Ponding water is a common problem on low-slope or “flat” roofs, and asphalt roofs are no exception. In this blog, roofing company Wood’s Roofing takes a look at the effects of ponding water on flat asphalt roofs and how it can be addressed.
How Does Ponding Water Form?
Ponding water is defined as water that stays on the roof for 48 hours or longer and can come from rain, snow, or runoff from HVAC drainage or some other rooftop equipment. Regardless of the roofing type, ponding water is never a good sign. It has adverse effects on flat asphalt roofs:
Deformation of the Deck or Substrate — It may not look like much, but ponding water can increase the live load (the downward force exerted by impermanent objects) on the roof, which makes the roof much heavier. If the force exceeds the roof’s maximum load limit, the deck or substrate can get deformed or damaged. A capable roofing contractor can conduct repairs,
Damage to the Roofing Surface — Flat asphalt roofs are similar to asphalt shingles, in that the outer layer is protected by granules. Ponding water, combined with extreme temperature changes, can damage the roof by wearing out adhesion between granules and expose the asphalt-soaked backing material.
Algae Growth — Long-standing ponding water allows algae and vegetation to grow. Certain types of algae feed on the limestone content of asphalt roofing and could accelerate wear. Vegetation can also clog the drains.
How to Manage Ponding Water
Managing ponding water begins with proper roof design. All types of flat roofs are not perfectly horizontal but are slightly sloped towards the drains. Secondary drains may also be added to the roof as permitted by local building codes.
While ponding water can be managed by regularly performing cleaning and draining the roof, this is in no way a permanent solution. Scrubbing the roof may accelerate wear on its protective layers. Unless the roof is up for replacement, wherein slope issues can be corrected, tapered roof insulation, along with crickets installed on the upslope, can help minimize ponding water.