--- title: 5 Steps To Calculate Total Roof Drains Needed description: The 5 steps you need to find out the number of roof drains needed for your commercial roofing project. image: https://resources.hy-techroof.com/hubfs/HTP_Resources-Steel-Head-Roof-Drains_600x400.jpg --- Search our inventory of drains, drain parts, safety and roofing equipment, and accessories. [Shop Now](https://hy-techroof.com/products/). - [ About ](https://www.hy-techroof.com/about-hy-tech/) - [ Contact ](https://www.hy-techroof.com/contact-hy-tech/) - - [![Hy-Tech Roof Products](https://resources.hy-techroof.com/hubfs/Logos/Hy-Tech%20Logo_rgb_maroon_480.png) ](https://www.hy-techroof.com) [**](https://www.hy-techroof.com/login.php) [**](https://www.hy-techroof.com/cart.php) ** ** [![Hy-Tech Roof Products](https://resources.hy-techroof.com/hubfs/Logos/Hy-Tech%20Logo_rgb_maroon_480.png) ](https://www.hy-techroof.com) [**](https://www.hy-techroof.com/cart.php) - - # 5 Steps To Calculate Total Roof Drains Needed August 15, 2022 [Roof Drains](https://resources.hy-techroof.com/blog/topic/roof-drains) [Subscribe](https://resources.hy-techroof.com/blog-subscribe)   ### 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](http://hy-techroof.com/manufacturers/zurn): 2, 3, 4, 5, 6, and 8 in. - [Josam](http://hy-techroof.com/manufacturers/josam): 2, 3, 4, 5, 6 and 8 in. - [Wade](http://hy-techroof.com/manufacturers/wade): 2, 3, 4 and 6 in. - [Smith](http://www.hy-techroof.com/manufactuers/j-r-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. ## Have a Drain Question? Have a question or in need of a specific roofing product or drain part? We have the resources to help you. **Reach out to the Drain Wizard.** [Call to Action](https://resources.hy-techroof.com/ask-the-drain-wizard) ![Ask the Drain Wizard](https://resources.hy-techroof.com/hubfs/2022%20Hy-Tech%20Website/Resources%20Pages/Drain%20Wizard%20CTA_800.jpg) ## Related Posts [![How To Purchase A Replacement Flat Roof Drain](https://resources.hy-techroof.com/hubfs/miscellaneous%20drains.png)](https://resources.hy-techroof.com/blog/how-to-purchase-a-replacement-flat-roof-drain) Aug 15, 2022 ###### [How To Purchase A Replacement Flat Roof Drain](https://resources.hy-techroof.com/blog/how-to-purchase-a-replacement-flat-roof-drain) ** [Roof Drains](https://resources.hy-techroof.com/blog/topic/roof-drains) Flat roof drains are often subjected to heavy weathering and regular wear and tear. 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[Continue Reading](https://resources.hy-techroof.com/blog/bid/56671/pros-and-cons-of-3-flat-roofing-systems) [![What Is A Retrofit Drain And How To Install One](https://resources.hy-techroof.com/hubfs/marathon-aluminator.jpeg)](https://resources.hy-techroof.com/blog/what-is-a-retrofit-drain-and-how-to-install-one) Aug 15, 2022 ###### [What Is A Retrofit Drain And How To Install One](https://resources.hy-techroof.com/blog/what-is-a-retrofit-drain-and-how-to-install-one) ** [Roof Drains](https://resources.hy-techroof.com/blog/topic/roof-drains) When reroofing or removing an existing drain, a replacement drain needed. This is where retrofit drains are used, as they replace the broken or corroded roof drains easily with almost no... [Continue Reading](https://resources.hy-techroof.com/blog/what-is-a-retrofit-drain-and-how-to-install-one) ![Background Pattern_Hy-Tech Logo_Gray](https://resources.hy-techroof.com/hubfs/2022%20Hy-Tech%20Website/Background%20Graphics/Background%20Pattern_Hy-Tech%20Logo_Gray.jpg) ## Find more resources Whether you’re in search of roof drain specifications, product catalog, or anything in between, we have the resources to answer your questions. 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