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HOW TO SELECT A ROOF DRAIN
 
To select the proper roof drain, the following information must be determined by the designer/specifier.
• Type of roof construction
• Roof pitch
• Maximum volume of expected rainfall and storm design criteria (This information
   must be obtained from your local weather bureau and/or local code authority)
• Desired rate of drainage
• Safety overflow requirements (Emergency/secondary overflow roof drains are
   recommended. Local codes vary but it is recommended to provide a 1 to 1 ratio)
• Roof load (The maximum possible rainwater [build-up] load should be determined
   and provided to the structural engineer for inclusion in the roof structure design)		 • Location of drains (Consult your local code requirements)
• Size
• Vandal-proofing
• NOTE: ALWAYS CONSULT YOUR LOCAL CODE FOR SIZING AND DESIGN
   CRITERIAWHEN DESIGNING THE ROOF DRAIN SYSTEM. LOCAL CODE
   REQUIREMENTS TAKE PRECEDENCE OVER CATALOG INFORMATION.
• DATA SHOWN IN TABLES 1 AND 2 BELOW ARE TAKEN FROM THE
   UNIFORM PLUMBING CODE (UPC) - 2006 EDITION.
SUGGESTED STEPS FOR SELECTING PROPER ROOF DRAIN LEADER SIZES AND NUMBER REQUIRED FOR A GIVEN ROOF
Steps: Example: Using a 4" Vertical Leader Example: Using a 6" Vertical Leader
1.  Calculate the total roof area. 1. Total roof area - 500' by 200' = 100,000 sq. ft. 1. Total roof area - 500' by 200' = 100,000 sq. ft.
2.  Determine the maximum hourly rainfall in inches. (The figure can be acquired from your local weather bureau and/or local code authority.) 2. Determine rate of rainfall - for this example use 4". 2.  Determine rate of rainfall - for this example use 4".
3.  Select leader size. 3. After studying building plan and physical arrangement, assume that 4" leaders are required for this project. 3. After studying building plan and physical arrangement, assume that 6" leaders are required for this project.
4. From Table 1, determine the number of square feet that can be drained by one roof leader at the local maximum rainfall rate. 4.  From Table 1 - one 6" leader at 4" rate of rainfall will take care 13,500 sq. ft. of roof area. 4. From Table 1 - one 6" leader at 4" rate of rainfall will take care of 10,200 sq. ft. of roof area.
5. Divide the total roof area by the area that one leader will handle. The above result is the number of roof drains required for the building. If the result is a fraction less, use the next higher number. 5. Number of roof leaders required is 29 (100,000 sq. ft. divided by 3,460 sq. ft.), Therefore 29 roof drains would be required. 5. Number of roof leaders required is 10 (100,000 sq. ft. divided by 10,200 sq. ft.), Therefore 10 roof drains would be required.
NOTE: It can readily be seen that if 4" leaders were used, the number of roof drains required would increase to 30 drains. (29.348 rounded off to next highest number.) If a smaller number of roof drains are required, then larger leaders would have to be chosen. Several small drains and leaders rather than one or two large drains will insure even safer yet adequate roof drainage. Drains should be spaced for uniform drainage.

ROOF DRAIN VERTICAL LEADER REQUIREMENTS FOR HORIZONTAL ROOF AREAS AT VARIOUS RAINFALL RATES

TABLE 1

STEPS FOR CALCULATING DRAINAGE REQUIREMENTS FOR ABOVE EXAMPLE USING G.P.M.
1. Use the following formula to determine G.P.M.:
    G.P.M. = .0104 x R x A
    G.P.M. = Gallons per minute
    R = Rainfall intensity - inches/hour
    A = Roof area - square feet
    .0104 = Conversion factor - G.P.M./sq. ft. for 1" (one) inch/hr. rainfall

2. Example:
   A. 4" rainfall inches/hr.
   B. 100,000 sq. ft. roof area
   C. G.P.M. = .0104 x 4" x 100,000 sq. ft. = 4,160 G.P.M.

3. Refer to table 2: a 4" leader [2] will handle 144 G.P.M.
   4,160 G.P.M/ ÷ 144 = (28.8) 29 - 4" vertical leaders required.

   Refer to Table 2: a 6" leader [2] will handle 424 G.P.M.
   4, 160 G.P.M. ÷ 424 = (9.8) 10 - 6" vertical leaders required.
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