Gutter Downspout Capacity

You should have enough gutter downspouts to effectively handle the size of your entire gutter system.

Gutter downspout capacity.

The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. Gutter downspout sizes are 3 to 6 inches. 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.

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. However too large a gutter downspout can look unattractive. The above recommendations assume that you have properly sized downspouts every 40 feet. Most downspouts are rectangular but they can also be round or square.

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. The larger a gutter downspout the more water it can drain quickly. Doubling the number of downspouts halves the required capacity for each. 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. Use the chart below to figure out how many extra downspouts you need. 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.

The size of the downspout should be. Downspout sizing in sizing downspouts the following considerations apply. Size downspouts to fit the gutter. Primary cause for water spillage is debri in the gutter.

The standard is about inch per 10 feet. Many factors can effect the proper size of gutter. Capacity from using table 1 1 and 1 2 should be com pared with local code requirements. 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.

Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. Problems with gutter sizing. Gutter professionals have experience matching the gutters and the downspouts. 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.

Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200. Most of all is location and your average rainfall during a large downpour. As with gutters a downspout s capacity must match or exceed the expected runoff. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.