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Rain Catching 102

August 15, 2010

This is #2 in a series of posts outlining the concept all the way through installation of my 275 gallon rain catch system last year.

My next step was to figure out the amount of water that drained through the convenient downspouts. There are three on the back of the house, one of each corner and one in the middle. The corner ones drain the back side gutters; basically these catch the outside slope of the roof for a little more than the back half. The middle one catches all the inside sloping roof on the house, which as pretty decent sized area.

For simplicity and volume the middle one seemed a good prospect (and it was the most hidden, another plus) so I refined my investigation a bit to see what volume of water would flow with 1 inch of rain.
The formula is approximately .623 gallons of water PER square foot of roof area PER inch of rain. The roof area measurement is constant regardless of the slope; for flat or sloped what you measure is the area under the roof, or more precisely the footprint of the area covered.

My home’s roof line is actually shaped like a “U” open to the back; this crates a canyon of sorts in the back that drains onto a flat roof and then down the central downspout. This would have been a difficult measurement except Big Brother had already made those measurements public record. I looked up my tax record online in the county database and right there for all aspiring rain catch engineers to see I found a sketch of the foot print of the house including the ridge lines of the roof. How convenient (and how scary, there was also a picture of the back of my house that was taken from inside my fence. Does the county send out ninjas to sneak these photos? Drones? ???)

Anyway… the area that dumped down the central downspout was approximately 700 square feet, so 1 inch of rain would theoretically provide 436 gallons for harvesting. If I connected my 5 remaining 55 gallon barrels, then a little more than 1/2 an inch of rain would fill them.

OK, so volume won’t be a problem.

I came up with a visual of what I thought I wanted for my multi-barrel rain catch system setup to do for me:

1. I wanted multiple barrels hooked up in parallel so that they all filled and drained at the same rate. This way there is no spillover or draining hierarchy to contend with and I would be able to drain all from a single point.
2. I wanted the hose connection to be @ the “front” garden beds (the back beds weren’t even conceived at that point) so that I wouldn’t have to drag hoses all over as before.
3. I wanted to have knockout cleaning ports to clean sediment and any other oddities out of the system if needed.

Easy enough, right?


Ok the first thing I built was a platform I could use to raise the base level of the barrels. This had to be stout as 4 x 55 gallons of water would weigh over 1700 lbs. (The 5th barrel was going to be in another area). Additionally there had to be gaps in the platform’s decking that allowed for fittings to go through below.

Here is my answer to these requirements:

For scale, a barrel has been placed on it:

Next: Fittings and the connections

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