Louisiana Administrative Code › Title 51 - PUBLIC HEALTH-SANITARY CODE › Part XIV - Plumbing › Chapter 18 - Appendices › Section XIV-1803 - Appendix A - Roof Drain Sizing Method

Current through Register Vol. 50, No. 6, June 20, 2024
A. Sizing Example. The following example
				gives one method of sizing the primary drain system and sizing the scuppers in
				the parapet walls. This method converts the roof area to an equivalent roof
				area for a 4-inch rate of rainfall so that Tables 1111.A and 1111.B of this
				code can be used as printed. The method described in §1111. B.2 of this code
				converts Tables 1111.A and 1111.B of this part to tables for the rate of
				rainfall specified for the area.
1. Problem:
				Given the roof plan in Figure 1803.A.1 of this Part and the site location in
				Birmingham, Alabama, size the primary roof drain system and size the scuppers,
				denoting the required head of water above the scupper for the structural
				engineer.
NOTE: For the purposes of this appendix the following metric
				  conversions are applicable:
1 in = 25.4 mm 1 ft = 305 mm 1
				ft2 = 0.0929 m2
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Figure 1803.A.1 - Example Roof
				  Plan
2. Solution:
Step a. From Figure 1111.E of this Part, the 100 year 60
				  minute precipitation is 3.75 inches per hour.
Step b. Each vertical drain must carry 2,500 sq ft of roof
				  area at 3.75 inches per hour of rainfall. To convert to an area for a 4 inch
				  per hour rainfall to enter LAC 51:XIV.Table 1111.A do this:
2,500 / 4 x 3.75 = 2,344 sq ft.
Enter LAC 51:XIV.Table 1111.A until you find a diameter pipe
				  that will carry 2,344 sq ft. A minimum 4-inch vertical drain is
				  required.
Step c. Horizontal Drain
2,500 sq ft.
To convert to an area for use in LAC 51:XIV.Table 1111.B do
				  this:
2,500 / 4 x 3.75 = 2,344 sq ft
Enter LAC 51:XIV.Table 1111.B until you find a diameter pipe
				  that will carry 2,344 sq ft.
A minimum 4-inch diameter pipe with a 1/4 inch per foot
				  slope will carry 2,650 sq ft. A minimum 4-inch diameter drain on a 1/4 inch per
				  foot slope is required.
Step d. Horizontal Drain
5,000 sq ft.
To convert to an area for use in LAC 51:XIV.Table 1111.B do
				  this:
5,000 / 4 x 3.75 = 4,688 sq ft.
Enter LAC 51:XIV.Table 1111.B until you find a diameter pipe
				  that will carry 4,688 sq ft. A 5-inch diameter pipe with a 1/4 inch per foot
				  slope will carry 4,720 sq ft. A minimum 5-inch diameter drain on a 1/4 inch per
				  foot slope is required.
Step e. Horizontal Drain
10,000 sq ft.
To convert to an area for use in LAC 51:XIV.Table 1111.B do
				  this:
10,000 / 4 x 3.75 = 9,375 sq ft.
Enter LAC 51:XIV.Table 1111.B until you find a diameter pipe
				  that will carry 9,375 sq ft. An 8-inch diameter pipe on 1/4 inch per foot slope
				  will carry 16,300 sq ft but a 6-inch will carry only 7,550 sq ft; therefore,
				  use an 8-inch diameter drain on a 1/4 inch per foot slope.
Step f. From Figure 1113.C of this Part, the rate caused by
				  a 100 year 15 minute precipitation is 7.2 inches per hour. The scuppers must be
				  sized to carry the flow caused by a rain fall rate of 7.2 inches per
				  hour.
Step g. Each scupper is draining 2,500 sq ft of roof area.
				  To convert this roof area to an area for use with LAC 51:XIV.Table 1803.A.2 do
				  this:
2,500 / 4 x 7.2 = 9,375 sq ft = 4,500 sq ft.
Enter LAC 51:XIV.Table 1803.A.2 to find a length and head
				  that will carry 4,500 sq ft or more. From LAC 51:XIV.Table 1803.A.2 a 12-inch
				  wide weir with a 4-inch head carries 6,460 sq ft. Use 12-inch wide x 5-inch
				  high scuppers at four locations. A height of 5 inches is needed to assure an
				  open area above the 4-inch head.
Step h. Notify the structural engineer that the design of
				  the roof structure must account for a height of water to the scupper entrance
				  elevation plus 4 inches for the required head to cause design flow.
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