Problems with gutter sizing.
Gutter downspout capacity.
The size of the downspout should be.
Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200.
The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website.
As far as downspouts go they run at 2 x3 and 3 x4 in size or 3 or 4 inches in diameter.
Size downspouts to fit the gutter.
Gutter downspout sizes are 3 to 6 inches.
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.
As with gutters a downspout s capacity must match or exceed the expected runoff.
Most downspouts are rectangular but they can also be round or square.
A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.
Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.
As with gutters a downspout s capacity must match or exceed the expected runoff.
Downspout sizing in sizing downspouts the following considerations apply.
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.
Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.
However too large a gutter downspout can look unattractive.
Doubling the number of downspouts halves the required capacity for each.
The larger a gutter downspout the more water it can drain quickly.
Most of all is location and your average rainfall during a large downpour.
Downspout sizes must not exceed the bottom width of the gutter.
Increasing the pitch increases a gutter s handling capacity but the gutter may look askew over a long run.
Primary cause for water spillage is debri in the gutter.
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.
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.
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.
For the vast majority of rains gutter and downspouts sizes considerably less than recommended will do the job.
Gutter professionals have experience matching the gutters and the downspouts.
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 above recommendations assume that you have properly sized downspouts every 40 feet.
The standard is about inch per 10 feet.
