ORIGIN
The modern roof drain must be designed to drain off rain water
in the most effective manner possible while maintaining an esthetic
appeal since it is placed in full view of the public in many instances.
HOW
TO SELECT A ROOF DRAIN
To
select the proper roof drain, the following information must be
determined by the specifier:
-
Type
of roof construction
-
Roof
pitch
-
Volume
of expected rainfall
-
Desired
rate of drainage
-
Roof
load and safety overflow requirements
-
Location
of drains
-
Size
-
Vandal
proofing
SELECTION
OF ROOF DRAIN BODY
For a heavy rainfall region use 12" diameter type for large or
small roof areas. For a light rainfall region use 8 1/2" diameter
type for small roof areas.
STEPS
FOR SELECTING PROPER ROOF DRAIN LEADER SIZES
1 . Calculate the total roof area.
2.
Determine the maximum hourly rainfall in inches. (The figure
can be acquired from your local weather bureau and or local
code authority.)
3.
Select leader size.
4.
From table 1, determine the number of square feet that can be
drained by one roof leader at the local maximum rainfall rate.
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.
NOTE:
It can be readily seen that if 4" leaders were desired the number
of roof drains required would increase to 22 drains. If a small
number of roof drains are required then a larger leader would
have to be chosen. Several small drains and leaders rather than
one or two large drains will ensure even, safer and adequate roof
drainage. Drains should be spaced for even drainage.
STEPS
FOR CALCULATING TOTAL G.P.M.
1.
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 Example: 4" rainfall with a roof area of 30,000
square feet .0104 x 4" x 30,000 = 1248 G.P.M.
Allowable
flow in G.P.M.
|
Pipe
Size
|
Vertical
Leader
|
Horizontal
Storm Drain
Slope
Per Foot
|
|
2
|
30
|
12
|
17
|
24
|
|
3
|
90
|
36
|
51
|
72
|
|
4
|
192
|
78
|
111
|
157
|
|
5
|
348
|
142
|
201
|
284
|
|
6
|
566
|
231
|
327
|
462
|
|
8
|
1220
|
498
|
705
|
996
|
|
10
|
2200
|
902
|
1275
|
1804
|
|
12
|
-
|
1467
|
2076
|
2934
|
|
15
|
-
|
2666
|
3774
|
5332
|
|
Roof
drain vertical leader requirements for horizontal roof areas
at various raninfall rates.
| Leaders |
Size
|
Hourly
Rainfall in Inches
|
|
|
Pipe
Size
Inches
|
Open
Area
Sq. In.
|
|
1
|
1
1/2
|
2
|
2
1/2
|
3
|
4
|
5
|
6
|
7
|
8
|
Roof
Area Sq. Ft.
|
|
2
|
3.14
|
2,880
|
1,920
|
1,440
|
1,150
|
960
|
720
|
575
|
480
|
410
|
360
|
|
3
|
7.06
|
8,880
|
5,860
|
4,400
|
3,520
|
2,930
|
2,200
|
1,760
|
1,470
|
1,260
|
1,100
|
|
4
|
12.56
|
18,400
|
12,700
|
9,200
|
7,360
|
5,130
|
4,500
|
3,680
|
3,070
|
2,630
|
2,300
|
|
5
|
19.60
|
34,600
|
23,050
|
17,300
|
13,840
|
11,530
|
8,650
|
6,920
|
5,765
|
4,945
|
4,325
|
|
6
|
28.30
|
54,000
|
36,000
|
27,000
|
21,500
|
18,000
|
13,500
|
10,800
|
9,000
|
7,715
|
6,750
|
|
8
|
50.25
|
116,000
|
77,400
|
58,000
|
46,400
|
38,680
|
29,000
|
23,200
|
19,315
|
15,570
|
14,500
|
| NOTE:
The above table is for leader sizes. Select drains with
adequate open free area in proportion to the leader sizes
consistant with code requirements. Based on National Plumbing
Code. |
|
