Ponding Water Basics

A low-slope asphalt roofing system is cost effective, durable and reliable. Multiple layers of weatherproof membranes protect a building, its residents and the property it houses. There are a few design elements that will help building owners get the most from their roofing system. Managing ponding water is essential to properly maintaining a roof. 

Ponding water is defined as the water which remains on a roof 48 hours or longer. Water may accumulate on a low slope roof due to rain, snow or runoff from rooftop equipment. 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.

The adverse effects of ponding water on roofs can 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.

The Asphalt Roofing Manufacturers Association (ARMA) recommends 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.  Model building codes also require a minimum ¼” per foot slope for new construction projects, and require positive drainage for reroofing projects.  These requirements are intended to prevent water from ponding on roof surfaces. 

Here are a few best practices to manage ponding water:

·        Adequate sloping should be taken into account during the design process. A roof’s structural frame or deck should be sloped, and drainage components like roof drains and scuppers should be included in the design.

·        In addition, secondary (or emergency) drains may be required by local plumbing codes to help reduce the risk of a structural failure due to clogged drainage systems.  Talk to your roof membrane manufacturer and/or roof system designer to determine the proper location of these components.

·        If a deck does not provide the necessary slope to drain, a tapered insulation system can be used. A combination of different approaches—single slope, two-way slope, and four-way slope—is often used to achieve the necessary slope and to allow for moisture drainage. 

·        Additionally, crickets installed upslope of rooftop equipment and saddles positioned along a low-point between drains, can help prevent localized ponding in conjunction with a tapered insulation system.

·        Building designers and owners should work with contractors and roof manufacturers to determine which methods are best and appropriate for a roof assembly’s long-term performance, whether it’s a new construction or reroof project.

The NRCA Roofing Manual; Membrane Roof Systems—2015, states the following:

“NRCA recommends that designers make provisions in their roof designs for positive slope.

Slope generally is provided by:

• Sloping the structural framing or roof deck

• Designing a tapered insulation system

• Proper location of roof drains, scuppers and gutters

• A combination of the above”

By following the proper drainage practices above, building owners can positively impact their low slope asphaltic roofing system and help to ensure it will remain durable and reliable throughout its service life. 

To obtain specific information about ponding water on particular products and systems, contact your roof material manufacturer. Visit asphaltroofing.org for more information about low-slope asphalt roofing systems.