Roof Ponding

Roof Ponding is the excessive accumulation of water on low lying areas of roof after rainfall. On low-slope or flat roofs, it may cause a deflection of the roof’s underlying structural components.

Low-slope roof systems are intended to be water-impermeable coverings. A waterproofing system, on the other hand, is designed to withstand hydrostatic pressure resulting from standing or ponding water. Roof systems generally are not designed as or intended to service as waterproofing systems.

The National Roofing Contractors Association suggests that no standing water remain on a roof membrane longer than 48 hours and considers ponding water on any roof unacceptable.

Ponding water on a roof causes leaks, algae and fungi growth. If left unattended, roof ponds can lead to the collapse of a roof.

Ponding Water can be detrimental to most roof membranes.

The size of roof ponds increases over time because the weight of the water causes additional deflection of the structural membranes underneath (above left). Eventually, the structure may not be able to carry the increased load. An infrared moisture survey will oftentimes reveal larger areas of wet insulation in the vicinity of the ponds (above right and inset).