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

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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