Gutters protect homes during rain storms. Without the gutters, water will fall and collect at the base of the home. This will, over time, cause the foundation to mould, rot and ultimately deteriorate. Instead, gutters route water away from the base of the home by collecting and directing water to an alternative location. The primary component of the gutter is the gutter spout. As a consequence, sizing the gutter spout is key to properly sizing a gutter system.

Determine the surface area of your roof. Use the formula A = L x W where A is area, L is length and W is width. For example, if your roof is 40 feet long and 20 feet wide, you area will be 800 square feet.

Determine your roof pitch. Roof pitch is expressed as the inches the roof rises for every 12 inches it runs; for example, a 6-in-12 roof rises 6 inches for every 12 inches the roof runs. You can see some roof pitch examples in the Resources section.

Determine your roof pitch factor. This is determined by your roof pitch: flat to 3-in-12, ×1; up to 5-in-12, ×1.05; up to 8-in-12, ×1.1; up to 11-in-12, ×1.2; 12-in-12 or more, ×1.3. For example, if our roof has a 6-in-12 roof pitch, its roof pitch factor is ×1.1

Calculate your roof watershed area by multiplying your surface area by the roof pitch factor. Using the example above, the watershed area is 800 square feet × 1.1 or 880 square feet.

Determine the rainfall intensity for your location. In the Resources section, refer to the Rainfall Intensity Map and choose the intensity applicable for your local area in inches per hour.

Determine the gutter capacity you need. Multiply the watershed area you determined in Step 4 by the rainfall intensity from Step 5. This will give you the maximum area of watershed your gutter has to handle with a rainfall rate of one inch per hour. For example, if your watershed is 880 square feet and your rainfall intensity is 6 inches per hour, your gutter system must have the capacity of 5280 square inches.

Choose your gutter size. Visit your local building supply store and use capacity from Step 6 to choose a gutter size. Gutters are rated in maximum area a gutter can handle with a rainfall rate of one inch per hour. This rating information is normally posted on the gutter packing or specifications. Using the 5280 square feet example from Step 6, you will choose a gutter size that can handle at least 5280 square feet area with a rainfall rate of one inch per hour.

Determine the size of your downspout. Match your downspouts to the gutter size you chose in Step 7. Refer to the gutter packaging or specification for the recommended downspout size. For example, a 5 inch gutter system requires at least 2 x 3 downspouts, and a 6-inch gutter system requires 3 x 4 downspouts. (Several examples are given below the rainfall intensity map in the Resources section, below.)

Determine the minimum number of downspouts. One square inch of downspout cross section can drain 100 square feet of watershed. This means a 2 x 3 spout drains up to 600 square feet and a 3 x 4 spout drains 1,200 square feet. Use this information to make sure you have the minimum number of downspouts. Using the watershed area from Step 4 of 880 square feet as an example, you will need at least two 2 x 3 downspouts or one 3 x 4 downspout to adequately drain an area of 880 square feet of watershed. As rule of thumb, insure you have much more than the minimum to be safe.