Deck Mount Flat Roof Skylight Installation

Last summer I pulled up to a beautiful Cobble Hill brownstone where the owner had installed a “deck mounted” skylight himself-placed directly on the flat roof deck like a ground-floor window, no raised curb, just a thin bead of caulk around the flange. Two hard rains later, water was running down the interior shaft walls. A deck mounted skylight on a flat roof needs a carefully built slope element or low curb to shed water away from the unit, plus flashing that ties into your membrane system at least six inches above the high-water mark. Without that integration, you’re just creating a permanent leak point. In thirteen years of roofing and daylighting work in Brooklyn, I’ve come to view every deck mounted skylight install on a flat roof as a mini roof project inside the roof-its own structure, slope strategy, and waterproofing plan.

What Is a Deck-Mounted Skylight on a Flat Roof?

A deck-mounted skylight is a factory-framed glazing unit that fastens directly to the roof deck through an integrated flange or attachment edge, rather than sitting on top of a tall raised curb like traditional dome skylights. On pitched roofs, the deck-mount flange simply lays over shingles and gets step-flashed. On true flat roofs in Brooklyn, however, deck-mounted units almost always require a small built-up curb or slope box underneath to keep rainwater and snowmelt moving away from the glass perimeter. The flashing kit-whether factory-supplied or site-built-must then integrate that curb into your existing membrane system (EPDM, TPO, PVC, or modified bitumen) to create a watertight transition.

Deck-mounted vs. curb-mounted at a glance:

• Deck-mounted: Lower profile, cleaner modern look, often used with fixed or venting glass skylights; attachment flange sits flat against the deck or a low curb.
• Curb-mounted: Glass or acrylic dome sits on a taller raised curb (8-12 inches or more); common on commercial buildings and older residential flat roofs.
• On flat roofs: Many so-called “deck-mounted” installs end up being hybrid-a low curb built to introduce slope, then deck-mounted flashing on top of that curb.

This guide focuses on how professionals plan, frame, and flash deck-mounted-style skylights on Brooklyn’s flat roofs. My goal is to help you evaluate contractor quotes, spot risky shortcuts, and understand why structural framing and waterproofing details matter more than the skylight model itself.

Is Your Brooklyn Roof a Good Candidate for a Deck-Mounted Skylight?

Not every flat roof and interior layout benefits from a deck-mounted skylight. Structural condition, membrane age, interior ceiling access, and surrounding building heights all play a role. Before you spend money on engineering drawings or skylight units, run through a basic feasibility check. Some roofs are naturals; others require significant remediation work before a skylight makes sense.

I once turned down a job on a Clinton Hill rowhouse where the owner wanted a 4×6-foot skylight directly above the kitchen island-but the roof deck had three layers of asphalt shingles over unknown sheathing, and water pooled six inches deep in that exact spot every storm. Adding a skylight there would have meant a full roof tearoff, new tapered insulation to eliminate ponding, structural assessment, and a raised curb tall enough to stay above snow. The homeowner ended up replacing the entire roof first, then we installed the skylight the following spring on a clean TPO membrane with proper drainage.

Anatomy of a Deck-Mounted Skylight on a Flat Roof

Understanding how the layers stack helps you see where leaks start and why each component matters. A deck-mounted skylight on a flat roof in Brooklyn isn’t just a hole with glass dropped in-it’s a carefully sequenced assembly where structure, air sealing, waterproofing, and glazing performance all have to work together. From the sky down to the room below, here’s what you’re building:

