NOAA (National Oceanic & Atmospheric Association)Â has produced historical rainfall Isopluvial Maps (in color) for most of the U.S. A square inch of downspout can drain 1,200 square feet of roof during a rain with an intensity of one inch per hour. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. TABLE 1106.6 SIZE OF SEMICIRCULAR ROOF GUTTERS For SI: 1 â¦ Downspout sizes must not exceed the bottom width of the gutter. By reducing the downspout spacing to perhaps 35lf, you could use a 6â³ gutter with (9) 4â³x6â³ downspouts in lieu of an 8â³ system. Calculations are based on level gutter capacity as experimentally determined by the National Institute of Standards and Technology (NIST). Metal Roofing Contractors. Gutter SizingÂ Step 3 –Â Calculate water flow during rainfallÂ extremes ©2020 SAF Perimeter Systems Division | www.saf.com/perimeter-systems/ | Privacy Statement, Metals Distribution, Finishing and Fabrication, For SAF Perimeter Systems Commercial Gutter Designers, Contractors and Building Owners this Gutter SizingÂ section provides, Gutter SizingÂ Step 1 – Determine roof area to be drained, Gutter SizingÂ Step 2 – Research historical rainfall data, Gutter SizingÂ Step 3 –Â Calculate water flow during rainfallÂ extremes, Gutter SizingÂ Step 4 – Calculate water handling capacity for gutter & downspout options, Round â Pipe & Drawn Tube Aluminum Extrusion, Dry Rain Screen Architectural Panel Systems. Gutter Sizing Guide Gutter & Downspout Sizing Methodology. 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.Â 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. Offset of more than 10 ft(3.0 m) can affect drainage capacity. The gutter â¦ Also, the gutter width in relation to height may also be adjusted. The roof pitch (slope) has to be accounted for when using our GPM tables. Gutter SizingÂ Step 2 – Research historical rainfall data The standards mentioned previously regarding container strength and size are standards set by the ISO organization and International Maritime Organization. Downspout Sizes are based on Table 1-3 Dimensions of Standard Downspouts, Page 1.4 of the Architectural Sheet Metal Manual 7th Edition. It should be used to calculate required gutter size, downspout size, and downspout spacing for a given â¦ â¦ Calculator is based on Chapter 1 of SMACNA's Architectural Sheet Metal Manual 7th Edition (ASMM). 8370 Highway 78, Villa Rica, GA 30180 | Phone: +1-800-334-9823 | Fax: +1-770-942-4173 Note: For the â¦ The gutter â¦ But this isnât the whole story. 4. For a canopy as shown in the photograph above, you would only take into account the area of the upper roof that drains onto the canopy. IBC Roofing. Size downspouts to fit the gutter. Continuing with our example in the last step, we know our gutter & downspout system may be subject to as much as 1,880 gallons per minute.Â For design purposes and aesthetics, let’s assume you have selected an 8â³ gutter.Â Also assume our guttered eave is 335 linear feet.Â From the table, we find that a 5â³x5â³ downspout will discharge 260 gallons per minute.Â At 1,880 gpmâs, we will need 8 downspouts to handle the rainfall (1,880/260=8 downspouts, rounded up). Sizing the Gutters Multiply the drainage area by the roof-pitch factor and rainfall intensity to find out the adjusted square footage. ( 15 m ) is consistent globally it is commonly believed the International Building 2018... Reviewing data from the noaa Databases you will need 60 minute duration data to use our GPM... Gutters shall be based on level gutter capacity determine how many square inches of can. Of Standard downspouts, Page 1.2-1.3 of the IBC area, according to a project roof slope are ibc gutter sizing... 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