Commercial, Mixed-Use and Strata Roof Maintenance in Vancouver, B.C.

Water Ponding On A Flat Roof, Is It Okay Or Bad?

Water Ponding On A Flat Roof, Id It Okay Or Bad?

Last week, we visited a client who had water ponding on her flat roof for a week after a rainstorm. She was worried about potential leaks and other damage to her roof. 

While working on her roof, she asked me, whether it was okay for water to puddle on a flat roof. And for how long can water stagnate on a roof without causing any damage?

Water ponding on a flat roof may or may not be bad. Generally, water collected on a flat roof for less than 48 hours is no threat. 

Provided, your roof was correctly installed and has a good drainage system to drain the water later. Water pooled for longer than that is cause for concern.

In this article, I have broken down all the aspects of water pooling on a flat roof. Along, with the solutions, you may need to deploy in case of excessive or chronic water puddling.

When can we say water is ponding on your roof?

When it rains, a flat roofing system is more susceptible to water pooling, compared to sloped or low-sloped roofing systems. This is due to the structure of the flat roof.

However, after a heavy downpour, it’s natural for water to stagnate on a flat roof for a day or two. As the moisture in the air increases, it reduces the rate of evaporation, which leads to brief water ponding. Eventually, the water drains by itself through the drains after 36-48 hours.

If the water is still puddling on the roof after two days. The Building Code and Roofing Associations, suggest the flat roof be examined by professional roofers, and to get rid of the water manually.

This is an ideal case of water ponding on a flat roof.

Common Causes of Water ponding over a flat roof.

Now, let’s see what are the causes of water puddling over a flat roof.

Poor Roof Design.

Almost all reasons for water ponding on a flat roof, circle back to the poor roof design. When the structure of the roof is not aligned with its pre-decided design plan. The roof is due for some serious repairs in future.

A flat roof is the topmost layer of a building/house. And the beams supporting the roof from beneath can sag over time. This gives a slightly uneven shape to the once flat roof. Resulting in water ponding.

Solution: Only an expert roofing contractor and a structural engineer are qualified enough to repair the structural design of the roof.

Depending upon the condition of your flat roof you may have to repair or completely replace it. To avoid water pooling.

Heavy Roof installations.

Every roofing system is designed to support a certain amount of weight. The flat roofing system is no exception to this.

When HVAC equipment, antennas, water tanks, etc are installed on a flat rooftop. They constantly exert some weight on the roof. When the building structure becomes old, the roof is no longer in its prime condition. Hence can’t support these additional installations, as well as they did back in the day.

These installations if not removed in time will create a crease in the roof around them. This crease eventually, will pond water. It may be condensation or rainwater.

Solution: Regular roof and building maintenance are the keys to solving this problem. Plus, make sure to not install anything too heavy over your roof.

Improper Roof Installations.

When drains, skylights, plumbing vents, etc. are improperly installed on the flat roof. They often have dips around them that worsen over time and cause water ponding. This is a pretty common occurrence in many flat roofing systems.

Solution: Contacting a professional roofer to correct the improper installations is the best solution. Also, make sure there are proper flashings around the installations, to avoid leaks.

Aged Roof.

Nothing can prevent a roof from getting old. Proper maintenance and timely repairs can extend its lifespan. But after some time, the roofing materials start to degrade. This causes a flat roof to sag and bends in some places.

These concave regions on a flat roof hold the rainwater for a long. As it prevents the water from reaching the drains.

Not only that, the drainage system of an old roof may be compromised, leading to extensive water pooling. There could be multiple reasons causing water ponding on an old roof.

Solution: Replace the roof on time with the help of an experienced contractor.

Lack of proper Drainage.

When water collects on these roofs after a rain storm. It often finds the nearest leakage and drains off the roof. However, some flat roofs face the problem of improper or lack of drainage.

When there is no proper drainage there is no way for the water to escape. So, it stagnates over the roof for days.

There should be a minimum of one drain, for every 10,000 sq feet area. Two drains should be not more than 80 feet apart. These are some simple rules to follow while laying an appropriate drainage system on flat roofs.

Solution: Timely cleaning of drains must be scheduled. Positive drainage is also an excellent solution. Positive drainage is when a flat roof has pre-built graded areas that lead water to the drainage

Crickets can also solve the water ponding issue. As crickets are slightly raised areas near roof fittings or other protrusions. The main purpose of crickets is to avoid water collection near these protrusions or fittings.

