What is the weight of 1.38 inches of rain water on a roof 114 feet by 150 feet.?

Answer 1

First, you must make all the units consistent. I chose to convert inches to feet. So, 1.38 inches equals 0.115 feet. Next, you must calculate the volume of water. To do that, multiply the three dimensions: 0.115 x 114 x 150 = 1966.5 cu ft. If we know how much a cubic foot of water weighs, we could directly calculate the weight of the water on the roof. But let's say that we only know the weight of a gallon of water. So, we must convert from cubic feet to gallons. A cubic foot of water contains 7.5 gallons. So, 1966.5 x 7.5 = 14748.75. Now, to calculate the weight, we multiply that figure by the weight of a single gallon of water, which is 8.34 pounds. So, 14748.75 x 8.34 = 123004.575, which we can round to 123,005 pounds. At first blush, that seems like a lot. But is it? Just to take this question a step beyond what the question asked, let's calculate the loading on the roof. The area of the roof is 114 x 150 = 17100 sq ft. If we divide the weight of the water by that amount, we can determine the load on the roof in pounds per square foot. So, 123005 / 17100 = 7.2 pounds per sq ft. That's less than the weight of one gallon per sq ft. Seems reasonable to me. Follow-up: I haven't done extensive research on this, so perhaps someone else will read this and modify it if it's wrong, but roofs are designed to withstand about 30 pounds per sq ft of loading due to snow or standing water. So, seven or eight pounds per sq ft should be no problem.