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How to Remedy & Prevent Ponding Water on a Flat Roof

February 24, 2023 6:36 pm

How to Remedy and Prevent Ponding Water on a Low Slope Roof


Ponding water on flat roofs becomes detrimental to the roof assembly after only 48 hours, reports the National Roofing Contractors Association (NRCA). Though you can’t completely avoid ponding water because flat roofs don’t have a steep enough slope, what can you do to protect your low slope roof system?

Learn why ponding water occurs on flat and low slope roofs and the associated dangers. More importantly, learn how to solve this issue fast without re-roofing and how to prevent ponding water in the future.

Why Ponding Water Accumulates on Roofs

Ponding water accumulates on flat roofs from rain, snow, or runoff from rooftop equipment like HVAC units.

The core problem is that the minimal slope does not always allow adequate movement of water to drains and scuppers. The presence of drains and scuppers isn’t enough, though. Drain systems must be cleaned and maintained to avoid getting clogged by leaves, branches, mud, or other foreign bodies, as clogs will impede the flow of water. Even with these best practices, not all ponding water issues are as simple to fix as unclogging the drain systems.

If the low slope/flat roof doesn’t have a proper slope, you are likely to experience ponding water because the rainwater is not drawn towards the drain systems by gravity.* 

Even if the roof has a correct slope, ponding water can still happen with penetrations. Curbs, mechanical equipment, or structures on the roof can impede water flow, and create areas for water to collect. The underlying structure supporting the roof can deflect (bend) and if structure deflection occurs, the flat roof membrane will sag, resulting in a pool of water after the next heavy rain. 

Another common source of ponding is compressed insulation beneath the flat roof surface. Installing heavy equipment and conducting intensive work on low slope roofs may cause localized insulation compression. This disrupts proper roof surface elevation and creates perfect spots for ponding water.

A bending structure, crushed insulation etc. are the negative effects of not correcting the ponding issues. A substrate should have at least a 0.25 in (6.4 mm) slope per foot to promote positive drainage before coating application.

The Danger of Ponding Water

  • The roof deck can deflect from ponding water, compromising the building’s structural integrity.
  • Frequent icing and thawing of ponding water can damage most flat roof types.
  • Algae and vegetation growth in standing water can damage flat roof membranes.
  • The ponding area accumulates dirt and debris that can also damage the membrane.
  • If the roof membrane is damaged, the water can leak through, negatively impact the insulation, and cause irreparable damage to the roof’s structural members.

Gaco Solutions for Fixing Ponding Water on a Flat Roof

If your roof has low areas where water ponds due to deck deflection, compressed insulation, or bad roof installation, you may not have to replace the entire roof. GacoRoofFoam is just one solution for addressing ponding water in these low areas. It can help properly divert water and improve the roof’s surface elevation consistency. 

GacoRoofFoam is a spray-in-place polyurethane foam that expands into sturdy, monolithic roof insulation once sprayed on a new or existing roof. You can use GacoRoofFoam to correct deflections and build up crickets necessary for water flow. GacoRoofFoam adheres to most substrates and can accommodate any roof shape and geometry.

Roof foam insulation still needs a protective coating layer. Gaco acrylic, silicone, or urethane roof coatings protect the foam and add reflectivity, which improves the building’s energy efficiency. 

Together, Gaco’s insulation foam and roof coatings can provide a complete roof system.

Roof Replacement Vs. Gaco Spray Foam and Coat Roofing System

Traditional tear-off and re-roofing jobs often require large crews, complex equipment, and a lot of new materials. The old roof must be torn off and disposed of, which increases landfill contribution and incurs disposal costs. Additionally, sometimes it’s necessary to disrupt road traffic if you are replacing a roof in an urban area. Boom trucks delivering fresh materials and removing the old roof are challenging to coordinate in dense urban environments. City regulations might require road closures or extra permits (and the additional costs incurred from that).

