Gutter Downspout Capacity

Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200.

Gutter downspout capacity.

The larger a gutter downspout the more water it can drain quickly. For the vast majority of rains gutter and downspouts sizes considerably less than recommended will do the job. Most downspouts are rectangular but they can also be round or square. Use the chart below to figure out how many extra downspouts you need.

Increasing the pitch increases a gutter s handling capacity but the gutter may look askew over a long run. Size downspouts to fit the gutter. As with gutters a downspout s capacity must match or exceed the expected runoff. You should have enough gutter downspouts to effectively handle the size of your entire gutter system.

As with gutters a downspout s capacity must match or exceed the expected runoff. Gutter professionals have experience matching the gutters and the downspouts. Most of all is location and your average rainfall during a large downpour. 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.

The above recommendations assume that you have properly sized downspouts every 40 feet. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Doubling the number of downspouts halves the required capacity for each. Many factors can effect the proper size of gutter.

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. 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. As far as downspouts go they run at 2 x3 and 3 x4 in size or 3 or 4 inches in diameter. Gutter downspout sizes are 3 to 6 inches.

The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. 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. Downspout sizes are based on table 1 3 dimensions of standard downspouts page 1 4 of the architectural sheet metal manual 7th edition.

Downspout sizes must not exceed the bottom width of the gutter. Capacity from using table 1 1 and 1 2 should be com pared with local code requirements. Downspout sizing in sizing downspouts the following considerations apply. Problems with gutter sizing.

The standard is about inch per 10 feet. However too large a gutter downspout can look unattractive. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. 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.