HOW TO SELECT A ROOF DRAIN |
To select the proper roof drain, the following information must be determined
by the designer/specifier.
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• 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. |
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SUGGESTED STEPS FOR SELECTING PROPER ROOF DRAIN LEADER
SIZES AND NUMBER REQUIRED FOR A GIVEN ROOF |
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Steps:
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Example: Using a 4" Vertical Leader |
Example: Using a 6" Vertical Leader |
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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. |
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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". |
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3. Select leader size.
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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. |
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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. |
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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. |
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ROOF DRAIN VERTICAL LEADER REQUIREMENTS FOR HORIZONTAL
ROOF AREAS AT VARIOUS RAINFALL RATES |
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TABLE 1 |
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STEPS FOR CALCULATING DRAINAGE REQUIREMENTS
FOR ABOVE EXAMPLE
USING G.P.M. |
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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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