Stephen Patterson and Madan Mehta - Unprecedented Change in Roof Drainage Design

By Colin Sheehan, RCS Reporter. 

The way roof drainage systems have been designed for over 70 years is proven incorrect. Learn what the entire contracting community missed for nearly a century. 

In Season 3, Episode 46 of Roofing Road Trips, Heidi J. Ellsworth meets with Stephen Patterson, RRC, PE and Dr. Madan Mehta, PE, PHD about the unprecedented change in roof drainage design. In this podcast, they share how they are responding to the shift out of a design that has been used for more than a half century. Until recently, the plumbing codes assumed that the flow through a roof drain is the same as the flow through a vertical leader of the same diameter. However, several investigations and roof-collapse forensics have shown that these assumptions are incorrect.  

Madan Mehta is a professor of architecture at the University of Texas at Arlington. He has been teaching in Arlington for 36 years and prior to that was the chair of architectural engineering at the University of Petroleum and Minerals in Saudi Arabia. Madan also wrote one of the most successful construction textbooks in the country. 

“My introduction with IIBEC and roofing in particular I will attribute it to Steve Patterson,” said Madan. “We worked together on a number of books. We did four editions of Wind Pressure monograph, and this is the second edition of Roof Drainage. And I must say that I've been very fortunate in having known Steve to have brought me into [roofing].” 

Their collaboration has been a recipe for success. The second edition of their book, Roof Drainage, provides a comprehensive drainage design manual that will benefit the entire industry. Most importantly, this book addresses the revelation of an error in previous drainage designs and provides a new solution. 

“The assumptions made on the drains, the flow rate through the drains were just simply wrong,” said Steve. “So, we've been designing these [drainage] routes for more than 70 years in a way that is no longer valid.” 

After discovering this, Madan and Steve went to work developing a new drainage monograph. Their second edition book, which is written like a textbook, is a practical resource for contractors, roofers, architects and engineers so they can design new routes with this additional information. The first edition, which came out in 2003, predicted some of these underlying issues, but failed to completely redesign the incorrect roof drainage systems. 

"What was known at that time (2003) was that the roof drains have virtually no role to play. They recognized the following fact, which we later on found was wrong, that the flow rate through a roof drain is the same as the flow rate through a vertical pipe,” said Madan, “The roof drains is important because it is the water from the roof drain that is the force behind all that goes into the horizontal pipes and all that goes into the vertical pipes, right? And the flow rate through a roof drain is not simply related to the outlet diameter of the roof drain, but [includes] many other factors that come into it.” 

Listen to the entire podcast to learn more about this roof drainage revelation that will change how roof drains are designed and understood for years to come.