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Downspout Sizing Chart Table 10.2. Sizing Roof Drain, Leaders, and Vertical Rainwater Piping (IAPMO, 2000)

This document provides sizing charts for roof drains, leaders, and vertical rainwater piping based on maximum allowable horizontal projected roof areas at various rainfall rates. The charts list pipe sizes from 2 to 8 inches and 50 to 200 mm and the corresponding maximum roof areas and flow rates. Notes explain that the sizing data assumes the pipes will flow 7/24 full and how to determine allowable roof areas for non-listed rainfall rates. It also provides guidelines for sizing non-circular pipes.

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750 views1 page

Downspout Sizing Chart Table 10.2. Sizing Roof Drain, Leaders, and Vertical Rainwater Piping (IAPMO, 2000)

This document provides sizing charts for roof drains, leaders, and vertical rainwater piping based on maximum allowable horizontal projected roof areas at various rainfall rates. The charts list pipe sizes from 2 to 8 inches and 50 to 200 mm and the corresponding maximum roof areas and flow rates. Notes explain that the sizing data assumes the pipes will flow 7/24 full and how to determine allowable roof areas for non-listed rainfall rates. It also provides guidelines for sizing non-circular pipes.

Uploaded by

ASHOK SUTHAR
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Downspout Sizing Chart

Table 10.2. Sizing roof drain, leaders, and vertical rainwater piping (IAPMO, 2000).
Drain, leader, Maximum allowable horizontal projected roof areas in
Flow
or pipe size square feet at various rainfall rates
Inches gpm 1 in./hr 2 in./hr 3 in./hr 4 in./hr 5 in./hr 6 in./hr
2 23 2,176 1,088 725 544 435 363
3 67 6,440 3,220 2,147 1,610 1,288 1,073
4 144 13,840 6,920 4,613 3,460 2,768 2,307
5 261 25,120 12,560 8,373 6,280 5,024 4,187
6 424 40,800 20,400 13,600 10,200 8,160 6,800
8 913 88,000 44,000 29,333 22,000 17,600 14,667
25 50 75 100 125 150
Millimeters L/S
mm/hr mm/hr mm/hr mm/hr mm/hr mm/hr
50 1.5 202 101 67 51 40 34
80 4.2 600 300 200 150 120 100
100 9.1 1,286 643 429 321 257 214
125 16.5 2,334 1,117 778 583 467 389
150 26.8 3,790 1,895 1,263 948 758 632
200 57.6 8,175 4,088 2,725 2,044 1,635 1,363
Notes: 1. The sizing data for vertical conductors, leaders, and drains is based on the pipes flowing
7/24 full. 2. For rainfall rates not listed, determine the allowable roof area by dividing the area
given in the 1 inch/hour (25 mm/hour) column by the desired rainfall rate. 3. Vertical piping may
be round, square, or rectangular. A square pipe should be sized to enclose its equivalent round
pipe. A rectangular pipe should have at least the same cross-sectional area as its equivalent round
pipe, except that the ratio of its side dimensions should not exceed 3 to 1.

These charts originally appeared in a Texas A&M AgriLife publication, B-6240, Rainwater
Harvesting: System Planning (2010), By Justin Mechell, Billy Kniffen, Bruce Lesikar, Douglas
Kingman, Fouad Jaber, Rachel Alexander, and Brent Clayton. The full publication is available
in print at AgriLifeBookstore.org.

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