Adding a skylight to a commercial flat roof can completely transform the atmosphere of your building. That’s because natural light makes interiors feel larger and more vibrant, boosting employee morale and, believe it or not, attracting more customers.
For instance, according to Kalwall, bringing in daylight through skylights can increase retail sales by up to 40%! Plus, the added sunlight can help lower energy costs by improving your building’s overall energy efficiency. So what’s not to love?
Well, a flat roof skylight needs the right design, installation, and maintenance to perform as beautifully as it looks. With that in mind, here’s everything property owners and facility managers should know before installing a skylight on a flat roof.
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Quick Answer
A flat roof skylight is a roof window designed to bring natural light into buildings with flat roofing systems. Installed either flush with the roof deck or on a raised curb, skylights improve energy efficiency while enhancing the comfort of the people below. When properly flashed and drained, they offer long-lasting performance with minimal maintenance, making them one of the most practical daylighting upgrades for commercial properties.
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Why Skylight Glazing Matters
A skylight’s glazing type (i.e., the material that lets light in) determines how the overall feature performs, at least in part. Because of that, the wrong choice can make a space too hot, too dim, or even damage interior finishes over time.
There are three main types of skylights for flat roofs:
- Clear skylights, which offer the most natural light but can create heat buildup, especially in warm Southeastern climates. They’re best for hallways, storage areas, or large open spaces.
- Opaque skylights, which filter and diffuse sunlight. Because they reduce glare and solar heat gain, they’re perfect for offices, classrooms, or other areas with computer screens.
- Amber skylights, which provide a warm, balanced glow that softens harsh sunlight while maintaining brightness. These are ideal for occupied spaces that don’t feature many screens, such as showrooms and lobbies.
When replacing a skylight, always match the glazing type to how the space below is used. A clear skylight might be great for a warehouse, but a tinted or amber option is better suited for offices or living spaces.
The Best Skylight Styles for Flat Roofs
A flat skylight for flat roofs needs to be watertight, durable, and compatible with your roofing membrane. Below are the most common types of skylights for flat roofs:
Curb-Mounted Skylights
The most popular choice for flat roofs, these skylights sit atop a curb, a type of raised frame that’s usually around 8 inches tall. The roofing membrane is flashed up the sides of the curb, keeping the assembly watertight.
These are the most common type of roof window for a flat roof, and for good reason. Some of their main benefits are as follows:
- Easy Replacement: They can be swapped out without re-flashing if the existing flashing is still in good condition, since the roof membrane is flashed up the curb rather than the skylight itself. This makes labor easier, reducing the cost you pay if and when you need to replace your skylight.
- Customizable Sizing: These skylights are easier to manufacture in custom dimensions to fit existing roof openings, further decreasing costs.
- Ideal for Retrofits: They are the perfect choice when the skylight itself needs replacement but the curb and flashing remain functional.
However, nothing is perfect, and curb-mounted skylights are hardly an exception. For instance, common issues include:
- Higher Profile: The raised curb adds height above the roofline, which can affect aerodynamics in windy areas. In addition, this can look somewhat jarring if seen from the ground.
- Reduced Energy Efficiency: Most existing curbs have minimal insulation (around 1½ inches thick), which can lead to greater heat transfer and cause energy loss.
So, while imperfect, curb-mounted skylights stand out as the most practical, and often most affordable, option for flat roofs. But what other option should you consider?
Deck-Mounted Skylights (Flat Ceiling Skylights)
Most commonly used on sloped roofs but still seen on flat roofs, these flat ceiling skylights sit flush with the roof deck, providing a sleeker look. However, they are more prone to ponding and leaks, especially on a flat roof.
Also, since replacing them requires re-flashing to avoid leaks (don’t worry, we’ll talk about this later), deck-mounted skylights can also be a bit more expensive than their curb-mounted counterparts. Consequently, they’re best for slightly sloped roofs where water drains naturally.
Other Types of Skylights for Flat Roofs
While curb-mounted and deck-mounted skylights are the most common options on the market, you can also find a few more niche options from time to time, such as:
- Tubular or “Sun Tunnel” Skylights: These compact, reflective tubes direct sunlight into small or interior spaces. They work well for bathrooms, hallways, or utility rooms.
- Custom or Structural Skylights: These are larger, custom-built installations most commonly used in atriums and showrooms. They require professional coordination between the roofer and the skylight manufacturer in order to be manufactured and installed properly.
Skylight Type Comparison
If that’s a lot to process, we’ve included this table detailing each skylight variety and its recommended use cases:
Type | Features | Use Cases |
Curb-Mounted Skylight | Raised curb design allows easy replacement and custom sizing but adds height and reduces insulation. | Best for flat roofs and retrofit projects where flashing is still in good shape. |
Deck-Mounted Skylight | Level with the roof deck for a sleek look but prone to leaks on flat roofs. | Ideal for slightly sloped roofs with good drainage or when aesthetics are a priority. |
Tubular Skylight | Compact, reflective tube that channels light efficiently into small spaces. | Great for bathrooms, hallways, or utility rooms needing daylight. |
Custom Skylight | Large, custom-engineered systems that usually require professional installation. | Suited for atriums, showrooms, or other architectural statement pieces. |
Of course, no matter which skylight style you choose, its long-term performance depends on how well it’s sealed and drained.