1. Glazing and skylight frame: Fixed or venting unit, typically dual-pane insulated glass with low-E coatings for energy performance and condensation control. Frame material is aluminum, vinyl-clad wood, or fiberglass.
2. Deck-mount flange or curb attachment surface: The skylight’s nailing or fastening flange sits on the horizontal surface of a built-up curb or directly on the roof deck if enough slope exists.
3. Flashing kit: Step and counter-flashing components (factory pre-formed or site-fabricated) tailored to flat or low-slope conditions; these redirect water down and around the skylight opening.
4. Primary roof membrane or cap sheet: Your existing EPDM, TPO, PVC, or mod-bit membrane is cut back from the opening, then turned up and onto the curb sides, lapping with the flashing layers to create continuous waterproofing.
5. Roof deck and framed opening: Structural opening with installed headers and trimmer joists that transfer load around the cut area; must meet span and bearing requirements per local code.
6. Insulated, air-sealed light shaft: Framed tunnel or shaft through the attic, plenum, or joist space; insulated to match surrounding roof/ceiling assemblies and air-sealed to prevent warm, moist interior air from reaching cold surfaces.
7. Interior ceiling trim and finish: Drywall, paint, and trim around the skylight opening at ceiling level; integrates with interior vapour control and provides a finished appearance.

Water moves down-slope and away from the glass perimeter because the curb or slope box underneath redirects it. Air and vapour control must be continuous-sealed at the roof deck level and again at the interior ceiling-so you don’t get condensation drips on cold winter mornings when warm air leaks into the cold shaft space.

Step 1: Structural and Layout Planning

Skylights add direct openings in your roof deck, which means you’re cutting through joists, beams, or concrete slabs that were designed as continuous structural elements. A Brooklyn building frame-especially in older rowhouses and mixed-use buildings-may have been altered over decades: sistered joists, notched beams, and shifted load paths. Before any cutting happens, you need to understand what’s carrying the roof load and how you’ll reroute forces around the new opening. Skipping this step is how you end up with sagging ceilings, cracked plaster, or worse.

Structural checks before you cut:

• Identify joist direction and spacing (typically 16 or 24 inches on center in wood-frame buildings) to minimize the number of members you need to cut.
• Plan double headers and trimmer joists to frame around the skylight opening and transfer loads to adjacent full-length joists or beams.
• Verify that bearing points in party walls, steel beams, or masonry are adequate for the re-routed loads; don’t assume old mortar or undersized plates can handle new concentrated forces.
• Get a licensed structural engineer to review and stamp drawings if you’re cutting more than one joist or if the building has any unusual framing or prior alterations; DOB inspectors will ask for stamped plans.

Interior layout and light planning:

• Align the skylight directly above the space that needs daylight-kitchen island, interior stairwell, hallway, or dark bedroom-not just wherever it’s easiest to cut.
• Confirm the shaft path is clear of ductwork, plumbing risers, electrical conduit, and structural cross-bracing; rerouting mechanicals adds cost and complexity.
• Consider how far natural light will spread from the opening and where direct sun or glare might create problems (computer screens, bedroom windows facing east).
• Factor in privacy from neighboring buildings; taller structures next door may look straight down into your skylight, especially in dense Brooklyn blocks.

On a Park Slope townhouse project, we planned a 3×5-foot skylight above a central stairwell that had been dark for a hundred years. The joist layout was perfect-opening fit between two joists with minimal cutting-but the shaft path ran right past a cast-iron waste stack from the third-floor bathroom. We ended up routing the shaft around the stack with angled drywall returns, which added two days of carpentry work but avoided replumbing the entire building. Layout planning caught that conflict before we cut the roof.

Step 2: Opening the Roof and Building the Opening

This is the messy, structural part that happens before any waterproofing or skylight setting. You’re cutting through your existing roofing system and deck in a controlled demolition, safely supporting any severed joists, and framing a box that matches the skylight’s rough opening dimensions. Precision here determines whether your skylight sits square, level, and at the correct height relative to the finished roof surface. Rush this step and you’ll spend the rest of the project fighting out-of-square gaps and ponding water around the unit.

2.1 Mark and confirm layout: Locate the opening from inside using measurements off walls or existing features, then drill pilot holes up through the roof at the four corners. Go back outside, snap chalk lines connecting the holes, and double-check dimensions and squareness before cutting. Make sure your lines avoid cutting through more joists than planned and that the opening aligns with interior shaft walls below.

