How To Size Number Of Downspouts Needed For Roof

How to calculate gutter and downspout capacity.

How to size number of downspouts needed for roof.

Also the gutter width in relation to height may also be adjusted. 3 by 4 inches 1 200 square feet. For example adding one 4 inch round downspout will increase the gutter. 2 by 3 inches 600 square feet.

Subtracting 3 4 for under the roof edge and 1 8 outside edge allowance this is the width that is available for the rainwater to enter the gutter total width of a flat piece of building material maximum total gutter drop before any downspout this assumes 4 7 per hour maximum rain intensity. The number of additional downspouts needed to increase the gutters capacity will depend on the size of the downspout. The size of the downspout should be constant through out its length. 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.

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. Multiply length peak to eave by width to determine. Adding additional sections of gutter reduces the length of each section and will reduce the gutter size. As with gutters a downspout s capacity must match or exceed the expected runoff.

In sizing downspouts the following considerations apply. If we have to sum it up in a single rule gutters need a downspout around every 30 to 40 feet. For most homes this means installing spouts at either end of a gutter section. Measure the roof to get the watershed area the area from which water will run off.

Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. According to the chart for 1 8 slope per 12 of pipe for 5 000 square feet of roof area with a 6 per hour of rainfall the required pipe size is 8. Find the rainfall intensity for your locale. Use the chart below to figure out how many extra downspouts you need.

Rainfall intensity is measured in a 5 minute. 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.