If your roof hasn’t suffered severe damage from ponding water, you can use Gaco’s sprayfoam and roof coating system to re-cover it. It’s significantly less costly than the traditional tear-off and replacement, requires a smaller crew, and has a lower staging footprint. Plus, not removing the current roof protects the building in case of sudden rain. 

Traditional tear-off/re-roof will disrupt the tenants and commercial activities. But applying GacoSprayFoam and a protective coating often allows continuing your day-to-day activities while the installation takes place. For example, Prince George’s County building saved thousands of dollars by restoring a leaking roof with GacoRoofFoam and the GacoFlex Silicone System. You can read many such success stories in our case studies.

If the ponding water area on your roof is not large, you don’t need to apply GacoRoofFoam over the entire roof. Instead, you can apply GacoRoofFoam as necessary to raise the deflected areas and provide an adequate slope. You can also use the GacoFlashFoam Kit for localized applications.

Gaco Coatings To Pair With GacoRoofFoam

The leading roof coatings used today are acrylic, silicone, and urethane. Each has advantages and disadvantages, so there isn’t a one-size-fits-all solution. Fortunately, Gaco is here to help you choose the right solution for your project needs. The most appropriate coating for your roof depends on the climate, building use, aesthetics, roof traffic, and other factors. 

After using the GacoRoofFoam to create an appropriate slope for water drainage, you will need to apply one of the following coatings. Request a free roof consultation, and we’ll help you narrow down your options. Depending on installation factors, a Gaco Licensed Applicator can provide up to a 20 year labor and material warranty.

Acrylic Coating

Acrylic roof coatings have an excellent cost-to-performance ratio compared to traditional re-roofing. They are highly reflective and can significantly reduce your building’s cooling demands. These coatings are water-based and should be used in dry, high heat, and low humidity environments. 

Acrylic coatings can be washed with plain soap and water to restore surface reflectivity if they get dirty. Additionally, these coatings also shed their top layer, which helps keep them shiny. Since they do shed their top layer over time, a proper maintenance schedule is suggested to ensure dry film thickness.

Given that only about 50% of roof surfaces installed onto industrial and low-slope commercial buildings in the US each year are reflective, acrylic elastomeric coatings are a smart option for turning a non-reflective roof into a reflective one. The use of reflective roofing is a widely recognized adaptive response to global warming, and urban heat island mitigation. 

Silicone Coating

Silicone roof coatings resist permanent ponding water and UV rays. They provide reflectivity to improve the building’s energy efficiency and don’t lose dry film thickness. Most substrates don’t require a primer for silicone coating, especially if using GacoFlex S42 coating**. 

Since silicones don’t contain water, they have a wide application window — you can apply them in temperatures from 32°F to 120°F. When applying in higher traffic areas, you have the option to apply GacoFlex WalkPad to create durable and visible walkways.

Urethane Coating

Urethane coatings offer maximum tensile strength and durability among roofing coatings. They are an excellent choice for high-traffic roofs. Plus, they have exceptional resistance to hail and extreme weather cycles.

Gaco’s urethane business dates back to the 1960s. We have a long history with this chemistry in decking and waterproofing, and with these strong performance attributes we’ve expanded the applications to include roofing, as well.

Urethane coatings are reflective and resist dirt pickup well, making them a good solution for improved energy efficiency with lower maintenance. Additionally, they are UV resistant, have high flexibility, adhere to most substrates, and won’t yellow over time.

Contact Gaco Today

Gaco’s sprayed insulation foam can help you correct your flat roof’s low areas and get rid of ponding water. Pair Gaco foam with a suitable coating, and you can save significant resources and valuable time compared to the traditional tear-off/re-roofing job. Plus, offering a simple solution to building owners can help you win more jobs if you are a contractor. 

Find your local Gaco area manager to learn more and get the best solution that meets your job requirements.


*We always recommend remediation on a roof without proper slope before coating it.

**Always perform an adhesion test to ensure compatibility.