2.2 Remove roofing and deck: Peel back your roof membrane or cap sheet at least twelve inches beyond the opening footprint on all sides-you’ll need that clean deck surface for new framing and flashing later. Cut the roof deck cleanly along your layout lines using a circular saw set to deck thickness; watch for hidden fasteners, old vent pipes, or electrical that shouldn’t be there. Protect the interior space below with tarps and have a pump and temporary cover ready in case weather moves in.

2.3 Frame the rough opening: Install double headers across the cut joist ends, fastening them with structural screws or bolts per your engineer’s details. Add trimmer joists along the sides of the opening, tying into the headers and running parallel to the existing full joists. Check the framed opening for square (measure diagonals), level (even if the roof slopes slightly), and correct dimensions per the skylight manufacturer’s rough opening spec-usually an inch or two larger than the unit itself. This framed box is what you’ll attach the skylight curb or flange to, so it has to be rock-solid and dimensionally accurate.

Step 3: Mounting the Skylight and Integrating With the Flat Roof System

On flat roofs, deck-mounted skylights almost always sit on a small, carefully built curb or slope box that introduces pitch even when the surrounding roof is dead flat. That curb might be only four to eight inches tall, but it’s critical: it keeps the bottom edge of the skylight above any standing water, snow, or leaf debris, and it gives you vertical surfaces to properly flash against. Proper fastening to the deck and flawless integration with your waterproofing membrane is what keeps this installation watertight for ten or twenty years. Skimp on flashing details and you’ll be back on the roof every spring fixing leaks.

3.1 Prepare the mounting surface: If your roof is truly flat or has minimal slope, build a small curb or pitch box around the rough opening. Frame it from treated lumber or use prefabricated skylight curbs if available for your unit. The top surface of this curb should be flat and level (even if the roof around it slopes slightly) so the skylight frame seats evenly. Make sure the curb height puts the skylight base at least four inches above the finished roof membrane level-higher in areas prone to heavy snow or ponding. Sheath the curb with plywood, fasten it securely to the roof deck and framing below, and ensure all four sides are straight and square.

3.2 Set and fasten the skylight: Lift the skylight unit onto the prepared curb, oriented correctly (most units are marked “top” or have weep slots at the bottom). Check that the frame sits flat on all four sides with no rocking or gaps. Fasten through the mounting flange or directly into the curb using stainless or coated screws as specified by the manufacturer-typically every six to eight inches around the perimeter. Don’t over-tighten; you’ll distort the frame and make the glass bind or crack. If it’s a venting skylight, test the operation now before flashing locks everything in place.

3.3 Flash into the roof system: This is where your roofing membrane type dictates the process. On single-ply roofs (EPDM, TPO, PVC), you’ll use compatible flashing membranes that are heat-welded or adhered to the main field membrane and turned up onto the skylight curb and over the mounting flange. Use factory pre-formed inside and outside corners to avoid wrinkles and fishmouths that trap water. On modified bitumen or built-up roofs, layer your base sheet and cap sheet up the curb sides with proper laps-base sheet first, then cap over it-and reinforce all inside corners with extra plies. If the skylight manufacturer provides a metal or plastic flashing kit, coordinate it carefully with your site-built membrane flashing so you don’t create reversed laps where water can run backward under the layers.

I always test the flashing by flooding the roof around the skylight with a hose for fifteen minutes, watching for any seepage at the curb corners or where the membrane meets the skylight frame. If water gets in during testing, it’s a twenty-minute fix. If it shows up after the shaft is drywalled and painted, it’s a day-long tearout and repair.

Detail Differences by Roof Type (EPDM, TPO/PVC, Mod-Bit)

Brooklyn’s flat roofs use a variety of membranes, and each requires different primers, adhesives, and flashing techniques around deck-mounted skylights. Using a generic detail-or worse, mixing incompatible materials-can void your roof warranty and create slow leaks that don’t show up until the skylight has been in place for a year. Here’s how the three most common systems differ when you’re tying a skylight curb into the field membrane.

