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.

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.