Design of Roof Drainage System

Once the roof type has been determined, careful attention must be paid to the type of roof drainage system to be installed.  The major factors to be considered in the selection of a roof drainage system are the area to be drained, size of gutters, downspouts and outlets, slope of roof, type of building, and appearance.  Whatever type of drainage system is chosen, be sure that all fabrication and installation work is done according to SMACNA standards and specifications.

In calculating the roof area and deciding on the size of gutter and downspouts to use, we must remember that rain does not fall completely vertically but rather at an angle and not perpendicular to the roof.  To determine the design area of a pitched roof, the use of the following chart should be used:

^{Source:  SMACNA  - Architectural Sheet Metal Manual, 1987}

To determine to design area of a steep slope roof, multiply the plan area by the appropriate roof area factor.  For example, to determine the design area of a roof 100 L/F by 80 L/F with a pitch of 6/12, the plan are would be 8000 sq. ft. (100 L/F x 80 L/F), and the design area for determining gutter size would be 8800 sq. ft. (100 L/F x 80 L/F x 1.10.)  Using this information, the architect or sheet metal contractor can determine the appropriate size gutter and downspout to be installed. 

Another factor which needs to be given special attention to appearance of the gutter system.  In steep slope applications, two basic choices exist:  A custom rectangular gutter design and roll-formed seamless gutters. 

The custom rectangular gutter is hand fabricated and installed in eight to ten foot sections.  The minimum ratio of the depth to width should be 3 to 4. Gutters should be properly supported with hangers, and support brackets if necessary, with enough strength to remain intact when full of water.   Gutter joints must be lapped 1”, riveted every 2” on center, and soldered if the metal can be soldered.  If cannot be soldered, a sealant must be continuously applied within a 1” lapped joint and the sealed lap must be riveted every 1” on center.  Selection of the gutter material must reflect the consideration of cost, similarity of metals between gutter, flashing, and roofing, and appearance.  The most common material choice for custom rectangular gutters are copper and galvanize.

Roll-formed seamless gutters are fabricated on the job-site in custom lengths with a smooth appearance.  Seamless gutters are available in 5”, 6”, 7”, and 8” widths, measured by the girth or by top opening.  Although gutters can be fabricated to extremely long lengths, expansion joints with 1” laps should be placed every 50 ft.  All gutter joints, at corners, elbows, outlet connections, and expansion joints, needs to be caulked and riveted for aluminum gutters, and soldered for copper gutters.