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5 Steps to Calculate TotalRoof Drains Needed

There are 5 easy steps to figure out how

many drains your commercial roofing

project will need.

Step 1: Calculate totalroof 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:

Step 3: Find rainfall rate

Find the rainfall rate for the buildings 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 ageneral reference

Step 4: Find the area handled by one drain

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7/25/2019 5 Steps to Calculate Total Roof Drains Needed

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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 23,05017,30013,84011,530 8,650 6,920 5,765 4,945 4,325

6 54,000 36,00027,00021,60018,00013,50010,800 9,000 7,715 6,750

8 116,00077,40058,00046,40038,66029,00023,20019,31516,57014,500

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

only.

Step 5: Calculate drains needed

Take the roofs 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.