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Ponding Water On Flat Roofs; Common Problems & Best Solutions

Water accumulation on a roof can lead to undesirable consequences for your property…

If you’re wondering how to drain water from a flat roof, there’s a high likelihood that you have sustained water retention issues that need to be dealt with. 

Unfortunately, the fix is not always straighforward, so let’s first define this problem then discuss frequent causes, solutions, and problems that can occur if you leave it untouched for too long.

What Is Water Ponding?

water ponding up on a tar and gravel flat roof after a rain storm
Ponding water on a flat roof as seen above can cause major damage to your home or commercial property.

Water ponding is when water builds up and stagnates on the surface of a roof for longer than 48 hours. Large puddles can form on the roof surface because the drainage system is not adequate for the structure, or due to a variety of other issues with the roof design or the building infrastructure. Because there is a high volume of liquid, it cannot evaporate off the roof and the stagnant water has nowhere to drain.

Standing water on a flat roof for 12-36 hours after a rain storm can be normal, but according to the National Roofing Contractors Association, any water found ponding on flat roofs for longer than 2 full days should be examined by a professional commercial roofing contractor.

The forces of gravity make significant water buildup on flat roofs a problem because the only place water can eventually go is down. Ponding water poses a significant risk to the flat roof system over time and must be fixed in order to protect the structural integrity of the entire building.

Importance of roofing material quality vs. craftsmanship

According to Martin J. Fradua of the University of Massachusetts, most roof membranes and roofing materials are water resistant and not completely waterproof. This is a critical point to understand when considering potential damage due to water accumulation. Even though most flat roof membranes seal water out to some extent, they can only hold up so long, especially of heavy water volumes are sitting on them.

This means that even the best materials will not help you avoid the ponded water problem. Roof design is more important, which is why we recommend choosing a good roofer who can do the work the right way the first time. While membrane, flashing, and other material quality is definitely important, you cannot simply buy top of the line materials and expect to avoid water ponding issues.

Choosing the best commercial roofer in your city will end up saving you money on unnecessary maintenance, repairs, and expensive phone calls down the road when avoidable damages aren’t covered under warranty because of the job’s craftsmanship.

If you are looking for flat roof replacement in Boston, you can contact us for a free estimate here.

Common Causes & Solutions For Ponding Water

contractor rolls out rubber roofing material onto flat roof
Avoiding flat roofing mistakes is especially important for rubber roof installation.

Now that we understand the definition of ponding water on roods and that it’s primarily related to craftsmanship errors, let’s cover some of the main causes and how they can be avoided.

Poor roof design

Mostly all of the causes for water ponding on a flat roof come back to poor roof design… it cannot be understated. If the architectural plans for the roofing project do not match the roof’s shape and building layout, then the project is doomed to have issues down the road. 

Potential complications are not always visible from the building’s exterior, which can complicate matters for a roofer who does not have experience with complex flat roof jobs. One common misconception is that flat roofs are easy to replace and repair, but this couldn’t be further from the truth. So if you do not have access to an experienced roofing company, then we recommend bringing a structural engineer into the fold.

Structural engineers can examine the roof from a systemic perspective, looking at more than just the roof to help your roofing contractor determine how the new roof should be designed to prevent pooling water. Examining joists and beams under the roof would all fall within a structural engineer’s scope. These joists and beams often sag over time, which can give a slight pitch to what seems like a completely flat roof.

Building age and foundation settling

There’s really no escaping father time. As the foundation of a building settles over the span of months, years, and decades, a well-intentioned roof design can be altered because of the way the foundation has settled on that particular plot of land. 

Sagging is natural, and thus is almost impossible for home builders to plan for. While this is more common in older buildings and structures, the settling can be a factor in newer construction too. 

Rubber roofs with multiple layers

Many flat roofs in Boston and other major cities have multiple layers. The first layer of a flat roof is almost always tar and gravel, with subsequent layers using a rubber material that you commonly think of when considering flat roof construction. So, what’s the problem?

When rubber is laid over the original tar and gravel base, it is rarely done in a perfectly level way and can often exacerbate existing slopes on the roof.

Crushed insulation causing uneven roof slope

Rubber roofing membranes have thick layers of insulation underneath to help keep heat inside the building during winter months and cool air inside during summer. Homes and buildings lose most of their energy from the roof, so keeping a well insulated roof is critical. This is especially true in climates like Boston, Massachusetts, where summers can be very hot and winters very cold.

A problem that can pop up with insulation is that it’s made of a malleable material. Sometimes the insulation can get unevenly depressed due to high foot traffic or excessive snow that was left to pile up. This leads to uneven surfaces and a perfect recipe for ponding water to form. When water accumulates during a heavy rain storm, the liquid can end up ponding in these sunken areas because they are too deep to evaporate quickly.

The solution for water ponding caused by pushed-in insulation is to add more insulation underneath, or change the insulation strategy altogether. Doing so comes with a cost, but can help you avoid more serious problems that water pooled in these indentations would cause over the years.

Tapered insulation helps control ponding water

Tapered polyisocyanurate (also known as tapered iso) is a type of flat roof insulation with built-in slope. Tapered insulation is used to strategically prevent ponding water because the slope helps the flowing water and debris navigate towards a roof drain. 

