My main building roof

I need to know the flow rate (FR, for example Litre per minute) of water coming out from the gutter of this roof. The easiest way would be catching the flowing water coming out from its downspout with a bucket and time it. However, the opening of the downspout pipe does not allow me to do that conveniently as the opening is very close to the floor.

My carport roof

I have my carport roofed with corrugated plastic sheet. The dimension of the roof is known. The number of the "valleys" is also known. I have measured flow rate of 1 valley during rain that i believe the maximum rain rate.

Note: my main building roof and carport roof have different slope.


Now I need to check if my understanding is correct:

  1. I can get the total FR of my carport roof by simply doing [the FR of 1 valley] x [ number of valleys]. Is this acceptable?
  2. I can approximate the FR of my main building by simply doing [Carport FR] x [ Area of main building roof] : [ Area of carport roof]. Is this acceptable?
  3. Roof flow rate does not have anything to do with roof slope. Is this correct?

Assuming all the water that hits the roof ends up in the gutter downspout, the flow rate will be the projected roof area times the rain rate, up until the rain comes so fast that that assumption is no longer good as the gutter overflows.

Projected roof area is pretty much the shadow the roof would cast on the ground if lit from directly overhead, or the area of the building it's on, plus any overhanging area. So a nearly flat roof and a steep roof would have a different number of "square feet (or meters) of roofing" for the same "projected roof area"

Anyway, I don't see a lot of benefit to "making a direct measurement" because that measurement will vary drastically with how fast the rain is coming down, which varies drastically. Depending on the purpose of the measurement you either need to calculate based on the highest rain rate for your area, or an average rate. Your local weather resources should have numbers for that sort of thing, which are probably more reliable than "I think it was the highest rate on the day I measured."

But the flow rate on the day you measured would be proportional, yes. It's just not as reliable as using actual meteorological data and a little bit of math.

  • Of course, all you need to do is get rainfall data for your exact location - and, from experience the closest available may be 5, 10 or 50 miles away... I had two sources and did an average between them... "ball park" ha...
    – Solar Mike
    May 10 '19 at 13:39

Seems a close enough approximation for 1 and 2.

You could check by getting the total roof output into a 50 gall container and timing it (between two points is usually reasonably accurate.)...

For 3, the slope and the roof surface will have an impact, but depending what you are doing it may not matter too much.

  • Why are you having problems? Or saving water.
    – user101687
    Jun 9 '19 at 22:34
  • @RobertMoody you need to work out who the OP is...
    – Solar Mike
    Jun 9 '19 at 22:57

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