![]() An expansion joint will be installed in each gutter between the downspouts. Maximum rainfall conditions will be used to determine downspout size.� It is decided to drain the building with 4 downspouts located at each corner of the building. The building is 100 x 85 ft.(30.5 x 26 m) with a double pitched roof having a slope of 6 in./ft.(152 mm/m).� The slope is toward the 100 ft.(30.5m) side. Select downspouts for a building in Boston, Mass. In making this calculation for a pitched roof, the plan area should be adjusted according to recommendations given. a scupper serving a designated roof area.� After the number and location of downspouts have been determined, the areas to be drained by each downspout should be figured. the risk of rain gutter overflow from insufficient drainage capacity. Water disposal at this location should be acceptable. the greater capacity of a pitched gutters or gutter. the appearance of the downspout system and a potential need for concealment. Locating downspouts on the north side of buildings is not recommended for such areas. Open, partially open or corrugated styles downspouts are suggested for areas subject to icing. potential for water freezing in downspouts and rain gutters. Unless special provisions are made for flexibility in downspouts, rain gutters and their support systems, gutters should expand away from downspouts and downspouts should not be located near gutter expansion joints. To limit the effects of thermal expansion in gutters 50 ft(15.3 m) is a practical maximum length of gutter to be served by a downspout. Assuming that using the fewest number of downspouts is desirable, their locations will be affected by.The downspout size must suit the bottom width of the rain gutter.The gutters outlet capacity should suit the downspout capacity.Offset of more than 10 ft(3.0 m) can affect drainage capacity.Downspouts should be constructed with conductor heads every 40 ft(12.2 m) to admit air and prevent vacuum.The size of the downspout should be constant throughout its length.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.In sizing downspouts, the following considerations apply: 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 in./hr.(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. Downspouts and rain gutters are sized in relation to rainfall on this basis. It takes 96.15 square feet(8.93 square meters) of surface with 1 inch per hour(25 mm/hr) of water to correspond with 1 gpm (0.063 l/s) flow rate. Local codes may require that drainage systems only be designed for the latter. New Orleans, Los Angeles, for example, may have 8 in./hr.(203mm/hr) for a five minute duration yet record only 4.8 in.(121 mm) in an hour over a 100 year period.� These rates correspond to 0.133 in./min.(3.4 mm/min.)and 0.08 in./min(2 mm/min.). Based on records through 1978, gives five minute intensities for selected cities. Rainfall intensity is usually given in inches per hour for a five minute duration or one hour duration based on U.S. Experience has taught that use of the true area of a pitched roof often leads to oversizing of the rain gutters or gutter, downspouts, and drains. 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. 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. The quantity of water in turn depends on the roof area, slope, and rainfall intensity. The design capacity for a roof drainage system depends on the quantity of water to be handled. 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.įactors to be considered in the design of roof drainage systems are the area to be drained, size of the gutter or gutters, downspouts, outlets, slope of roof, type of building, and appearance. 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. PROPER GUTTER AND DOWNSPOUT SIZING DESIGN OF ROOF DRAINAGE SYSTEMS ROOF DRAINAGE
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