While they are a staple of the commercial world, flat roofs are becoming more popular on residential structures. You may have purchased a home with a flat roof or added one to your garage or home addition. While flat roofs can be beautiful and modern, many homeowners are unprepared for how different these systems are from sloped roof systems.
For example, flat roofs may develop a problem called ponding that doesn’t occur on sloped roofs. Ponding is when water collects in puddles on the roof instead of draining off in a timely manner. Homeowners may think this small amount of water is harmless, but ponding tends to get worse over time and can have a negative impact on the roof and your whole home.
If you own a flat roof, it is essential to understand what counts as ponding, why it happens, and how to resolve it before it damages your roof and home. We’ll explain the basics of ponding and why it’s a wise idea to get a professional involved as soon as possible when your flat roof has ponding.
What is Ponding?
Roof ponding is where water gathers on a flat roof. So how much ponding is acceptable on a flat roof? Specifically, water must remain on the roof for 24-48 hours for the National Roofing Contractor’s Association (NRCA) to consider it ponding. If you have this leftover water, you may notice it collects in dips in the roof or around drains.
Why Shouldn’t Flat Roofs have Puddles?
Even minor ponding can have a serious impact on your roof and your home. The excess weight from the water may cause damage to the building’s structure, especially if it exceeds the safe weight that the home can carry. Even if it does not, increased weight can stress the roof trusses and other parts of the building.
Constant exposure to water is also a problem for roofing materials. Water can erode your roofing materials, which may result in early roof failure.
Water may also enable the growth of algae, moss and lichen. This vegetation can stain or damage your roofing materials. Standing water may also attract animals or insects who are looking for a drink, and they may damage the roof or the rest of your home. Or, they may simply become a nuisance that impacts the quality of your home and your quality of life in it. For example, mosquitoes may breed in standing pools of water on the roof. This may become a nuisance when you try to enjoy your outdoor space.
Damage from ponding may result in a roof leak. One way it can cause a leak is when the depth of the ponding exceeds the height of the flashing on a roof feature, thereby allowing water to penetrate the structure. In cold climates there is also the possibility that the water will freeze in winter. Freeze/thaw cycling can erode the mineral surface on some roofing materials, such as modified bitumen, which can reduce the life of the roof.
For all of these reasons, ponding may also significantly shorten the lifespan of the roof, so do everything that you can, as a homeowner, to avoid it.
Why Does Ponding Happen?
Why does ponding happen, and why does it tend to get worse over time? There are several possible causes of this problem, including:
- Improper installation: A flat roof isn’t perfectly flat. It must be installed on a slight pitch to allow for positive drainage of the roof. The slope should gently guide water towards the drains. If it slopes away from the drains instead or has an inconsistent slope, you may get ponding at the lowest points on the roof.
- Drainage issues: Clogged or slow-moving flat roof drains can create ponding issues. Some flat roofs are so large that they need siphonic drains to move water in a timely manner. On residential roofs, a more common issue is the connection point between flat and sloped roofs. Improper installation of this connection can result in poor drainage and ponding issues.
- Compressed insulation: Some flat roofs are not built for foot traffic or for the storage of heavy items. When these roofs are used for that purpose, the insulation can compress. This compression will create a low spot which may naturally collect water. The weight of the water can exaggerate this problem over time. You should not store objects or place heavy decoration on your flat roof unless the roof is specifically designed and protected to accommodate these loads.
- Dips near roof features: Certain roof features, such as skylights or plumbing vents, may have been installed with small dips that allow for ponding and become worse over time. It is essential to get an experienced roofing professional to install these features and their flashing to prevent this problem.
- Structural issues: Buildings settle over time, and sometimes this settling can create problems with the slope of the roof and cause ponding.
- HVAC leaks: Sometimes HVAC equipment that is installed on the top of the roof causes ponding because it is too heavy and creates a dip in the roof. The ponding water may also be normal condensation or an abnormal leak from the air conditioner itself. The water may be slowly draining, but resupplied by the machinery. In this case you will need an HVAC professional to fix the equipment’s leak or reroute the normal condensation.
How to Fix Ponding on a Flat Roof
You will need a professional to help you fix ponding on your flat roof. There are several flat roof ponding solutions that your roofer may discuss with you. Which one is right for your roof will depend on what’s causing the ponding and the specifics of your residential roof system. However, to give you an idea of what might be involved, we’ll discuss some common strategies that might be useful to fix ponding on your flat roof.
- Conduct the Roof Inspection: When you call roofers about your flat roof’s ponding, the first thing they should do is inspect the roof to discover what is causing the ponding. Your roofers may also need the help of an architect or structural engineer if there has been structural damage to the home. Your roofers should explain what the problem is and give you a quote before they begin work.
- Fix Structural Issues: When ponding has caused structural damage to the building, that damage should be addressed first. Your roofers may need to work with structural engineers or architects to make a plan to address structural issues.
- Fix Slope Issues: If your roofers found that your previous roofers installed the flat roof without the proper slope, they may need to replace the entire roof. Unfortunately, this is one of the more expensive and time-consuming solutions to ponding, but if the roof is improperly sloped, you may have no other choice to solve the ponding for the long-term.
- Fix Drainage Issues: When the drains are the problem, your roofers have a wide variety of solutions available to them. It may be that you need to add in a new drain or change the drainage system so that it can accommodate more water. If the drain is frequently getting clogged, your roofers may recommend a better drain screen, or for you to get the roof cleaned more regularly. In cold climates, meltwater from snow may freeze in the drain. In this case, your roofers may recommend an electric cable to re-melt any ice in the drain. Or, they may suggest a new schedule for snow removal that may reduce ice development.
- Fix Low Points on the Roof: If your roof has a correct slope overall but has developed low points from improper installation, misuse or damage, your roofers can correct those specific low spots on the roof. They may install a roof cricket to redirect water away from a low point. They may use a flat roof puddle filler to bring low points back up to the appropriate level. Or, if the low spot is near a roof feature or a drain, they may need to reinstall the flashing on that feature.
- Fix Other Roof Damage: Once they have fixed the underlying cause of the ponding, your roofers should also be careful to replace any materials that were damaged by the ponding. They may need to replace sections of your cap sheet that were constantly exposed to water or replace insulation that was compressed under the weight of the water, or any other part of the roof.