Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.
Gutter length per downspout.
Increasing the pitch increases a gutter s handling capacity but the gutter may look askew over a long run.
The gutter outlet capacity should suit the downspout capacity.
Doubling the number of downspouts halves the required capacity for each.
15 m is a practical maximum gutter length to be served by a downspout.
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.
There is no linear foot of gutter per downspout calculation that can be made since it is the size of the area draining into that downspout that will tell you how much volume of water will typically drain through it.
You can t do the math like that.
The standard is about inch per 10 feet.
The above recommendations assume that you have properly sized downspouts every 40 feet.
The capacity of a sloped rectangular gutter may be approximated by using a gutter cross section area not less than that of a semicircular gutter and a depth to width ratio of at least 0 75.
Offset of more than 10 ft 3 0 m can affect drainage capacity.
Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.
Design of roof drainage systems.
The above recommendations assume that you have properly sized downspouts every 40 feet.
The downspout size must suit the bottom width of the gutter.
To limit the effects of thermal expansion in gutters 50 ft.
The standard is about inch per 10 feet.
A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.
Size downspouts to fit the gutter.
For example adding one 4 inch round downspout will increase the gutter capacity by 1 255 square feet and one 3 inch round downspout would increase capacity by 706 square feet.
However too large a gutter downspout can look unattractive.
If we have to sum it up in a single rule gutters need a downspout around every 30 to 40 feet.
The larger a gutter downspout the more water it can drain quickly.
Most downspouts are rectangular but they can also be round or square.
The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts.
I m not sure what the answer is but i know you can t do it per foot of gutter.
Gutter downspout sizes are 3 to 6 inches.
Downspouts should be constructed with conductor heads every 40 ft 12 2 m to admit air and prevent vacuum.
For most homes this means installing spouts at either end of a gutter section.