Flashing and Drainage: Your Skylight’s First Line of Defense
The biggest issue with flat roofs is that they don’t shed water like steep roofs. As a consequence, the flashing and drainage system around your skylight are essential.
For reference, drainage is your roof’s system of getting rid of water, often including gutters, roof drains, and scuppers. Similarly, flashing is the metal or membrane material that waterproofs vulnerable points on your roof, like where your roof meets your chimney or parapet wall. If you want to learn more about flashing, check out this article:
Most professional roofing contractors worth their salt will use base flashing made of the same material as your roof membrane (like TPO, PVC, or EPDM) to ensure chemical compatibility. Then, they’ll line the skylight or curb with counter flashing to protect the edges from UV damage and high winds, all while ensuring rainwater always meets a flashing surface to direct it off your roof.
Even a small dip near the curb can trap water, leading to water infiltration and membrane deterioration over time. Because of that, we recommend that you have a licensed commercial roofing contractor handle the complex installation process to avoid leaks and costly headaches in the future.
Energy and Comfort Considerations
Speaking of headaches, a skylight’s biggest strength, sunlight, can also be its weakness if not properly managed. For instance, like we’ve discussed already, clear skylights maximize daylight but also let in more heat, increasing air conditioning demand. Conversely, opaque and amber skylights filter sunlight, improving comfort and reducing glare.
Low-E coatings on modern skylights further cut UV rays and heat transmission. For example, VELUX reports that their Low-E glass skylights can achieve a Solar Heat Gain Coefficient (SHGC) as low as 0.26, meaning they block about 75% of incoming solar heat. So, choosing the right glazing not only saves energy but also prevents overheating, making your interior space feel more balanced year-round.
Integrating Skylights with Membrane Roofing Systems
On a membrane-based flat roof, your skylight should be installed using materials that match your existing roofing membrane:
- TPO or PVC roofs: Use heat-welded thermoplastic flashing for the best protection.
- EPDM roofs: Require uncured EPDM flashing for flexibility as the roof expands and contracts.
- Modified bitumen roofs: Can use torch-applied or cold-process flashing, though torching should only be done by certified professionals.
Improper materials or methods can void your roof warranty, so it’s critical to work with a qualified commercial roofing contractor. To learn more about a couple of types of roof membranes and the materials that comprise them, check out this article:
Flat Roof Skylight Maintenance
Like any roof feature, skylights need occasional care. That’s because dirt buildup or worn sealants can impact performance as they worsen over time. So, your routine maintenance checklist should include these items:
- Inspect 1-2 times a year for cracks, leaks, or damaged flashing.
- Clean debris and dirt from the glazing and curb area.
- Ensure there’s no dirt buildup blocking roof drains and scuppers near skylights.
- Reseal or replace aged caulking as needed.
Regular attention to these simple tasks helps extend your skylight’s lifespan while keeping natural light shining in year-round.
Frequently Asked Questions (FAQs)
Installing a skylight on a flat roof requires attaching the skylight frame, then flashing it with the same membrane as your roof and ensuring proper drainage around it. For a curb-mounted skylight, the feature should be attached on top of an 8-inch-tall frame; otherwise, it should be embedded directly into the roof. Either way, skylight installation is not a DIY-friendly project, and professional installation is the best way to ensure watertight performance.
Remove the existing skylight and old flashing, then inspect the curb and rebuild it if it’s worn or damaged. Afterwards, install the new skylight with compatible flashing and sealants, matching your roofing system. No matter what, try to coordinate replacements during broader roof repair or reroofing to make the process more efficient, potentially bringing down costs.
Use a mild soap-and-water solution with a soft brush or sponge. Avoid abrasive cleaners, which can scratch the glazing. If safe roof access is limited, hire a professional to clean and inspect the skylight at the same time; otherwise, a DIY cleaning is entirely practical.
Leaks often stem from damaged flashing or sealant failure. A licensed commercial roofing contractor can reseal seams, replace flashing, or even rebuild the curb if needed. If you need an immediate solution, you can apply some temporary caulking to alleviate the problem for a time, but it’s crucial that you seek professional repair work to patch the leak in the long term.
A curb-mounted skylight with opaque glazing is best for most commercial applications, especially in the Southeast. This system lifts the skylight above the roof surface and reduces heat gain while keeping interiors illuminated with natural light.
Professional installation typically ranges from $1,200 to $3,000, depending on skylight size and roof complexity. The average cost is around $1,800, but custom roof windows for a flat roof may cost more.
Final Thoughts
A flat skylight for flat roofs can brighten interiors and reduce energy costs, but only when it’s chosen for the right application and installed correctly. Remember to always take the following steps when considering skylights for your property:
- Match the skylight glazing to the space below.
- Use proper flashing and drainage to prevent leaks.
- Schedule annual or biannual inspections to extend the lifespan of your roof and skylight.
When designed and maintained by professionals, your skylight will illuminate your building, and the people within it, for decades to come.
Baker Roofing: Commercial Roofing Services Throughout the Southeast
Considering a flat roof skylight for your commercial property? For more than 110 years, Baker Roofing has delivered trusted skylight inspection, repair, replacement, and installation services across the Southeast.
Our century of experience means you can count on premium workmanship that will stand the test of time. Click below to schedule your free roof estimate and protect your property with peace of mind.
