PROPER GUTTER AND DOWNSPOUT SIZING

1 PROPER GUTTER AND DOWNSPOUT SIZING DESIGN OF ROOF drainage SYSTEMS ROOF drainage The roof is one of the most essential parts of a building as it protects occupants, contents, and interior of the structure from the elements. Once an architect has determined the kind of roof he intends to use, he must give equal attention to the design of the roof drainage system. Factors to be considered in the design of roof drainage systems are the area to be drained, size of gutters, downspouts, outlets, slope of roof, type of building, and appearance. ROOF AREA TO BE CONSIDERED The design capacity for a roof drainage system depends on The quantity of water to be handled. The quantity of water in turn depends on the roof area, slope, and rainfall intensity.

2 In considering the roof area, it must be remembered that rain does not necessarily fall vertically and that maximum conditions exist only when rain falls perpendicular to a surface. Since the roof area would increase as its pitch increases, then it would not be advisable to use the plan area of a pitched roof in the calculation of a drainage system. Experience has taught that use of the true area of a pitched roof often leads to oversizing of gutters, downspouts, and drains. To determine the design area for a pitched roof, Table 1-1 is used. TABLE 1-1 DESIGN AREAS FOR PITCHED ROOFS PITCH mm/mm *B Level to 3 76/305 4 to 5 102-127/305 6 to 8 152-203/305 9 to 11 229-279/305 12 305/305 To determine the design area multiply the plan area by the factor in B column These areas are then divided by the PROPER factor given in Table 1-2.

3 Thus obtaining the required area in square inches (square mm) for each DOWNSPOUT . From Table 1-3 select the DOWNSPOUT . PROPER GUTTER AND DOWNSPOUT SIZING RAINFALL INTENSITY - DOWNSPOUT CAPACITY Rainfall intensity is usually given in inches per hour for a five minute duration or one hour duration based on Weather Bureau records. Table 1-2 based on records through 1978, gives five minute intensities for selected cities. New Orleans, Los Angeles, for example, may have 8 (203 mm/hr) for a five minute duration yet record only in. (121 mm) in an hour over a 100 year period. These rates correspond to ( mm/min.) and (2 mm/min.). Local codes may require that drainage systems only be designed for the latter. It takes square feet( square meters) of surface with 1 inch per hour(25 mm/hr) of water to correspond with 1 gpm ( l/s) flow rate.

4 Downspouts and gutters are sized in relation to rainfall on this basis. Plumbing codes typically use the vertically projected roof area for drainage design and they often use a square foot allowance per square inch of DOWNSPOUT for 1 (25 mm/hr) rainfall that varies with diameter, for example, 3 in.(76 mm): 911(85); 4 in.(102 mm): 1100 (102); 5 in.(127 mm): 1280 (119); 6 in.(152 mm): 1400 (130) and 8 in.(203 mm): 1750 (163) sq. ft.(sq. m). Net drainage capacity from using Table 1-1 and 1-2 should be compared with local code requirements. DOWNSPOUT SIZING In SIZING downspouts, the following considerations apply: 1. Downspouts of less than sq in.(4515 sq mm) cross section should not be used except for small areas such as porches and canopies.

5 2. The size of the DOWNSPOUT should be constant throughout its length. 3. Downspouts should be constructed with conductor heads every 40 ft( m) to admit air and prevent vacuum. 4. Offset of more than 10 ft( m) can affect drainage capacity. 5. The GUTTER outlet capacity should suit the DOWNSPOUT capacity. 6. The DOWNSPOUT size must suit the bottom width of the GUTTER . TABLE 1-2 RAINFALL DATA AND drainage FACTORS A STORMS WHICH SHOULD BE EXCEEDED ONLY ONCE IN 10 YEARS B STORMS WHICH SHOULD BE EXCEEDED ONLY ONCE IN 100 YEARS Intensity lasting 5 minutes Calculated roof area drained per DOWNSPOUT area Intensity lasting 5 minutes Calculated roof area drained per DOWNSPOUT area in/hr mm/hr sq/ft sq m/ sq in 100 sq mm in/hr mm/hr sq/ft sq m/ sq in 100 sq mm ALABAMA: Birmingham Mobile ALASKA: Fairbanks Juneau ARIZONA: Phoenix Tucson ARKANSAS.

6 Bentonville Little Rock CALIFORNIA: Los Angeles Sacramento San Francisco San Diego COLORADO: Denver Boulder CONNECTICUT: Hartford DISTRICT OF COLUMBIA FLORIDA: Jacksonville Miami Tampa GEORGIA: Atlanta HAWAII: Honolulu Kahului Hilo Lihue IDAHO: Boise ILLINOIS: Chicago INDIANA: Indianapolis IOWA: Des Moines KANSAS: Wichita KENTUCKY: Louisville LOUISIANA: New Orleans MAINE: Portland MARYLAND: Baltimore MASSACHUSETTS: Boston MICHIGAN: Detroit MINNESOTA: Minneapolis MISSOURI: Kansas City Saint Louis MONTANA: Helena Missoula NEBRASKA: Omaha NEVADA: Reno Las Vegas NEW JERSEY: Trenton NEW MEXICO: Albuquerque Santa Fe NEW YORK: Albany Buffalo 191 208 53 43 141 155 187 187 124 64 68 57 146 164 158 180 200 195 212 186 221 177 442 265 46 172 173 186 191 175 211 136 160 150 570 700 220 200 160

7 160 250 480 450 540 210 190 190 170 150 160 140 160 140 170 70 110 660 180 180 160 160 170 140 220 .170 256 274 97 57 224 232 259 253 170 100 93 78 232 238 221 247 256 250 274 251 305 305 488 366 84 236 239 262 267 238 277 192 120 110 310 530 140 130 120

8 120 180 310 330 390 130 130 140 120 120 120 110 120 100 100 60 80 360 130 130 120 110 130 110 160 120 New York City NORTH CAROLINA: Raleigh NORTH DAKOTA: Bismark OHIO: Cincinnati Cleveland OKLAHOMA: Oklahoma City OREGON: Baker Portland PENNSYLVANIA: Philadelphia Pittsburgh RHODE ISLAND: Providence SOUTH CAROLINA: Charleston TENNESSEE: Memphis Knoxville TEXAS: Fort Worth Dallas Houston San Antonio UTAH: Provo Salt Lake City VIRGINIA: Norfolk WASHINGTON: Seattle Spokane WEST VIRGINIA: Parkersburg WISCONSIN: Madison Milwaukee WYOMING.

9 Cheyenne 181 134 162 178 187 181 46 46 188 57 53 170 102 115 165 152 169 185 167 172 160 193 56 53 172 163 143 184 187 169 193 194 208 193 75 71 230 190 170 160 170 660 660 160 530 570 180 300 270 190 200 180 160 180 180 190 160 550 570 180 190 210 170 160 180 193 160 160 150 160 410 430 247 183 226 253 265 251 77

10 61 267 114 133 236 171 169 232 213 235 250 250 236 223 267 97 76 238 224 198 238 253 229 267 267 274 267 131 170 140 120 110 120 390 500 110 270 230 130 180 180 130 140 130 120 120 130 140 110 310 400 130 140