Gutter Downspout Capacity

For the vast majority of rains gutter and downspouts sizes considerably less than recommended will do the job.

Gutter downspout capacity.

Use the chart below to figure out how many extra downspouts you need. Now that we are well aware of the various gutter downspout sizes and styles let us dig deeper into the materials used for effective downspouts and how much they shall cost. Most downspouts are rectangular but they can also be round or square. Many factors can effect the proper size of gutter.

As with gutters a downspout s capacity must match or exceed the expected runoff. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. The second most common style of downspouts in urban homes is the rectangular downspout coming under a range of sizes such as 2 3 3 4 and 4 5. Downspout sizing in sizing downspouts the following considerations apply.

Downspouts of less than 7 00 sq in 4515 sq mm cross section should not be used except for small areas such as porches and canopies. Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200. You should have enough gutter downspouts to effectively handle the size of your entire gutter system. The above recommendations assume that you have properly sized downspouts every 40 feet.

Primary cause for water spillage is debri in the gutter. Doubling the number of downspouts halves the required capacity for each. As far as downspouts go they run at 2 x3 and 3 x4 in size or 3 or 4 inches in diameter. The larger a gutter downspout the more water it can drain quickly.

Downspout sizes must not exceed the bottom width of the gutter. Gutter professionals have experience matching the gutters and the downspouts. Capacity from using table 1 1 and 1 2 should be com pared with local code requirements. However too large a gutter downspout can look unattractive.

The standard is about inch per 10 feet. Gutter downspout sizes are 3 to 6 inches. As with gutters a downspout s capacity must match or exceed the expected runoff. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.

For example a 6 inch k style gutter requires a 3 x 4 downspout while a 5 inch k style gutter uses a 2 x 3 downspout. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Problems with gutter sizing. Size downspouts to fit the gutter.

Most of all is location and your average rainfall during a large downpour. Increasing the pitch increases a gutter s handling capacity but the gutter may look askew over a long run. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. The size of the downspout should be.