The Effects of Ponding Water
Ponding water can have major negative consequences, regardless of the type of roofing system. Proper design, installation and maintenance of roofing structures can prevent this condition and its associated problems.
Ponding water is defined as the water which remains on a roof 48 hours or longer. The Asphalt Roofing Manufacturers Association has been joined by many reputable organizations, such as the National Roofing Contractors Association, the Midwest Roofing Contractors Association, and the American Institute of Architects, in recommending that roof designs provide adequate slope (min. ¼” per foot) to ensure that the roof drains freely throughout the life of the building and to thereby avoid the effects of ponding water.
The known adverse effects of ponding water on roofs include:
- Deformation of the deck structure: Ponding water can substantially increase the load on roof decks. As water accumulates, deck deflections can increase, thereby resulting in additional ponding water which could compromise the structural integrity of the deck.
- Damage to the roof surface: Ice formations develop and move constantly with changes in temperature. This movement can “scrub” the roof membrane to such an extent that considerable physical damage to the membrane can occur.
- The growth of algae and vegetation: When water stands for long periods of time, algae and vegetation growth will likely occur, and may cause damage to the roof membrane. Additionally, vegetation and other debris can clog drains and cause additional ponding.
- Accumulation of dirt, debris and other contaminants in the ponding area: These elements can affect and damage the membrane surface.
Ponding water may lead to accelerated erosion and deterioration of the membrane surface that can result in failure of the roof system. Allowing even relatively small amounts of moisture beneath the roof membrane may reduce the thermal efficiency of the insulation. More importantly, moisture intrusion can cause serious damage to the deck, insulation, and membrane as well as the building’s interior.
To obtain specific information regarding the effects of ponding water on particular products and systems, contact the individual roofing material manufacturer.
*DISCLAIMER OF LIABILITY: This document was prepared by the Asphalt Roofing Manufacturers Association and is disseminated for informational purposes only. Nothing contained herein is intended to revoke or change the requirements or specifications of the individual roofing material manufacturers or local, state and federal building officials that have jurisdiction in your area. Any question, or inquiry, as to the requirements or specifications of a manufacturer, should be directed to the roofing manufacturer concerned. THE USER IS RESPONSIBLE FOR ASSURING COMPLIANCE WITH ALL APPLICABLE LAWS AND REGULATIONS.
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