Install Curb Mount Flat Roof Skylight

Bringing Light Into a Flat Roof With a Curb-Mounted Skylight

On a flat roof, the curb-not the skylight itself-is what decides whether your new source of daylight stays dry for twenty years or starts dripping after the first real snow. A curb-mounted skylight sits on a raised box that lifts the vulnerable joint above ponding water, drifting snow, and the debris that collects on every Brooklyn flat roof. The curb gets integrated into your roof membrane before the skylight unit ever touches down. Get that curb and flashing right, and you’ll never worry about water. Treat it like an accessory, and you’ll be patching leaks before the paint dries inside.

Most flat roofs in Brooklyn need curb-mounted skylights because the roof holds water-sometimes for hours after a storm, sometimes in the same low spot all summer. A skylight sitting close to the deck has nowhere to go when water backs up. A skylight on a proper curb stays high and dry while the membrane wraps up the sides and keeps the seal well above the problem zone.

On this page, you’ll walk through:

• When a curb-mounted skylight is the right choice for a flat roof
• How the curb, skylight, and roof membrane work together
• The step-by-step installation sequence a pro in Brooklyn will follow
• Local details-code, weather, and building quirks-that affect your project

Is a Curb-Mounted Skylight Right for Your Flat Roof?

Curb-mounted skylights are usually the best choice when your roof is truly flat or very low-slope and sees ponding water during storms. If you have a membrane roof system like EPDM, TPO, PVC, or modified bitumen, the curb gives you a clean way to tie the skylight into that membrane without creating a weak spot at deck level. Buildings with parapet walls and thicker insulation build-ups also benefit-curb height accounts for that extra depth and keeps the skylight opening well above the finished roof surface.

If you’re replacing an old dome or glass unit that’s already sitting on a curb, staying with curb-mount is almost always easier and safer than trying to switch to a deck-mounted product. The existing curb may need rebuilding or adjustment, but the roof structure and flashing are already set up for that style of installation.

How Brooklyn building stock shapes the decision: Rowhouses, brownstones, and low-rise apartment buildings in Brooklyn often have multiple roof layers and parapets. Some have been re-roofed three or four times over seventy years, with each new layer adding height. Curb-mounted skylights make it much easier to tie new windows into these thicker, older roof assemblies while keeping water away from the opening. Deck-mounted products expect a clean, thin roof deck and predictable framing-conditions you rarely find on an older Brooklyn flat roof.

Curb-Mounted vs. Deck-Mounted Skylights on Flat Roofs

Deck-mounted skylights work beautifully on pitched roofs where gravity moves water quickly down and away. On a flat roof, gravity doesn’t help. Water sits. Snow melts slowly and refreezes at night. A skylight that meets the roof membrane at deck level is always fighting that water, and the flashing has no room for error.

Rule of thumb: If your roof is flat, uses a membrane, or holds water for more than a few hours after a storm, a curb-mounted skylight is almost always the safer and more durable choice in Brooklyn’s climate. I’ve seen deck-mounted units perform well on commercial roofs with perfect slope and drainage, but those conditions are rare in residential Brooklyn.

Anatomy of a Curb-Mounted Skylight on a Flat Roof

A curb-mounted skylight isn’t just a window on a box. It’s a series of layers that work together to keep water, air, and heat where they belong. From inside your room to open sky, the assembly stacks up like this:

• Interior ceiling and light well or shaft
• Roof deck-wood planks, plywood, steel, or concrete
• Insulation and vapor control layers
• Roof membrane: EPDM, TPO, PVC, modified bitumen, or built-up roofing
• Wood or metal curb fastened to the structure and wrapped in membrane
• Curb-mounted skylight unit attached and sealed to the top of the curb

Each layer has to do its job correctly, but the curb is where most failures happen. If the curb is too short, water reaches the joint. If the curb isn’t square or level, the skylight won’t seal evenly. If the membrane doesn’t run up and over the curb properly, you get a weak spot that will leak within a year.

