Skylight Construction for Flat Roofs

When you add a skylight to a flat roof, you’re removing a section of the structural system that’s supporting your building-and creating a permanent opening where water naturally wants to collect. Adding skylights to a flat roof can completely change how a Brooklyn home feels-suddenly dark kitchens, hallways, and top-floor rooms become bright and livable. But cutting a hole in a flat roof is not like dropping a window into a wall. The entire skylight construction-from framing and curb height to flashing and drainage-determines whether you get years of daylight or a recurring leak every time it snows or pours.

Design Skylights for Flat Roofs That Let in Light, Not Water

The reason skylight construction on flat roofs is so unforgiving is simple physics: gravity doesn’t help you here. On a pitched roof, water runs down and away from the skylight naturally. On a flat roof (or low-slope roof), water sits, pools, and tests every joint until it finds a way through. That’s why proper skylight construction on a Brooklyn flat roof starts with structural planning, continues through waterproofing design, and ends with thermal details that prevent condensation from making you think you have a leak when you don’t.

This page is for you if you want to:

• Understand how skylights are built into flat roofs structurally and waterproofed correctly
• Compare different skylight construction options for flat roofs (curb-mounted, flat glass, lanterns)
• Know what to ask your roofer or designer so the details get done right in Brooklyn’s climate
• See where skylights belong in the sequence of a flat roof or extension project

Be Clear What Your Flat Roof Skylights Need to Do

Before anyone specifies curb heights or membrane types, you need to define what the skylight is for. Different goals produce very different construction approaches.

I’ve seen too many Brooklyn projects where the homeowner picked a skylight purely on looks, only to discover mid-install that it required joists to be cut, curbs to be rebuilt, or that it ponded water at every corner because no one designed drainage around it. Function drives construction details-get that order right.

Flat Roof Skylight Systems: What You’re Actually Building

There are four main skylight constructions used on flat roofs in Brooklyn, and they differ in how they integrate with your roof structure and waterproofing system:

Curb-mounted skylights: A skylight unit sits on a raised curb that’s framed and flashed as part of the flat roof. The curb height and waterproofing become part of the roof design, not just the window spec. Best for most Brooklyn EPDM, TPO, or modified flat roofs over living space.

Flat glass roof windows (with integrated upstands): Slim-profile units designed for flat roofs, often supplied with their own upstand. The upstand needs to coordinate with insulation depth, membrane, and ceiling height. Best for modern extensions, sunrooms, and minimalistic interiors.

Roof lanterns on flat roofs: Multi-pane, pitched glass structures mounted on a rectangular upstand. Essentially a small roof on top of a roof-upstand and framing must be engineered like any other roof opening. Best for feature spaces like rear kitchen and dining extensions.

Glazed roof hatches: Access hatches with glass lids for light and roof access combined. They behave like heavy skylights structurally and need solid support and careful curb detailing. Best for lofts and roofs used for maintenance or decks.

Anatomy of a Skylight in a Flat Roof: Layer by Layer

To understand skylight construction on a flat roof, you need to see the vertical build-up from room to sky. Here’s what’s actually there, from ceiling to outside:

1. Finished interior ceiling and light shaft (drywall or plaster, paint, trim)
2. Roof deck structure (joists, beams, or slab around the opening)
3. Vapor control layer (if part of the roof build-up)
4. Insulation (warm roof or cold roof configuration)
5. Roof deck sheathing (plywood or OSB for timber roofs, boards for steel or concrete)
6. Waterproofing membrane (EPDM, TPO, PVC, modified bitumen, or liquid system)
7. Upstand or curb built above the membrane plane
8. Skylight frame and glazing fixed to the curb

Why the curb is the pivot point: The upstand or curb is where structure, insulation, membrane, and skylight all meet. Its height, width, and how it’s fixed to the deck are decided with both the roof system and the chosen skylight in mind-not as a last-minute box someone nails on top. I’ve repaired more skylight leaks caused by weak curbs than by bad skylights.

Structural Design: Framing the Opening Before the Skylight Goes In

When I arrive at a Brooklyn rowhouse to plan skylights, the first thing I do is go into the attic or top floor and look at the joists. Which direction do they run? What’s the spacing? Are they original 2x8s from 1920 or newer engineered lumber?

Structural questions answered before ordering skylights:

• Which direction do your existing joists run, and what is their span and spacing?
• Can we place skylights between joists, or do we need to trim and double them?
• Will the opening affect any main beams, bearing walls, or load-bearing party walls?
• Is there enough depth for the light shaft without clashing with ductwork or pipes?

