The Evils of Ponding Water

What is ponding water?  Ponding water as defined by the NRCA, National Roofing Contractors Association, as water that remains on a roof surface longer than 48 hours after the termination of the most recent rain event.

Long-term ponding water on a roof surface is one of the most destructive conditions that can adversely affect the life cycle of a flat roof system.  The UBC, Uniform Building Code, and the IBC, International Building Code, require that a flat roof surface deck is provided with a minimal sloping of ¼” per foot for proper roof drainage.  Most buildings are built with that requirement, however, while this sloping is provided at the building framing stage some sloping is lost when the with the adding of additional construction components that typically are suspended from roof joists or trusses such as ceiling, plumbing and electrical systems. Heavy roof top equipment such as HVAC, refrigeration and air circulation systems may also contribute to loss of sloping to the roof deck.

There are several methods of correcting water ponding issues on a flat roof.  The most effective is to provide proper sloping with tapered insulation or re-decking with rip-strip framing.  For moderate to severe localized ponding, layering additional roofing membrane can produce limited success, however, care must be exercised not to just to move the ponding issue to another location of the roof.  Another method to correct moderate to severe ponding, is installing an additional surface drain recessed in to a sump box at the deepest point of the ponding.  This method requires the enlistment of a plumbing contractor for feasibility and installation of a drainpipe to the exterior of the building.

Most roofing material manufacturers will void their product warranties when ponding water conditions exist.  There are a limited type of roofing systems were ponding water does not affect the roof system life cycle or manufacturers warranties. The use of these systems should always be explored.

One should always consult with a qualified professional roofing contractor when dealing with ponding water conditions on a roof.   Budget restraints may leave both the building owner and roofing contractor with limited options on the decision to choose the lesser of two evils in choosing the better solution to the condition.

MT