Curb height really matters. NYC and Brooklyn conditions-and manufacturer guidelines-generally call for a curb high enough to stay above likely snow and water levels. That often means 8 to 12 inches above the finished roof for true flat roofs. I usually spec 10 inches minimum on Brooklyn rowhouse roofs where snow drifts against parapet walls and ponding happens in the same spots every spring. Too low, and meltwater and ponding will challenge the flashing every winter. An extra two inches of curb height costs almost nothing and buys you decades of reliability.

Plan the Skylight Before Anyone Cuts the Roof

The time to fix skylight problems is before the saw comes out. Once the opening is cut, your options narrow fast. Smart planning means deciding on paper-or at least with a tape measure and a flashlight in the attic-where the skylight will go, how big it will be, and how interior framing will create the light shaft from your ceiling up to the curb.

Decisions to make on paper first:

• Which room are you trying to light, and where is the safest spot in the roof above it?
• Will the skylight be fixed or venting? Manual crank or electric motor?
• How will interior framing create the light shaft from ceiling to curb?
• Does the chosen location conflict with structural members, ducts, or existing wiring?
• How will water drain around the curb-does it sit in a high point or a natural pond?

That last question matters more than most people expect. If you drop a skylight in the middle of an existing ponding area, you’re asking for trouble no matter how good the flashing is. We try to place skylights in naturally higher spots on the roof, or we build up tapered insulation to push water around the opening instead of against it.

Permits and approvals in Brooklyn: Cutting a new opening in a flat roof or enlarging an existing one generally requires design and filings, especially in multi-family or mixed-use buildings. Even simple replacements should respect DOB requirements and any co-op, condo, or landmark rules for your block. I’ve seen projects stall for months because the skylight went in without approvals and the building couldn’t get a sign-off later. It’s worth sorting out permits before the work starts.

Professional Installation Sequence for a Curb-Mounted Skylight

Here’s how a Brooklyn roofer typically installs a curb-mounted skylight, step by step. This is the sequence I follow on rowhouse and low-rise projects where the roof is accessible and the interior is either vacant or carefully protected.

1. Layout and structural check. The installer marks the skylight location from inside and on the roof, confirming that joists or beams can be framed around safely and that the opening aligns with the interior room below. On older buildings, we often find joists running the wrong direction or unexpected blocking that forces us to shift the layout a few inches. Better to find that now than after the cut.

2. Cut the roof opening. Roof membrane and deck are cut carefully to the layout, with temporary protection set up so no debris or water falls into the space below. If the building is occupied, we coordinate with tenants and stage tarps inside before the saw touches the roof. Weather matters here-we don’t cut openings with rain in the forecast.

3. Build or set the curb. A wood or prefabricated curb is fastened to the structure, squared, and checked for level. Height is set to keep the future flashing well above the roof surface and snow line. I typically build curbs from pressure-treated 2×10 or 2×12 lumber, lag-bolted through the deck into solid framing. Prefab metal curbs work too, but wood gives you more flexibility to adjust for out-of-square conditions on older roofs.

4. Wrap the curb into the flat roof system. Roof membrane is brought up the sides of the curb, with corners, seams, and transitions flashed using details specific to your roof type-EPDM, TPO, modified bitumen, or whatever’s already up there. This step makes the curb part of the roof, not an add-on. The membrane should run at least six inches up the curb on all sides, with proper inside and outside corner detailing so there are no gaps or fish-mouths where water can sneak in.

5. Install the skylight onto the curb. The curb-mounted skylight is set in place, fastened through its flanges into the top of the curb, and sealed per manufacturer instructions. Most units come with a foam or rubber gasket that compresses between the skylight frame and the curb. Care is taken not to over-tighten fasteners and crush that seal. If the skylight is venting, we check that the operator works smoothly and that rain sensors and motors are wired correctly.

6. Exterior and interior finishing. Any exterior trim or metal cladding is installed to protect the top of the curb from UV and weather. Inside, the light shaft is framed, insulated, and finished so the skylight doesn’t become a condensation or draft point. That interior work is often underestimated-it takes as much time as the roof work and matters just as much for comfort and durability.

