Ponding is the unwanted pooling of water, typically on a flat roof.
Most flat roof systems are designed with a slight pitch to shed water off the sides, usually into a scupper system or into an internal drainage system. When a scupper or drain is clogged or fails for other reasons, storm water tends to pool around that low area. Over time, with each passing storm, the weight of the storm water will deflect the structural system beyond the structures bending point, thus allowing a bigger puddle to form. As a bigger puddle forms more weight is applied to the structural system causing more bending, allowing an even bigger puddle, then more weight, until the structure fails.
In the construction industry, the National Roofing Contractors Association (NRCA) defines roof ponding as "water that remains on a roof surface longer than 48 hours after the termination of the most recent rain event".
According the 2009 International Building Code Chapter 15 "Roof Assemblies and Roof Top Structures" & Chapter 16 "Structural Design";
"When scuppers are used for secondary (emergency overflow) roof drainage, the quantity, size, location and inlet elevation of the scuppers shall be sized to prevent the depth of ponding water from exceeding that for which the roof was designed ... Ponding instability. For roofs with a slope less than 1/4 inch per foot [1.19 degrees (0.0208 rad)], the design calculations shall include verification of adequate stiffness to preclude progressive deflection in accordance with Section 8.4 of ASCE 7."
Ponding on Land
When water is diverted into a lower area that has no outlet or is not suitable for drainage, water will begin to pool, and over time the weight of the water will create a deeper pool allowing more water to sit, eventually creating a permanent water feature. Some municipalities recognize this as an issue on private land, such as the City of Indianapolis.
Other municipalities see this as a great concern, such as the Kapiti Coast District, New Zealand where, "groundwater ponding is a chronic problem, that results in damp housing and water logged sections. The damage that it causes is less apparent than the damaging events associated with floods, but the duration of groundwater ponding, which can last for several months, makes it a serious issue for those affected".
Ponding that forms on paved surfaces, like streets or parkinglots that are not properly pitched will cause issues, such as, deep puddles and Crocodile cracking.
- "Investigation, analysis and design of an experiment to test ponding loads on flexible roof systems" by Duncan Stark, published by Oregon State University, June 2008
- "The Evils of Ponding Water" October 31, 2009, by Paul Graham
- 2009 international building code, by International Code Council in Books
- Urban planning - managing surface water and groundwater ponding" by Michelle Malcolm, Craig Martell, and Brydon Hughes, Sinclair Knight Merz Ltd. Kapiti Coast District Council 2008