Ponding
Encyclopedia
Ponding is a term that refers to the unwanted pooling of water, typically on a flat roof
Flat roof
A flat roof is a type of covering of a building. In contrast to the sloped form of a roof, a flat roof is horizontal or nearly horizontal. Materials that cover flat roofs typically allow the water to run off freely from a very slight inclination....

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Cause

Most flat roof systems are designed with a slight pitch to shed water off the sides, usually into a scupper
Scupper
A scupper is an opening in the side walls of an open-air structure, for purposes of draining water. They are usually placed at or near ground level, and allow rain or liquids to flow off the side of the open-air structure, instead of pooling within the walls.There are two main kinds of 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
Bending
In engineering mechanics, bending characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element. The structural element is assumed to be such that at least one of its dimensions is a small fraction, typically...

 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.

Construction Codes

In the construction industry, the National Roofing Contractors Association
National Roofing Contractors Association
The National Roofing Contractors Association , established in 1886, is one of the construction industry's oldest trade associations in the United States....

 (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
International Building Code
The International Building Code is a model building code developed by the International Code Council . It has been adopted throughout most of the United States.-History:...

 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
Indianapolis
Indianapolis is the capital of the U.S. state of Indiana, and the county seat of Marion County, Indiana. As of the 2010 United States Census, the city's population is 839,489. It is by far Indiana's largest city and, as of the 2010 U.S...

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Other municipalities see this as a great concern, such as the Kapiti Coast District
Kapiti Coast District
The Kapiti Coast District is a local government district in the lower North Island of New Zealand 50 km north of Wellington.The district is named after Kapiti Island a prominent landmark 5 km offshore. The population of the district is concentrated in the chain of coastal settlements...

, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 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
Crocodile cracking
Crocodile cracking, also called fatigue cracking or alligator cracking, is a common type of distress in asphalt pavement. It is usually studied under the transportation section of civil engineering. Crocodile cracking is characterized by interconnecting or interlaced cracking in the asphalt layer...

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