7. Final checks and water test. The roofer visually inspects all seams and penetrations, then may perform a controlled water test, especially on roofs prone to ponding. We’ll flood the area around the curb with a hose and watch for any drips inside. It’s a simple test, but it catches problems while the crew is still on site and tools are handy.

Tying the Curb Into Different Flat Roof Membrane Types

Not all flat roof membranes flash the same way. The curb-wrapping details change depending on whether you’re working with EPDM rubber, heat-welded TPO, or torch-down modified bitumen. Here’s what matters for each type:

EPDM (rubber roofs): Use compatible adhesives and primers around the curb-mixing product lines is a common mistake that leads to failed bonds. Avoid sharp edges on the curb that can cut or stress the membrane. Pay special attention to inside and outside corner patches, which are the most vulnerable spots. EPDM corners are usually built up with two or three layers of material, each one lapped and adhered carefully.

TPO and PVC single-ply: Curb wrap is typically heat-welded for a monolithic seal. Use preformed corners where possible for cleaner detailing-they’re faster to install and more reliable than field-cut patches. Keep fasteners concealed and properly covered at the curb base. TPO and PVC are strong membranes, but they’re unforgiving if the welds aren’t done right. A good installer will probe every seam with a blunt tool after welding to check for voids.

Modified bitumen and built-up roofs: Layer base and cap sheets up the curb in a staggered pattern, so seams don’t line up and create a path for water. Use appropriate torches or cold-applied methods with fire safety in mind-open flame on a Brooklyn roof with wood framing and tarred shingles below is something we take seriously. Integrate the curb flashing with any existing cant strips or base flashings at parapets so the whole system flows together.

Avoiding Drainage and Ponding Problems Around the Skylight

Even a perfectly installed skylight will struggle if it’s sitting in standing water half the year. Curb height helps, but placement and slope matter just as much. Here’s what works and what doesn’t:

Good practices for skylight placement on flat roofs:

• Place the curb in a naturally higher spot on the roof whenever possible.
• Use tapered insulation or crickets to push water around the skylight, not against it.
• Keep skylights away from primary drains and scuppers so they don’t become debris traps.

Bad practices to avoid:

• Don’t drop a skylight directly in the middle of an existing ponding area.
• Don’t let water shed from a higher wall directly into the uphill side of the curb.
• Don’t rely solely on sealant where proper slope and flashing are missing.

Designed for Brooklyn rain and snow: We design curb height and drainage details with heavy summer downpours and freeze-thaw winters in mind, so melting snow and ice don’t sit and work their way into the opening. I’ve repaired skylights on Bedford-Stuyvesant rowhouses where the original installer used a four-inch curb and no slope correction-those units leaked every March when snow melted and refroze against the curb. Raising the curb to ten inches and adding a small cricket solved the problem permanently.

What You Can Decide Yourself vs. What the Roofer Must Do

There’s a natural dividing line between planning decisions you can make and technical work that belongs to a professional crew. Here’s how that breaks down:

Decisions and prep you can handle:

• Choosing which room you want to brighten.
• Deciding between fixed and venting, and glass options like tinted or low-E.
• Collecting photos of the existing roof and interior so the roofer can plan access and protection.
• Noting any history of leaks, condensation, or drafts that might affect placement or detailing.

Tasks for a Brooklyn flat roof skylight pro:

• Cutting openings in the roof deck or structure.
• Framing and anchoring the curb safely to existing joists.
• Integrating curb flashing with the existing membrane.
• Working at height on unprotected flat roofs.

Flat roof work is fall and puncture risk work. Skylights-especially old acrylic domes-are not meant to be stepped on. Professional crews use proper access, protection, and harnesses where needed to avoid accidents and damage. I’ve seen homeowners punch through old skylights trying to clear snow or inspect a leak. The fall risk is real, and the repair bill is high.

