5 Steps to Calculate Total Roof Drains Needed

There are 5 easy steps to figure out how many drains your commercial roofing project will need.

Step 1: Calculate total roof area

Calculate the total roof area by multiplying its length by width. Example:  A 200 ft. x 250 ft. = 50,000 sq. ft. area.

Step 2:  Determine leader size

Determine leader or pipe size. Following are the standard sizes for some of the top manufacturers:

  • Zurn: 2, 3, 4, 5, 6, and 8 in.
  • Josam: 2, 3, 4, 5, 6 and 8 in.
  • Wade: 2, 3, 4 and 6 in.
  • Smith: 2, 3, 4, 5, 6 and 8 in.

Step 3: Find rainfall rate

Find the rainfall rate for the building’s location. The most accurate and up-to-date resource for rainfall rates is going to be your local codes.

However, there are resources like the International Code Council that list the rainfall rates for larger cities in all 50 states. This should be used only as a general reference 

Step 4: Find the area handled by one drain

Using the chart below, find the total square footage one drain can handle based on hourly rainfall rate.

Leader / Pipe Size (in.) Hourly Rainfall (in.)
1 1.5 2 2.5 3 4 5 6 7 8
Total Sq. Ft. Covered Per Drain
2 2,880 1,920 1,440 1,150 960 720 575 480 410 360
3 8,800 5,860 4,400 3,520 2,930 2,200 1,760 1,470 1,260 1,100
4 18,400 12,700 9,200 7,360 6,130 4,600 3,680 3,070 2,630 2,300
5 34,600 23,050 17,300 13,840 11,530 8,650 6,920 5,765 4,945 4,325
6 54,000 36,000 27,000 21,600 18,000 13,500 10,800 9,000 7,715 6,750
8 116,000 77,400 58,000 46,400 38,660 29,000 23,200 19,315 16,570 14,500

 

This table is based on Zurn's Roof Drain Vertical Requirement for Horizontal Roof Areas at Various Rainfall Rates table. The above sizing data is offered as a guide only. 

Step 5: Calculate drains needed 

Take the roof’s total square footage and divide by the total square footage handled by one drain. The result is the number of drains needed.

For example: 50,000 / 4,400 = 11.36; or 12 drains required.