Calculate How Many Downspouts Gutters Need

As with gutters a downspout s capacity must match or exceed the expected runoff.

Calculate how many downspouts gutters need.

5 gutters typically use 2 3 downspouts. Minimum top opening width max. 3200 square feet of roof is calculated as 3200 600 5 33 rounding up you would need 6 2x3 downspouts. Measure the roof to get the watershed area the area from which water will run off.

Remember two gutter lengths can share the same downspout so if you calculate the max gutter to be 40 feet you could put a downspout halfway in an 80 foot length. To limit the effects of thermal expansion in gutters 50 ft. 3 by 4 inches 1 200 square feet. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.

If we have to sum it up in a single rule gutters need a downspout around every 30 to 40 feet. The code limit is 150mm for dp s and 26 800 sq mm for flat eaves gutters the program will calculate the size and number of downpipes downspouts leaders required and the eaves gutter sizes for any roof. There are many more important factors that can change this broad rule. It is used in 95 of all installations and works for all sloped roofing materials shingles metal slate wood etc.

Refer to as nzs 3500 3. 6 gutters typically use 3 4 downspouts to accommodate higher volume of water. For most homes this means installing spouts at either end of a gutter section. Multiply length peak to.

How many downspout installations should your gutters have. Use the chart below to figure out how many extra downspouts you need. Downspout sizes must not exceed the bottom width of the gutter. Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200 square feet.

For hipped roofs with eaves gutters a simplified formula can be used to calculate catchment area but the slope of the roof must be known. The number of downspouts can be determined by the surface square feet of your roof. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. 15 m is a practical maximum gutter length to be served by a downspout.

Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. There is no limit to downpipe size or eaves gutter size. 2 by 3 inches 600 square feet. 6 k gutter is used for higher volume of water runoff to a single point typically on flat roofs or larger commercial buildings.

Find the rainfall intensity for your locale. Rainfall intensity is measured in a 5 minute interval translated. The catchment area of a roof and any vertical walls is greatly influenced by the direction of wind driven rain therefore the combined catchment area must be used for sizing purposes. Once you add up the square footage of your roof you can then determine the number of downspouts.

How to calculate gutter and downspout capacity step 1. Enter larger gutter widths in the yellow boxes to find out how wide your gutters need to be given the length that you want to go between downspouts.