Many brownstones and rowhouses have older, sometimes irregular joist layouts and multiple generations of roof framing. Before we cut anything, we confirm what’s actually there-not just what’s shown on old drawings. I’ve seen “non-structural” ceilings that were actually holding up roof rafters, and “simple joist bays” that had hidden steel beams running through them.

Curb Design: Height, Material, and Integration with the Flat Roof

The curb is the most critical piece of skylight construction on a flat roof, and it’s where most amateur installations fail. Here’s what we’re deciding:

Curb height above finished roof: Needs to clear typical snow and ponding levels. On true flat roofs in Brooklyn, we typically build curbs 8-12 inches high. Too low and water, ice, and snow challenge the joint constantly. I repaired a Bed-Stuy skylight last year where the curb was only 4 inches high-it leaked every winter because snow piled against it and melted into the flashing seams.

Curb width and construction: Wide enough for proper fastening and insulation, but not so wide that it becomes a cold bridge. Material choices include treated timber, steel curb, or proprietary upstands. We typically frame curbs from double 2x lumber or build them from plywood boxes, depending on the skylight weight and roof system.

Insulation at the curb: Avoids cold spots and condensation around the light shaft. Detailing must tie roof insulation into wall and shaft insulation smoothly. This is where condensation problems start if you skip it-the curb becomes a cold frame that drips onto your new ceiling.

Membrane turn-up and termination: The roof membrane needs to wrap up the curb with neat, reinforced corners, then terminate under the skylight flange or separate flashings. This is membrane origami-every fold, every corner, every fastener placement matters.

Waterproofing Details Around Flat Roof Skylights

How we waterproof curbs depends entirely on your flat roof membrane type. Here’s what changes:

EPDM: Use compatible adhesives and primers; avoid petroleum-based mastics. Form continuous corner patches to avoid slicing and patching at every corner. Keep mechanical fasteners away from critical corner upturns. EPDM is forgiving in some ways-it stretches-but unforgiving if you puncture it or use the wrong tape.

TPO and PVC: Heat-weld seams at curb faces; avoid tight folds that can’t be welded properly. Use pre-formed outside and inside corners where possible for cleaner transitions. Confirm curb substrate is suitable for welding-separation sheets as needed. These systems create the strongest joints when done right, but there’s no margin for error on weld quality.

Modified bitumen and liquid-applied: Step multiple plies up the curb for redundancy. Maintain safe clearances and fire safety when torching near skylight units. With liquids, pay attention to recommended thickness and curing around verticals. I prefer modified bitumen around skylights when the rest of the roof is mod-bit-it’s repairable and inspectable in ways that single-ply isn’t.

Never make the skylight the lowest point: We design slope and sometimes add crickets around the uphill side of skylights so water flows around them, not toward them. Skylights should never sit inside a natural ponding area. That’s asking for leaks and shortened membrane life. On one Prospect Heights project, we added a small saddle behind a large skylight to divert water-it cost an extra half-day of framing but eliminated the ponding issue completely.

Thermal and Condensation Control Around Skylights

You can have a perfectly watertight skylight and still have water dripping into your room. That’s condensation, and it happens when warm interior air hits cold surfaces around the skylight frame or shaft walls.

Construction mistakes that lead to condensation and drafts:

• Light shaft walls not insulated to match surrounding roof and wall assemblies
• Gaps in vapor control where the shaft meets ceilings or walls
• Metal curbs or frames with no thermal break, creating cold lines above drywall
• Single-glazed or poor-quality units creating cold interior glass surfaces

How we build to avoid those issues:

• Wrap shaft walls with continuous insulation where feasible
• Seal air leaks around the top of the shaft and at ceiling junctions before finishes go in
• Specify double- or triple-glazed skylights with low‑E coatings for habitable rooms
• Coordinate HVAC and ventilation so moist air from bathrooms or kitchens doesn’t just hit cold skylight glass

Where Skylight Work Fits in Your Flat Roof or Extension Project

Skylight construction isn’t a single trade-it touches framing, roofing, waterproofing, and interior finishes. Here’s when each decision and task happens:

Design and permitting: Decide skylight size, type, and approximate locations. Engineer openings and confirm with DOB, your co-op board, or landmarks where required. Coordinate skylight placement with beams, joists, and duct runs. This is when we catch conflicts-not mid-demo.

Structural framing: Cut and frame openings in decks and roofs to match skylight layout. Build rough light shafts where necessary. Frame curbs or prepare to install proprietary upstands. At this stage, the skylight is just a hole with blocking around it.

Roofing and waterproofing: Install roof insulation, deck, and membrane. Wrap and flash curbs into the flat roof system. Install skylight units once membrane work is watertight. The skylight goes in after the roof is done enough to protect the opening-never before.

