Sizing

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 if drainage Roof load and safety overflow requirements Location of drains Size Vandal-proofing Local Code Requirements.

Steps for Selecting Proper Roof Drain Leader Sizes and Number Required for a Given Roof Calculate the total roof area. Determine the maximum hourly rainfall in inches.(The figure can be acquired from your local weather bureau and/or local code authority.) Select leader size. From Table 1, determine the number of square feet that can be drained by one roof leader at the local maximum rainfall rate. 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. Example: Total roof area - 500' by 270' equals 135,000 sq. ft. Determine rate of fall - -for this example use 4". After studying building plan and physical arrangement, assume that 6" leaders are required for this job. From Table 1 - one 6" leader at 4" rate of rainfall will take care of 13,500 sq. ft. of roof area. Number of roof leaders required is 10 (135,000 sq. ft. divided by 13,500 sq. ft.), thus 10 roof drains would be required. Note: It can readily be seen that if 4" leader were used, the number of roof drains required would increase to 30 drains. (29.348 rounded off to next highest number.) If a small number of roof drains are required, then larger leaders would have to be chose. 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.





Table 1

Roof Drain Vertical Leader Requirements For Horizontal Roof Areas at Various Rainfall Rates




Table 2

Steps for Calculating Drainage Requirements




Example

Flat Roof Using Smith 4" Fig. 1010 Roof Drain




Example

Flat Roof Using Smith 5" Fig. 1010 Roof Drain




Example

Flat Roof Using Smith 6" Fig. 1010 Roof Drain