Did you know that 1 inch of ponding water can weigh more than 5.2 pounds per square foot? This means even a small area of standing water on your roof can carry significant extra weight from rain, snow, or other backed-up water due to clogged water outlets such as roof drains, scuppers, or even rain gutters. When this weight applies downward force due to gravity day after day, it can slowly stress the underlying structure causing permanent deflection of the roof deck and impacting its lifespan.
Low-sloped roofs for commercial spaces and contemporary homes alike are robust and consider live and dead load weight, ponding water on a continual basis is not part of that equation and over time can result in unwanted consequences.
What is Water Ponding?
When water fails to drain from a roof surface effectively, any standing water remaining after 48 hours is referred to in the industry as ‘ponding water. Normally, when it rains, the surface water should flow off the roof without being inhibited by low areas or depressions in the roof/roof deck. By Code newly designed structures are to have a roof slope of a minimum of ¼ inch per foot to help drain rainwater. Depending on the complexity of the building and roof deck design, often water must be diverted laterally to roof drains or roof scuppers. When improperly designed, crickets can be ineffective at properly directing water, allowing it to stand atop the roof surface.
Now, when the roof doesn’t have enough slope, or if something is blocking the water’s flow, the water cannot drain as it should. This might happen if the roof deck depressions or roof top penetrations, where the water can stagnate. When the water starts ponding on a flat roof it can result in damage to the various roof and other building components.
If you notice visible puddles forming after rain, dark stains where water pools regularly, or membrane surfaces may be blistered, or have algae buildup, it’s an indication that that there may be something wrong with either the drainage system or roof design. Since the water being accumulated is of a high volume, it’s unable to easily evaporate off the roof.
In case of a heavy downpour of rain, or a variety of other causes, it is not abnormal for water to stand for several hours, but when two full days have passed, it’s critical to reach out to a qualified roofing professional for assistance in evaluating the issue.
What Causes Ponding Water on Low-Sloping Roofs?
A rainstorm is not the only reason for water on a flat roof. Numerous issues could be contributing to roof-top drainage issues.
Improper Roof Replacement or Installation
Despite its name, no roof deck and roof membrane system can perform to its maximum potential and should be perfectly flat. Roof decks that are truly dead-level are older per-1970 constructions of heavy concrete design specifically intended to retain water atop the roof as most storm drain systems were not developed at that time to handle the water run-off from every building. It is critical to know that the Code no longer allows even existing roof designs to be dead-level and all must have positive drainage. If the slope and roof drainage design of the roof doesn’t have an adequate slope, surface water will not be able to move along the roof and reach the drainage outlet locations.
Choosing roofing materials that are specifically able to handle the water management design of the roof is important. Heavier-duty redundant systems such as three-layered modified bitumen systems are more adaptable for keeping water on the roof and out of the building’s interior.
Compressed Insulation
Have you ever noticed that when a home’s roof is used as a storage area for a bunch of equipment or large mechanical units, it more often than not, leads to a sagging roof deck? This can be caused by a couple of usual conditions. Roof designs with above-deck roof insulation can become compressed under all that load. Some low-slope roofs are not designed to handle the kind of weight that comes from heavy storage or excessive foot traffic unless they have been explicitly reinforced for it. This may be exacerbated when the water also starts pooling in these areas, this continuous exposure to the weight load can create depressions in the roof deck causing these to become permanent deflections. Conditions like these continue to get worse over time.
Improper Designed or Installed Roof Drains
Low-slope or low-pitched roofs rely on roof drains or scupper outlets to create a pathway for water to exit from the roof. If the installed flat roof drains are improperly sized or get clogged, water moving too slowly will not exit the roof as needed. If you have a larger commercial or industrial roof, it is critical that roof drains be properly sized and located to handle the amount of water.
Roof transitions can become challenging regarding roof drainage. When lower-sloped roofs interface with steep-sloped roof areas, the transitions between the two areas can be tricky for some inexperienced contractors. If this connection isn’t done correctly, the water may become trapped and fail to drain from the rooftop.
Building Movement or Settling
Older buildings tend to shift slowly because of weight, ground movement, age, and foundation settling. This changes the way the entire structure stands. If the roof was originally built with a slight slope for water drainage, the slope may change over time. Even if it’s an extremely small and unnoticeable movement, it can create noticeable problems on a low-sloped roof if not addressed. Some parts of the roof might dip lower than others and result in low spots where water ponds and deck deflection can increase over time causing greater issues.
Roof-Top HVAC Leaks
Roof-top mechanical units during warmer months of the operation can create significant condensation. Without a proper plumbing system to remove this condensation water, it is too often allowed to drain on the roof ultimately causing concerns and damage. Proper condensation piping is important to drain water and not simply allow it to gather atop the roofing system.
Restrictions Near Roof-top Structures
The areas around skylights, vent pipes, scuppers, parapet walls, roof hatches, and other structures is also prone to poor drainage and retaining water. When these features are installed, the process can sometimes cause depressions in the surrounding area because the base is raised somewhat higher than the adjacent surface. As a result, rainwater or snowmelt gathered there has nowhere to go. And if the material around the feature becomes misaligned or degrades, the dip will likely deepen even further.
How to Prevent Water Ponding on Roofs
Polyglass® offers an extensive range of high-end modified bitumen membranes for waterproofing low-slope and steep-slope roofs of all designs, styles, and sizes, all of which are engineered to endure even when drainage is not optimal. That said, we do recommend weighing material quality vs craftsmanship in your roof design to proactively reduce the risk of ponding, such as ensuring the structural design and slope of the structure are correct for conditions.
The placement of roof drains or other water outlets is ideal for the building and roof design. When needed utilize materials such as tapered insulation to correct slope issues. Areas at or around roof outlets can be reinforced with fluid-applied reinforced systems such as Polyflash 1C and Polyflash Polyester.
Our technical support team is always happy to help both homeowners and contractors with expert advice on how to avoid water ponding by considering roofing material quality vs. installation techniques and drainage solutions. Get in touch with us today.