How Strong Is Your Roof?

We don’t get much snow in central Alabama. What we get is rain, and increasingly these days, it comes in the form of torrential downpours. But this week we got freezing rain, then sleet, then snow and sleet, and finally snow.

Whatever fell stayed on the metal roofs of the barn and the lodge for two days and began flowing down today as the temperatures thawed. We’re in an unusual position of having all our gutters drain to a single point for (we hope) future rainwater harvesting, so I’m able to guess with some accuracy just how much water was up there. Here’s what I know:

  • Both the barn and the lodge run exactly east and west.
  • The barn and the lodge together have about 10,500 square feet of roof space, divided equally between south-facing surface and north-facing surface.
  • I watched the flow rate all day today (yeah, probably should have been doing something more constructive). I measured it by timing the speed with which it filled up a five gallon bucket. I did this measurement seven times during the day. There was a little variation in the flow rate, but less than I expected.
  • The water flowed faster (about four gallons per minute) for the first couple of hours as the sun hit the south-facing surfaces of the buildings and melted the ice and snow on them rapidly. Then as the south-facing surfaces slowed down in their rate of draining and the temperature rose enough to begin melting the north-facing surfaces, the flow rate slowed down to about 2.5 gallons per minute.
  • I estimate the slower rate continued for five hours and is in fact continuing as I post this.
  • So here are my calculations:

    Hours of flow at 4 gal/min 2
    Minutes of flow at 4 gal/min 120
    Total gallons 480
    Pounds per gallon 8.34
    Total pounds first two hours 4003.2

    Hours of flow at 2.5 gal/min 5
    Minutes of flow at 2.5 gal/min 300
    Total gallons 750
    Pounds per gallon 8.34
    Total pounds next five hours 6255

    Total gallons of water 1230
    Total pounds 10258.2

    That is to me a staggering amount of water, and a staggering amount of weight that the roofs were bearing. Apparently they were up to the task. Are yours? Obviously we have elected to present an unusually large amount of roof space to the storm. If you want to extend my calculations to your roof, figure about 1/2 ton per thousand square feet of roof space.

    Here’s what the flow from the north side of the buildings looked like. Doesn’t seem like a lot, of course, but think about five hours of this rate of flow and then that much again. That’s a lot of water.

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