How To Size Number Of Downspouts Needed For Roof

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

How to size number of downspouts needed for roof. The number of additional downspouts needed to increase the gutters capacity will depend on the size of the downspout. For most homes this means installing spouts at either end of a gutter section. 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. To size the vertical roof drain for the same parameters 5 000 square feet of room area and 6 per hour of rain consult table 1106 2 figure 3.

Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. How to calculate gutter and downspout capacity. 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. 2 by 3 inches 600 square feet.

If we have to sum it up in a single rule gutters need a downspout around every 30 to 40 feet. 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. Multiply length peak to eave by width to determine. In sizing downspouts the following considerations apply.

The size of the downspout should be constant through out its length. 3 by 4 inches 1 200 square feet. Measure the roof to get the watershed area the area from which water will run off. Find the rainfall intensity for your locale.

Also the gutter width in relation to height may also be adjusted. The gutter and downspout size requirement depends upon the number of sections and length of the gutter sections as well as the number of downspouts. 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. Use the chart below to figure out how many extra downspouts you need.

For example adding one 4 inch round downspout will increase the gutter. Adding additional sections of gutter reduces the length of each section and will reduce the gutter size.