Brooklyn-Specific Experience Matters With Curb-Mounted Skylights

Installing a curb-mounted skylight on a brochure-perfect new flat roof is one thing. Doing it on a seventy-year-old Brooklyn walkup with three past roof layers, parapet walls, and shared access is another. Local experience shows up in the details:

• We’re familiar with older joist layouts-16-inch centers, 24-inch centers, or no consistent pattern at all-and how to frame around them without cutting structural members.
• We know how to tie into patchwork roofs where one section is EPDM and another is still torch-down, without creating weak spots.
• We understand co-op, condo, and DOB expectations for penetrations and curbs, and we know which details trigger scrutiny and which sail through.
• We plan access and protection so tenants and neighbors aren’t surprised by noise, dust, or equipment staged in shared spaces.

On a Park Slope project in 2019, we installed two curb-mounted skylights in a building where the roof deck was ship-lap boards over 3×10 joists running front-to-back. The original skylight had been framed into the deck with no headers, and the surrounding boards were rotted from decades of slow leaks. We ended up sistering new joists, replacing twelve feet of decking, and rebuilding both curbs from scratch. That’s typical for older Brooklyn buildings-the skylight install is straightforward, but the prep work takes longer than anyone expects.

Looking After Your New Curb-Mounted Skylight

Once the skylight is in and sealed, a little attention goes a long way. Flat roofs need regular care anyway, and the skylight area should be part of that routine.

A quick care routine for Brooklyn flat roof skylights:

• Have the roof and skylight area checked annually for membrane cracks, loose flashing, or ponding that’s getting worse.
• Keep leaves and debris from piling up at the uphill side of the curb, especially in fall when gutters and drains clog.
• Have seals and moving parts on venting units inspected every few years-operators, hinges, and rain sensors wear out.
• Watch for interior condensation that could signal humidity or insulation issues, not just skylight problems. A skylight that’s dripping in winter isn’t always leaking-it might be condensation from a poorly insulated shaft.

Curb-Mounted Skylight Installation FAQs

Can I reuse my existing curb for a new skylight? Sometimes. If the curb is solid, properly flashed, high enough above the roof, and sized to match the new unit, it may be reused. On many older Brooklyn roofs, we replace or adjust the curb to correct height and repair hidden damage that shows up once the old skylight comes off.

How long does it take to install one curb-mounted skylight? Once the unit is on site and weather cooperates, many single-skylight installs take about a day-longer if structural changes, interior shaft work, or roof repairs are part of the job. Two-day installs are common when the interior isn’t ready or the roof needs prep work before we can start.

Will installing a skylight weaken my flat roof? Not when it’s framed and supported correctly. The opening is reinforced and trimmed so loads move around it safely. That’s why structural checks and proper curb fastening are so important. A skylight opening is smaller and simpler than most stair or HVAC penetrations, and those don’t weaken roofs when done right.

Is there more leak risk with a venting skylight? Venting units add seals and moving parts, so they need careful installation and occasional maintenance. When installed properly with a good curb and flashing, they can be just as watertight as fixed units. The risk comes from operators that aren’t fully closed or rain sensors that fail-both fixable with basic service.

Do I need interior work as well as roof work? Yes, usually. The shaft from your ceiling to the curb needs framing, insulation, and finishing so the opening looks clean and doesn’t create condensation or thermal bridges. Budget for interior work that’s roughly equal in time and cost to the roof work-it’s part of the project, not an add-on.

Plan Your Curb-Mounted Skylight Install on a Brooklyn Flat Roof

If you’re ready to bring natural light into a Brooklyn flat roof, start with an on-roof review of structure, membrane type, and the best skylight locations for your building. We’ll walk the roof with you, check joist direction and spacing, note any ponding or drainage issues, and recommend sizing, curb height, and drainage improvements tailored to your building.

What you get from a skylight planning visit:

• On-roof review of structure, membrane type, and best skylight locations
• Sizing, curb-height, and drainage recommendations tailored to your building
• Clear install sequence and coordination plan for tenants or interior trades

Already working with an architect or contractor? We can coordinate details of the curb, flashing, and membrane integration so your curb-mounted skylight works perfectly with the rest of the project. Skylight installs go smoothest when the roofer, framer, and interior finisher talk to each other before anyone cuts anything.

Call or email to schedule a planning visit, or send photos of your roof and the room below for a preliminary review. Most skylight decisions can be made in a fifteen-minute conversation once we see the roof and understand what you’re trying to light.