Roofing Materials Can Play a Role in Addressing Ponding Rain Water

Ponding water is water that remains on a roof for over 48 hours. This may be rain water, runoff from rooftop equipment like HVAC units, skylights, and vent stacks, or melting snow/ice. Ponding should be addressed the moment you notice it to avoid deflection/deformation of the roof due to the extra load, to prevent ice damage and growth of vegetation and algae which can cause clogging and chemical/physical damage, to prevent insect infestation and the roof from acting as a magnifying glass to the sun’s UV rays which can damage the surface, and to prevent water infiltration.

It is recommended that you install your flat roof with a slope (with the minimum recommended slope being ¼” per foot) to reduce the risk of ponding. Ensuring drains remain unobstructed and connection of HVAC condensate lines to proper drains help prevent ponding. Your choice of roofing material can also help address the problem. 

In vulnerable areas such as penetration areas, water ponding cannot be addressed simply by diverting water flow. Property owners should work with roofing providers to install reinforced materials. The factors to consider when picking reinforced materials are:

  • Specific gravity: The chemical composition of the roofing material, particularly its specific gravity, porosity, adhesion, and tensile strength impacts its ability to shed water. Go for roofing materials whose specific gravity is higher than that of water, that have low porosity, high tensile strength, and that has superior adhesion.

  • Thickness: The thickness of the membrane is an important factor because the polymer deteriorates over time when exposed. The deterioration is even quicker when there is ponding water. Select a thick membrane in areas that are susceptible to ponding to increase the life of the roof.

  • Color: It might not seem like it, but the color of a roof has an impact on ponding. Darker colors like dark grays have higher heat retention capabilities compared to white and other lighter colors, which means water evaporates quicker.

  • Seamless materials: Some materials come in large sheets, which allows for a seamless roof. Go for a roofing contractor who avoids seams as these are weak points. A good contractor will also reinforce areas where sheets are overlapping.

  • Reinforcement: A layer of polyester reinforcement on the membrane helps strengthen it and reduces the risk of water penetration. The investment is well worth it given the possible impact of ponding.

  • Identify and fix low spots: Do a regular roof inspection to avoid low spots. A roofing contractor can help fix such depressions with roofing plaster.

  • Adherence to code: The roof should comply with the International Building Code (IBC) when it comes to the materials, design, and installation and the International Plumbing Code when it comes to the drainage. The IBC is very detailed on how roofing should be done and all good roofing contractors adhere to it.

  • Manufacturer and workmanship warranties: The best time to identify and rectify the problem is immediately the roof is installed since it will prevent the exacerbation of the problem. The roof will most likely also be under manufacturer and workmanship warranty.

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