) = Gallons/Year
For example, a 500 sq. ft roof that gets 36 in/yr. will produce 1,500 Cubic Feet or 11,145 Gallons
of water per year.
Come on Commercial Warehouse Owners-Collect your rainwater
Given the example above if one is to extrapolate the rainwater yield to the millions of sq ft of flat roof space in the heavy rain areas ie > 40inches/ year of the USA it is a tragedy that this water is not collected and used for at least irrigation and possibly potable water consumption.
America is funny really. It would rather codify the use of an irrigation system, often using potable water, and not take advantage of what nature gives us.
We will investigate the codes in certain progressive US communities to see what is allowed for commercial warehouse rain water catchment or even harvesting and get back to you. We will also write some progressive US businesses such as Wal Mart, Target and the like to see what their policy is.
Stay tuned we'll let you know the results