Is It Acceptable to Have Ponding On My Flat Roof?
By Allen Sensel
There is a lot of speculation with older building designs and how their flat roofs have a tendency to collect pools of standing water referred to as ponding. The purpose of this report is to provide clarity for a commonly misunderstood notion that a flat roof, whether it’s asphaltic built-up, modified bitumen, or any kind of single-ply membrane, should not bear any kind of collection of rain water. I will provide brief examples from the IBC, U.S. Department of Housing and Urban Development, and Factory Mutual, all of whom mandate construction and renovation standards to comply with regional climates,weather patterns, and safety.
Ponding water, as described by the IBC and recognized by the U.S. Department of Housing and Urban Development, is water that remains on a roof surface longer than 48 hours after the termination of the most recent rain event.
This explanation from the IBC can also point out that directly following persistent rainfall, water is not considered standing until after 48 hours of dry weather, after which, can evaporate efficiently at temperatures above 45 degrees Fahrenheit. Only when the same volume of water exists after this period can there be a concern for potential leaks, providing that the roof system was properly installed. The U.S. Department of Housing and Urban Development elaborates on how building inspectors approach ponding after an installation:
“48 hours before conducting a physical inspection, check the local weather forecast to see if precipitation is expected. If there has been precipitation within 48 hours prior to your inspection, use your judgement in deciding if observable ponding is due to the recent precipitation or because of an ongoing problem. Keep in mind that some flat roofs are designed to allow ponding.”
Concurrent with the size of the roof, Factory Mutual Global explains rain load on a dead flat roof:
“220.127.116.11.7 Roofs should be designed with positive drainage: however, dead-flat roofs consistent with this guideline are acceptable.”
An older building will lack modern specifications for framing slopes a minimum of 1/4” rise per 12” run. The solution to controlling water flow on a dead flat roof, if ponding is still a noticeable concern after following the aforementioned provisions, is designing a framed slope for positive drainage to scuppers and/or bowl drains.
“Definitions Supplement.” U.S Department of Housing and Urban Development Real Estate Assessment, October, 2001: 3
“Roof Loads for New Construction.” FM Global Property Loss Prevention Data Sheets, September 2006: 3-5
International Code Council. 2009 International Building Code. 2009