Gutter Downspout Capacity

Use the chart below to figure out how many extra downspouts you need.

Gutter downspout capacity. 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. Size downspouts to fit the gutter. As with gutters a downspout s capacity must match or exceed the expected runoff. As far as downspouts go they run at 2 x3 and 3 x4 in size or 3 or 4 inches in diameter.

The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. However too large a gutter downspout can look unattractive. The size of the downspout should be. Capacity from using table 1 1 and 1 2 should be com pared with local code requirements.

Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. Downspout sizes are based on table 1 3 dimensions of standard downspouts page 1 4 of the architectural sheet metal manual 7th edition. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

Downspout sizes must not exceed the bottom width of the gutter. 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. Most of all is location and your average rainfall during a large downpour. Now architects engineers designers and contractors can easily and accurately size downspouts and gutters according to the specifications in smacna s architectural sheet metal manual 7th edition 2012.

Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200. As with gutters a downspout s capacity must match or exceed the expected runoff. The standard is about inch per 10 feet. The above recommendations assume that you have properly sized downspouts every 40 feet.

Downspout sizing in sizing downspouts the following considerations apply. 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. For the vast majority of rains gutter and downspouts sizes considerably less than recommended will do the job. Many factors can effect the proper size of gutter.

Increasing the pitch increases a gutter s handling capacity but the gutter may look askew over a long run. Gutter professionals have experience matching the gutters and the downspouts. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Primary cause for water spillage is debri in the gutter.

Gutter downspout sizes are 3 to 6 inches. Doubling the number of downspouts halves the required capacity for each. 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. The larger a gutter downspout the more water it can drain quickly.