Ponding is the (typically) unwanted pooling of water, typically on a

flat roof A flat roof is a roof which is almost level in contrast to the many types of sloped roofs. The slope of a roof is properly known as its pitch and flat roofs have up to approximately 10°. Flat roofs are an ancient form mostly used in arid c ...

or roadway. Ponding water accelerates the deterioration of many materials, including seam adhesives in single-ply roof systems, steel equipment supports, and particularly roofing asphalt. On low-slope asphalt roofs, ponding water allows the oil solvent components of the asphalt to leach out and evaporate, leaving the roof membrane brittle and susceptible to cracking and leaking in the ponding location. The time taken for water to saturate a zone, usually from

rainfall Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water ...

, causing a pond to form, is referred to as the "ponding time" or "time of ponding".

Cause

Most flat roof systems (properly called "low-slope roof systems") are designed with a slight pitch to shed water off the sides, usually into gutters,

scuppers A scupper is an opening in the side walls of a vessel or an open-air structure, which allows water to drain instead of pooling within the bulwark or gunwales of a vessel, or within the curbing or walls of a building. There are two main kinds of ...

, internal drains, or a combination of these. 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 structure's

bending In applied mechanics, bending (also known as flexure) 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 ...

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 (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 to the 2009

International Building Code The International Building Code (IBC) is a model building code developed by the International Code Council (ICC). It has been adopted for use as a base code standard by most jurisdictions in the United States. The IBC addresses both health and s ...

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 .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 (), colloquially known as Indy, is the List of U.S. state and territorial capitals, state capital and List of U.S. states' largest cities by population, most populous city of the U.S. state of Indiana and the county seat, seat of ...

. Other municipalities see this as a great concern, such as the

Kapiti Coast District The Kapiti Coast District is a local government district of the Wellington Region in the lower North Island of New Zealand, 50 km north of Wellington City. The district is named after Kapiti Island, a prominent island offshore. The popu ...

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island country ...

where, "groundwater ponding is a chronic problem, that results in damp housing and waterlogged 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".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 Ponding that forms on paved surfaces, like streets or parking lots that are not properly pitched, will cause issues such as deep puddles and

crocodile cracking Crocodile cracking, also called alligator cracking and perhaps misleadingly fatigue cracking, is a common type of distress in asphalt pavement. The following is more closely related to fatigue cracking which is characterized by interconnecting ...

References

{{Pond Construction

Cause

Construction codes

Ponding on land

See also

References