Gutter Downspout Capacity

Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.

Gutter downspout capacity. The larger a gutter downspout the more water it can drain quickly. 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. 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.

Size downspouts to fit the gutter. Most of all is location and your average rainfall during a large downpour. Use the chart below to figure out how many extra downspouts you need. Downspout sizing in sizing downspouts the following considerations apply.

Gutter professionals have experience matching the gutters and the downspouts. Increasing the pitch increases a gutter s handling capacity but the gutter may look askew over a long run. Many factors can effect the proper size of gutter. The size of the downspout should be.

For the vast majority of rains gutter and downspouts sizes considerably less than recommended will do the job. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. As far as downspouts go they run at 2 x3 and 3 x4 in size or 3 or 4 inches in diameter. Downspout sizes are based on table 1 3 dimensions of standard downspouts page 1 4 of the architectural sheet metal manual 7th edition.

As with gutters a downspout s capacity must match or exceed the expected runoff. 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. 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. Doubling the number of downspouts halves the required capacity for each.

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. 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. 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 must not exceed the bottom width of the gutter.

You should have enough gutter downspouts to effectively handle the size of your entire gutter system. Primary cause for water spillage is debri in the gutter. Most downspouts are rectangular but they can also be round or square. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

However too large a gutter downspout can look unattractive. The standard is about inch per 10 feet. Problems with gutter sizing. Capacity from using table 1 1 and 1 2 should be com pared with local code requirements.