Interior finishes: Insulate and line light shafts. Finish ceilings and integrate with lighting plans. Caulk and paint; final clean-up of glass and frames. This is when the skylight stops looking like construction and starts looking like part of your home.

Brooklyn-Specific Skylight Construction Challenges

Brooklyn flat roofs come with constraints you don’t find in newer suburban construction:

• Historic or soft brick around skylight curbs on older rowhouses: We can’t always anchor curbs into masonry the way we’d like, so we design structural ties back to the deck framing instead.
• Close neighbors and party walls shading roofs or funneling wind-driven rain: That narrow alley between buildings becomes a wind tunnel during storms-flashing details need to account for lateral water movement, not just gravity.
• Multiple existing roof layers that must be cut and rebuilt cleanly around openings: I’ve opened roofs in Greenpoint and found four generations of roofing. You can’t just flash into that-it all has to come off around the skylight zone.
• Access through narrow stairs and hatches: Affects skylight size and installation method. If your building’s stair is 32 inches wide and turns twice, you’re not getting a 4-foot roof lantern up there in one piece.
• Co‑op, condo, or landmark rules about how visible skylights and lanterns can be: Some buildings allow skylights only if they’re invisible from the street. That changes placement and sometimes rules out certain designs entirely.

Examples from our Brooklyn flat roof skylight builds: We installed two curb-mounted, double-glazed units aligned over an island in a top-floor brownstone kitchen in Park Slope. Curbs were integrated with a new warm-roof system, and interior shafts were flared to spread light across the room instead of creating two bright spots. In a rear Bushwick extension, we placed three slimline flat glass skylights between joists over a dining area-drainage slope was carefully coordinated so no ponding formed at the upstands.

What You Decide vs What Your Roofer and Designer Engineer

You’re in charge of the vision and comfort goals. Your roofing and structural team handles the technical execution. Here’s how that breaks down:

Design and comfort choices you can lead:

• Which rooms and zones you want to light from above
• Preference for flat glass vs domes vs lanterns
• How bold you want the skylight to appear from outside
• Tolerance for heat gain, glare, and maintenance demands

Construction decisions for the pro team:

• Sizing and reinforcing openings in existing roofs
• Curb height and integration with membrane and insulation
• Flashings, terminations, and drainage design around skylights
• Compliance with NYC code, fire, and energy requirements

Skylight Construction on Flat Roofs: Common Questions

Are skylights more likely to leak on a flat roof than on a pitched roof?
They can be if they’re detailed like pitched-roof skylights. On a flat roof, the upstand height, membrane integration, and drainage around the skylight are more critical. When those are designed correctly, skylights can be just as reliable as on a pitched roof.

Can I add skylights to my existing flat roof without replacing the whole roof?
Sometimes. If the roof is in good condition with enough remaining life, we can cut and frame openings, build curbs, and tie into the existing membrane. If the roof is near the end of its life, adding skylights at the same time as a reroof is usually smarter and more cost-effective.

How many skylights can I add before it weakens the roof?
There isn’t a fixed number. It depends on the roof size, joist layout, and opening placement. Multiple smaller units between joists can be structurally easier than one huge opening, but this should always be checked by someone who understands the existing structure.

Do I need a permit to add skylights to my flat roof in Brooklyn?
Cutting new openings typically does, especially in multi-family or mixed-use buildings. Replacement of same-size existing skylights may be simpler. It also depends on whether any structural changes are involved. A local architect or expeditor can clarify based on your building type.

Will skylights make my top floor too hot in summer?
They can if you choose low-spec glazing and don’t manage shading. Using low‑E and solar-control glass, considering orientation, and sometimes adding interior blinds or external shading can keep the space comfortable while still bringing in light.

Plan Skylight Construction for Your Brooklyn Flat Roof

Good skylight construction on a flat roof starts with understanding what you’re cutting into, how water will behave around the new opening, and how all the layers-structure, insulation, membrane, glass-work together instead of fighting each other.

Get skylights that work with your flat roof, not against it:

• On-site review of your existing flat roof structure and drainage
• Advice on skylight types, sizes, and placement specific to your rooms and roof
• A construction plan that integrates framing, waterproofing, and interior finishes

Already working with an architect or interior designer? FlatTop Brooklyn coordinates structural openings, curb details, and roofing work so your skylight concept on paper becomes a watertight, well-built reality on your Brooklyn flat roof. Request a flat roof skylight consultation and we’ll walk your roof, review your plans, and show you exactly how the skylight construction will be sequenced and detailed for your building.