One of the more popular products on the commercial roofing market is GAF’s EnergyGuard™ Tapered Polyiso Foam Roof Insulation. Like many tapered insulation materials, GAF’s Tapered Polyiso comes in a variety of slopes. The two most popular tapered iso’s apply a 1% (⅛ inch per foot) and 2% (¼ inch per foot) grade to the roof area where the polyiso is used. 

As you can imagine, a project that uses this type of insulation can completely change the roof design to favor positive water navigation towards drains and rain gutters. 

Your best opportunity to do this successfully is to call Roof Hub or another knowledgeable roofing name you can trust. If you are doing a roof repair or full roof replacement yourself, call your local roof supplier for pricing on these products.

Lack of drainage or improperly installed flat roof drains

In order to maintain a dry surface after storms, any flat or extremely low slope roof needs adequate drainage. Besides poor roof design (including the use of tapered iso), a lack of sufficient drainage is one of the main culprits of ponding water and roof damage. 

Because of the negative effects caused by improper draining, you’ll find many local building codes that regulate these matters throughout the United States. According to Martin J. Fradua, P.E. at the University of Massachusetts, the following conditions should be considered for proper drainage to prevent water ponding:

  • Maximum area per drain is 10,000 square feet, with a minumum of two drains per roof area.
  • The maximum spacing between drains should not exceed 80 feet.
  • Parapet wall scuppers may be used in lieu of an inboard drainage system, provided that they are properly sized to permit the proper flow (please see resource above for additional information about parapet wall drainage requirements).

Positive drainage & how to slope a flat roof

Unlike swimming pools, roofs are not built to hold water… If property owners have a roof where water pools for more than 48 hours time, then they risk voiding their warranties. A positive drainage system is another way to avoid this problem.

Positive drainage is when a flat roof is built with graded areas that help lead water away from areas where water commonly pools. Quite often, the positive drainage will allow water to easily flow into areas with large, easily accessible drains. 

Think of a low slope roof as positive drainage in action. The area at the top is higher, allowing gravity to pull water into rain gutters. When used on a large rubber roof, positive drainage systems effectively turn sections of the roof into a slight low slope in order to avoid ponding that may happen in problem areas.

Using crickets to stop water pooling up

Crickets are raised up areas near a roof protrusion that helps divert water. You may be familiar with a chimney cricket, which is used on roofing projects where the slope of one roof section leads directly to the chimney. In these cases, a small wooden structure built up against the chimney redirects water away from the back of the chimney and off the building. 

Crickets can be used to help keep water away from not only chimneys, but also HVAC units and other tough to drain areas on commercial rooftops.

Keeping drains clean and clear of debris

It may sound simple, but keeping drain areas clear of debris can play a major role in avoiding ponding in the low points of some roofs. 

According to John Lawson at California Polytechnic State University, “drains with domed strainers and scuppers through parapet walls are susceptible to clogging because debris builds up in these areas, causing a backup in water that is supposed to flow into them”. If these drain areas are not regularly cleaned and maintained, you’ll find water pooling near the drains. The longer this goes on, the larger the puddles will get and the more weight will be put on a roof. Eventually (and sometimes rather quickly), the excess water will reverse direction, going up the drain and causing a leak in the building.

Fortunately, cleaning drain areas is a simple fix that can be done by almost anyone and should be kept in mind if you want to avoid some of the problems posted below.

Problems With Large Roof Puddles

dark brown stains on flat roof caused by ponding water
Water ponding can lead to dark stains; a tell-tale sign of weakening roof materials underneath.

Now that we understand frequent causes and best ways to avoid ponding on a flat roof, let’s touch on damaging effects that can occur if you let the problem get out of hand.

Vegetation growth

The longer water builds up on a surface, the more likely you are to experience vegetation growth. Moss, algae, and other plants can do long term damage because they turn the roof’s surface into it’s own ecosystem. The plant’s roots dig into the roof materials and if gone untreated for a long period, can begin burrowing into the insulation and substrate underneath, which could be made of wood, steel, or concrete.

If you have plant growth, the roots could be bringing moisture into the structure. It’s best to get out in front of this problem before it can cause unnecessary damage. Many building owners learn this hard way, when the cost of roof repair turns into a cost for both repairs and potentially mold and mildew removal. 

Sagging roof deck and potential collapse

Roof collapses are the worst case scenario for any property owner. Besides the need for tenets and/or businesses to evacuate, there’s a serious risk of property damage and in extreme cases, the loss of human life. But what does this have to do with long-term water accumulation?

When water begins ponding on a roof, the areas underneath start to deteriorate. Because the roof materials themselves are not completely waterproof like a swimming pool liner would be, they start to break down over time. Water saturation increases, leading to more weight for the structure to hold. Rotting materials and extra weight are not a good mix! And this is exactly where the risk of a cave-in is heightened.

Frequent roof leaks

Water ponding increases the likelihood of leaks because it deteriorates roof materials, as we discussed above. Once you start seeing a roof leak on the interior of the home or building, the issue is much greater than one small leak. So if you have a roof leak issue under ponding water, it may not be a simple repair and we recommend contacting an expert to do 

So, Is Standing Water On A Flat Roof A Problem?

Prolonged standing water on a flat roof is a serious problem. As we covered above, water that does not drain or evaporate after 48 hours must be dealt with in order to avoid more serious issues like leaks or compromising the structure of your building. 

However as we discussed, it is best to attack these problems from the beginning rather than fight an endless battle of repairs and maintenance. Make sure you hire a roofing contractor who knows how to design a roof replacement with positive drainage that takes all factors of the building into consideration.

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