How to Calculate the Square Footage of a Roof Vs. the Size of a Drain Pipe

Determining the size of drain pipe you need for your roof requires finding the total surface area of the roof and the highest rate of rainfall you can expect per hour. This will help you determine the number of gallons of water that may go through the drain system. Once you've made your calculations, picking the correct drain size is as simple as matching the flow of water to the drain capacity.

Instructions

- 1
  Determine the surface area of your roof. This involves determining the square footage of each area of roofing. Square footage is determined with a simple length times width formula.
- 2
  Determine the maximum rainfall per hour for your area of the country. The best way to do this is to contact a local television station or weather monitoring station. They will be able to give you a rough estimate of annual rainfall as well as the heaviest rains that the area regularly experiences.
- 3
  Use the following calculation to determine the maximum gallons per minute flow off from your roof. Most drainage systems have 4-inch leader channels (eaves) so this is what we will base our calculation on. If you have different sized eaves see the first reference for a calculation chart.
  
  Gallons per minute = .0104 x maximum inches per hour x roof area
- 4
  Take the gallons per minute calculation and divide it by the maximum flow rate of a 4-inch drain pipe (144 gallons per minute). If the answer contains a fraction, round it up to the next whole number. This is the number of downspouts needed to prevent eaves from overflowing.

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