Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.
Gutter downspout capacity.
Most downspouts are rectangular but they can also be round or square.
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.
For the vast majority of rains gutter and downspouts sizes considerably less than recommended will do the job.
As with gutters a downspout s capacity must match or exceed the expected runoff.
Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.
The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website.
Doubling the number of downspouts halves the required capacity for each.
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.
Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200.
Primary cause for water spillage is debri in the gutter.
You should have enough gutter downspouts to effectively handle the size of your entire gutter system.
A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.
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.
Downspout sizing in sizing downspouts the following considerations apply.
Size downspouts to fit the gutter.
The larger a gutter downspout the more water it can drain quickly.
Capacity from using table 1 1 and 1 2 should be com pared with local code requirements.
Problems with gutter sizing.
Downspout sizes must not exceed the bottom width of the gutter.
However too large a gutter downspout can look unattractive.
The size of the downspout should be.
Gutter professionals have experience matching the gutters and the downspouts.
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.
Downspout sizes are based on table 1 3 dimensions of standard downspouts page 1 4 of the architectural sheet metal manual 7th edition.
Gutter downspout sizes are 3 to 6 inches.
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 standard is about inch per 10 feet.
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.
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.
