Gutters And Downspouts Gutter Sizing Table

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.

Gutters and downspouts gutter sizing table.

1 inch 25 4 mm 1 square foot 0 0929 m 2. 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. Doubling the number of downspouts halves the required capacity for each. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

In response smacna s technical resources department has created a free downspout and gutter sizing calculator. Downspouts should be constructed with conductor heads every 40 ft 12 2 m to admit air and prevent vacuum. Use the chart below to figure out how many extra downspouts you need. Table 1 2 rainfall data and drainage.

The size of semicircular gutters shall be based on the maximum projected roof area in accordance with table 1106 6. 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. The saf perimeter systems gutter downspout capacity table click to enlarge relates downspout size to gutter sizes. Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200 square feet.

Offset of more than 10 ft 3 0 m can affect drainage capacity. Use the chart below to figure out how many extra downspouts you need. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. As with gutters a downspout s capacity must match or exceed the expected runoff.

Gutter dimensions are selected from the horizontal gutter sizing table based on the slope of the gutter to the downspout s and the gutter capacity based upon the roof flow rate in gallons per minute. To limit the effects of thermal expansion in gutters 50 ft. Size downspouts to fit the gutter. The resultant calculation q cia will be the flow rate for the roof design.

2 by 3 inches 600 square feet. It is possible to use a mix of gutter sizes a mix of downspout sizes and quantities to accommodate the theoretical gallons per minute draining from a roof during in climate weather. 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.

Capacity from using table 1 1 and 1 2 should be com pared with local code requirements. Table 1106 6 size of semicircular roof gutters for si. Water capacity values of a gutter downspout system is calculated using the height head of water a gutter can hold the outlet orifice size of the downspout and the quantity of downspouts used. As with gutters a downspout s capacity must match or exceed the expected runoff.

Downspout sizes must not exceed the bottom width of the gutter. The size of the downspout should be constant through. The downspout size must suit the bottom width of the gutter. 3 by 4 inches 1 200 square feet.

Downspout sizing in sizing downspouts the following considerations apply.