Addressing Water Ponding on Flat Roofs: Causes, Prevention, and Solutions

Water ponding on top of a flat room is a common problem to have. It’s also one of the most frustrating things to deal with because it is so hard to avoid. Knowing when water ponding on a flat roof a serious issue is can help you address the problem before it gets worse.

If you have water ponding on your flat roof, you will need to consider clearing the drainage channels, adding more drains, or having your roof re-sloped to direct the water downward. You can also replace your roof membrane or use roof crickets to help disperse to ponding water. In some extreme cases, you may need to consult a professional to handle your ponding water problems.

This guide will walk you through everything you need to know about ponding on a flat roof and how you can resolve and avoid this problem in the future.

How Long Should Water Remain On Roof After Rain?

If water builds up on your roof for longer than 48 hours, it’s called water ponding. Roofs can form huge puddles when the drainage system doesn’t work well or when the roof design or infrastructure doesn’t work. Due to the high volume of liquid, it can’t evaporate off the roof and the stagnant water can’t drain.

A flat roof may have standing water for 12 to 36 hours after a storm, but if the water persists for more than two full days, it’s time to call a pro.

Because water can only go down, gravity makes significant water buildup a problem on flat roofs. Over time, ponding water can damage the flat roof system, so it must be fixed to keep the building’s structural integrity.

What Damage Is Caused By Ponding Water?

Your roof and your home can get seriously damaged by even minor ponding. Excess water weight can damage the building’s structure, especially if the weight exceeds the safe weight. Weight can stress roof trusses and other parts of the building, even if it doesn’t.

Roofing materials can also get ruined by constant water exposure. Roofs can be eroded by water, resulting in early roof failure. Algae, moss, and lichen may also grow in water. Vegetation like this can stain or damage your roof.

Animals and insects may also get attracted to standing water and damage your roof or home if they’re looking for a drink. They may also just be a minor nuisance that affects the quality of your home and your life. Standing pools of water on a roof, for example, may breed mosquitoes. When you’re trying to enjoy your outdoor space, this can be annoying.

Ponding can cause roof leaks. Water can leak when it penetrates a roof feature when the amount of the ponding is greater than the height of the roof flashing. It’s also possible for the water to freeze in cold climates. Some roofing materials, like modified bitumen, can get damaged by freeze/thaw cycles, reducing their life expectancy.

Do everything you can as a homeowner to avoid ponding, as it can shorten your roof’s lifespan for all these reasons. Now that we have discussed the possible damages, we can discuss the main causes of roof ponding in the next few sections of this guide.

Six Causes Of Roof Ponding

The Foundation Has Settled

Time isn’t gonna stop. In the decades and years ahead, the foundation of a building can settle, altering a well-intentioned roof design. Almost no one can plan for sagging because it’s natural. It’s usually more common in older buildings, but it can happen in newer ones too.

Crushed Insulation On Roof

Insulation layers underneath rubber roofing membranes help to keep heat contained during winter and cool air in during summer. The roof loses most of its energy, so keeping it insulated is important. Summers can be really hot in Boston, and winters can be freezing.

Having a malleable material can cause problems with insulation. Oftentimes, insulation gets unevenly depressed from high foot traffic or excessive snow that piled up. Because of this, ponding water forms on uneven surfaces. During a heavy rainstorm, water can accumulate in these sunken areas because they’re too deep to evaporate fast.

For water ponding caused by pushed-in insulation, add more insulation underneath or change the insulation strategy. While it costs you, you can avoid more serious problems that water pooling in these indentations would cause.

Lack Of Drainage

It’s important to have adequate drainage on flat and low-slope roofs after storms. Water ponding and roof damage are commonly caused by poor roof design (including tapered ISOs) and inadequate drainage. 

In the United States, you’ll find many local building codes that regulate draining because of its negative effects. Water ponding can be prevented by considering the following conditions: There can be up to two drains per roof area, with a maximum of 10,000 square feet per drain.

Drains shouldn’t be more than 80 feet apart. Scuppers can be used in place of an inboard drainage system, provided they’re properly sized to allow enough flow.

Drainage Is Blocked

When installed correctly, positive drainage moves water toward drains and scuppers. Incorrectly installed drains and scuppers can actually block water from exiting, creating large pools. Water can be blocked by even a slight lip.

Owners and property managers need to proceed with caution before trying to resolve the issue because sometimes fixing the problem makes it worse.

Too Many Layers Of Roofing On The Roof

There are multiple layers on many flat roofs in Boston and other big cities. When it comes to flat roof construction, the first layer is usually tar and gravel, followed by rubber material. So what’s the deal? Rubber is never laid perfectly level over the original tar and gravel base, and it can worsen existing roof slopes.

