Ponding Water – How Much is Too Much

May 15, 2020
01

Ponding water on flat roof systems is one of the most misunderstood topics. Opinions range from, “It’s a flat roof, it’s meant to hold water,” to, “No water should ever sit on a flat roof for more than 24 hours.” The truth is in the middle.

As a general rule of thumb, most single-ply manufacturers require 80% of water to be off of a roof within 48 hours of a rain event. So, if there are a few places throughout the roof that have some ponding water after this 48-hour point, it is within the design of the roof system.

On most coating systems, ponding water is less flexible. Requirements vary, but generally, if ponding water exists, coating warranties either exempt these areas or require a more expensive and more heavy-duty flashing grade coating to be used.

Asphalt roof systems tend to be in the middle. The 80% rule within 48 hours of a rain event still applies, but they typically prefer ponding to be as limited as possible.

So what can be done to reduce ponding water? There are a few options:

  • Consider a full or partial tear off, even if only 1 roof exists. Ponding water can be created from wet or damaged insulation, and so removing and replacing these areas can help.
  • Consider a tapered roof system. Tapered insulation has a slight pitch to it, typically 1” over 8’, 1” over 4’, or 1” over 2’. While this sounds small, it makes a big difference on completely flat roofs that hold water. The downside of tapered insulation is that it is very expensive. Creating a pitch like this on a flat commercial roof system can increase the amount of insulation used dramatically. Additionally, installing a tapered roof system is like installing a puzzle, each piece has a place.  So, installing a tapered roof system can add a substantial amount to the cost. Typically, it adds 30% to 50% to the total amount of the project.
  • Add additional drains. If interior plumbing can be added easily, installing additional drains is one of the easiest and most cost-effective solutions, especially if ponding water is concentrated in a few areas. Basically, we would install a new drain with sumped insulation. This makes the roof area slightly lower around the drain. Then, a licensed plumber would connect the drain inside the building.
  • Add a scupper box. If the ponding water is located near the edge of the building, or if interior plumbing is not an option, consider a scupper box and downspout. This would involve installing a new scupper box, downspout, and sumped insulation near the edge. Sumped insulation basically means that the insulation is lower around the scupper box than the surrounding roof, which helps promote drainage. If needed, we can also cut in a larger relief, so that water has a path to the downspout.

These are the most common steps that we at Chaffee Roofing take to reduce ponding water. If you have an issue with ponding water and want to review ideas, reach out to us and we would be happy to review your options!

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One Comment

  1. Adrian May 19, 2020 at 8:59 am - Reply

    Now this is a VERY interesting post. Especially this….
    “As a general rule of thumb, most single-ply manufacturers require 80% of water to be off of a roof within 48 hours of a rain event. So, if there are a few places throughout the roof that have some ponding water after this 48-hour point, it is within the design of the roof system”
    Thats a real gem. Thank you for posting this. Seriously good information

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