Step by Step Instructions
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.
Hourly Rainfall (in.)
| 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.
