How to Size Roof Drains

Divide the roof into sections. Climb a ladder and spread a measuring tape from one end of the front roof line to the other end. Have someone else hold the measuring tape in place if needed. Write down this number in feet. Measure the roof line along the side of the house and write down the feet. Multiply these two numbers together to get the roof area. Repeat for smaller sections of roof, if needed.

Figure out the maximum amount of rain that will fall in your area per hour. Get this number from your local codes department or weather bureau. You may also access the National Standard Plumbing Code. For example, houses in Erie, Pennsylvania, can expect up to a 4-inch hourly rainfall.

Choose a leader size. A 4-inch or 6-inch leader is typical on most homes. The larger the drain, the fewer drains are needed.

Calculate the roof area that can be drained by each leader during the maximum rainfall situations. Charts are available from plumbers and pipe companies. For example, one 6-inch leader is sufficient for a 13,500 square foot roof during 4 inches of rain per hour.

Divide the square footage of your total roof by the roof area the leader can handle (from Step 4). The answer is the number of drains you need to install on your roof.