Chimneys often have crickets. However, they can be multipurpose or used wisely and can prevent water clogging around roof installations and HVAC equipment.

Flawed Insulation.

Every roof has a layer of insulation to regulate the inside temperature according to the outside. The outer layer of the flat roofs is mostly made up of rubber, glue and tar. And have thick insulation inside.

When this insulation gets unevenly dented, due to various factors. It leads to water puddling in these dents. These dents are sometimes too deep for water evaporation. Hence, they remain full for a long time.

Solution: The easiest way to solve this crushed insulation problem, is to add more insulation underneath it. This is a costly solution, but in the long haul, it will prevent the problem of water pooling and all the damage that comes with it.

To avoid water ponding a more advanced form of insulation, called Tapered Insulation could be used. This

Insulation is purposely sloped, to allow water and other dirt to gravitate towards the drains.

Roof Layering Issues.

The roof contains many more layers apart from the insulation. These layers are made of tar rubber gravel. When the roof is freshly installed, all these layers are in perfect shape. However, with passing seasons and constant use of the roof. These layers lose their original shape and give rise to uneven slopes.

Which results in water stagnation.

Solution: Regular roof inspection and maintenance is the best fix to the problem. You can also re-pitch the roof. This can be done as both, a solution and a preventive measure.

Effects of Water Ponding On Flat Roofs.

Why should a flat roof not have water puddles over it?

When water collects over a flat roof it is likely to compromise the structure of the roof. These are some of the effects of puddled water on a flat roof.

Vegetation Growth.

When water is stagnant in a place, be it in a small pond or on your flat roof. Plants like algae, moss and others and bound to take advantage of it.

This stagnant water and the concrete/ gravel surface below. Offers a perfect habitat for these plants to thrive. They turn your flat roof into their personal ecosystem.

These plants may seem small but grow at an exponential rate. And burrow their roots deep into the flat roof surface.

If this issue goes unattended for long, it may cause some major structural damage to your roof. Leaks, molds, and mildew are some common side effects of vegetation above the roof.

Moreover, the repair cost is high.

Frequent Leaks.

If water finds a permanent home on your roof, then it is bound to seep through the roofing layers. Eventually, you will see leaks on your ceiling caused due to water ponding.

If your roof is old, the cracks and aged roofing material will allow the water to penetrate and cause leaks.

Sagging Of The Roof.

A new roofing system is capable of supporting the added weight of the pooled water. Some areas on the roof adjust to brace the weight. However, with time the roof loses its natural flex and isn’t capable of supporting the water as well as it used to.

This causes sagging in the areas where the water pools. This sagging prevents the water from reaching the drains, stagnating the water for longer. Which in turn increases the weight on the roof, which leads to more sagging.

This circle of events continues till the roof is completely compromised, or until repairs take place.


Will water pooling damage the roof membrane?

Yes and No! Temporary water pooling does not damage the roof membrane.

A flat roofing membrane in general is flexible and shouldn’t tear with a bit of additional stress. Seams are typically bonded together with strength. If you feel the flex of the membrane may be a bit much, we recommend an inspection from a qualified roofing contractor.

My window or deck is beside the ponding water and it’s an unsightly site. What should I do?

Not only can it be unsightly, but it can also emit a foul smell. These pools can be a breeding ground for mosquitos. You should get rid of the ponded water as soon as possible.  

How is this problem solved on a new roof?

Depending on the membrane chosen, adding a few extra plys on the top sheet in the area or adding insulation or a roofing board below could alleviate the problem of water ponding on a new flat roof.


So, is water ponding a real problem on flat roofs? Maybe, maybe not… It solely depends on how long the water stagnates on your roof and whether it creates any new roofing issues.

If you are unsure, it’s best to call in a roofing professional to take a look. If you are looking for an experienced roofer, look no more!

Leak Stop is a one-stop destination to meet your roofing-related requirements. We offer roof repair, replacement, maintenance and inspection services all over GVRD (Greater Vancouver Regional District).

We offer a free inspection to first-time clients. Connect with us TODAY at 1 866 777 0084 for immediate assistance!

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