Poor Installation Of Roof

Poor roof design and installation is the main cause of water ponding on a flat roof, it cannot be overstated. The roofing project is doomed to fail if the architectural plans don’t match the roof’s shape and building layout.

A roofer who doesn’t have experience with complex flat roofs may not see potential complications from the exterior of the building. There’s a common misconception that all flat roofs are easy to replace and repair. We recommend bringing in a structural engineer if you don’t have access to an experienced roofing company.

Your roofing contractor can use structural engineers to tell how the new roof should be designed to avoid pooling water by looking at more than just the roof. A structural engineer would look at joists and beams under the roof. Over time, these joists and beams can sag, giving a slight pitch to what looks like a flat roof.

Now that we have gone over the major causes of water pooling on a roof, we can start to get into the different fixes you can use to address them. This will be covered in the next section.

Fixes For Ponding Water

Use Roof Crickets

Crickets live near roof protrusions that divert water. Chimney crickets are used on roofing projects where the slope of one roof section leads directly to the chimney. A small wooden structure built up against the chimney directs water away from the building and away from the chimney.

On commercial rooftops, crickets can help keep water away from chimneys, HVAC units, and other hard-to-drain areas.

Keep Drains Clean

Keeping drains clear of debris can help prevent ponding in low spots on some roofs. You’ll find water pooling near drains if these areas aren’t cleaned and maintained. The longer this goes on, the bigger the puddles will get and the heavier the roof will be.

 Eventually, this excess water will change direction, going up into the drain and leaking. Cleaning drain areas is a really simple thing that almost anyone can do and should be done regularly to avoid these problems.

Add More Drains

You can look into a few things to help your current drainage system remove water from your flat roof more effectively. Adding more drains is one of the most effective ways to deal with this problem, even if it costs more.

In areas with high precipitation, flat roofs sometimes need those extra drains to keep water from building up. In the aftermath of a big storm, ponding water is a significant issue that most building owners don’t realize.

A good roofing contractor can install extra drain lines or even widen your existing building’s drains to keep your roof dry.

Obviously, this is an extra cost, so most building owners won’t do it. However, there might be a better option out there.

Reslope The Roof

Another way to avoid this is with a positive drainage system. Having graded areas on a flat roof helps keep water away from pools. There are often large, easy-to-access drains in areas with positive drainage. 

Positive drainage is what low-slope roofs do. Rain gutters are higher at the top, so gravity pulls water into them. In problem areas of most large rubber roofs, a positive drainage system works well to turn sections of the roof into a slight slope to prevent ponding.

Replace The Roof Membrane

Your roof membrane keeps things like moisture from getting into your roof, attic, or other areas of your building. Moisture can cause serious structural damage when it builds up in your building.

The membrane can get damaged for various reasons, including ponding water, so you might want to just get a new one. You can do this if you can’t afford to replace the roof.

There are a lot of membranes out there that can handle ponding water, so replacing your old roof with a newer, more effective one might be a good thing. A flat roof inspector will know how much ponding is okay.

Should You DIY Or Calls A Professional?

Knowing whether you should try and tackle this issue yourself or whether you need a professional is important. In the next few sections, we will try and help you make this decision.


Roofers make products that even out sagging and ponding spots so that when it rains, you don’t collect water on your roof. For DIYers who want to fix ponding issues on their own, this is an easy-to-apply product; it’s also an auxiliary sloping compound.

Flat roof drainage problems can be fixed with one application of this product. You can match it to your existing roof. UV-stable products are best. The compound shouldn’t get less effective after sitting in direct sunlight for hours, days, or years.

You can also figure out where your roof’s low spots are. Put in a drain once you know how water puddles on your roof, so you don’t need to spend a lot of money on repairs. Your handiness and the severity of the issue will determine how long it takes.

Professional Costs

The fact that there are some simple and effective roofing solutions available for those who wish to do their own repairs doesn’t mean these solutions are right for everybody.

You should call in a roofing pro if you don’t feel comfortable applying a compound or if you aren’t sure, you can fix your ponding issue by yourself. The issue of a ponding flat roof might be a lot more serious than what appears at first glance, even for those who feel confident doing their own repairs.

You should call a professional roofing contractor if you see water leaking from the roof, even if just to assess how bad it is.

You’ll probably just get stained ceilings if you’re lucky. It’s really dangerous to have water leaking in from ponding on the roof because you have to deal with damaged electrical systems, damaged furniture and possessions inside the room, and serious carpet damage, just to name a few things. Depending on how bad the damage is, it could cost you thousands of dollars to fix them all.

Take care of your ponding issue right away if it’s all you have. It’s possible to fix it yourself, but if the ponding water breaches the roof and leaks into your space, you’ll need professional help and advice. You may not be aware of the full extent of the problem at the moment.

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