How to Calculate Roof Drains

Measure the total square footage of your roof. Measure each side and then multiply to get the total area.

Contact your local code department or weather service and inquire about the maximum annual rainfall per hour. The roof drains will need to be able to handle this amount, in inches, of rainfall.

Select a 6-inch or 8-inch size leader to drain the most amount of water from the roof. The drain pipes are manufactured in sizes ranging from 2 to 8 inches. Choose a larger size so you can install fewer of them.

Determine how many drain pipes are needed. Plumbers and pipe companies have charts based on the Uniform Plumbing Code that tell you how many square feet the drain pipes can handle based on your maximum volume of rainfall per hour. For example, one 6-inch leader can handle 10,200 square feet of roof if it is raining 4 inches an hour.

Do some math to determine how many drain pipes you need to install. Divide the total roof area by the square footage one leader can handle. This is how many leaders you must install.