Warranty impact: Most roof system warranties require skylight flashing details to follow the membrane manufacturer’s published guidelines exactly. If you use off-brand flashings, incompatible sealants, or skip specified reinforcements, the warranty administrator will exclude leak coverage around the skylight-even if the field membrane is fine. Always get your skylight flashing plan approved in writing by the roof system manufacturer or their certified contractor before you start cutting.

Interior Shaft, Condensation, and Comfort

How you finish the inside of a deck-mounted skylight matters as much as the exterior flashing. A poorly insulated or leaky shaft can sweat, stain ceilings, and make rooms drafty-even if the roof above never leaks a drop. Brooklyn winters bring cold outdoor air against warm, humid indoor air, and any thermal bridge or air leak in the skylight shaft becomes a condensation factory. I’ve seen brand-new skylights blamed for “leaking” when the real problem was condensation running down uninsulated shaft walls and dripping onto the ceiling below.

Moisture and condensation control:

• Air-seal the shaft at both ends: Use spray foam, caulk, or gaskets where the shaft meets the roof deck and again where it meets the interior ceiling. Warm, moist air from the living space should never reach cold surfaces inside the shaft.
• Insulate shaft walls: Pack fiberglass batts or spray closed-cell foam into the shaft framing to match or exceed the insulation level of your surrounding roof or attic assembly. Don’t leave gaps at corners or around framing members.
• Vapour control: In NYC’s mixed climate, install a vapour retarder (poly sheet or foil-faced insulation) on the warm side of the shaft insulation-usually the interior face-to stop moisture diffusion into cold cavities during heating season.

Light and heat comfort:

• Choose appropriate glazing: Low-E coatings and low solar heat gain coefficients reduce summer overheating. If the skylight faces south or west, consider tinted or spectrally selective glass.
• Diffuse direct sun: In bedrooms, home offices, or TV rooms, use skylights with built-in diffusers, frosted glass, or add interior cellular shades to control glare and heat.
• Size for the room: Multiple smaller skylights (two 2×4 units instead of one 4×4) often distribute light more evenly and reduce hot spots under a single large opening.

On a Bed-Stuy duplex, the owner installed a big 4×6 skylight above the main bedroom without any interior shades or low-E glass. By July, the bedroom was ten degrees hotter than the rest of the apartment, and condensation pooled on the glass every winter morning, dripping onto the bed. We retrofitted cellular honeycomb shades and added storm glazing on the interior side of the shaft-problem solved, but it cost more than doing it right the first time.

Brooklyn-Specific Constraints for Deck-Mounted Skylights

Installing a deck-mounted skylight on a Brooklyn flat roof isn’t just a technical exercise-it’s also a logistical and regulatory challenge shaped by tight urban lots, shared walls, landmark rules, and access limitations. I’ve hand-carried skylight units up four flights of stairs in a Brownstone with no roof hatch, worked around active restaurant exhaust fans on mixed-use buildings, and coordinated crane lifts over sidewalks with NYPD and DOT permits. Every Brooklyn job has site-specific constraints that affect timing, cost, and design choices.

On a recent Carroll Gardens rowhouse project, we needed to install two 3×5 skylights on a landmarked block. The Landmarks Preservation Commission required that the units be invisible from the street, which meant positioning them on the rear slope and using low-profile glass rather than raised domes. The interior shafts had to route around original plaster crown molding that couldn’t be disturbed, and we could only work between 9 a.m. and 4 p.m. to avoid disturbing tenants in the adjacent building. The neighbor’s fourth-floor windows looked directly down onto the roof, so we scheduled the open-roof framing phase during the week they were away on vacation. Every detail was negotiated, planned, and sequenced around the realities of a dense, regulated, historic Brooklyn block.

Local factors to consider:

• Landmark Preservation Commission approval for any visible skylights on historic blocks; review can take months and may limit size, materials, and placement.
• Party wall setbacks: Keep skylights at least three feet from shared property lines to avoid water runoff or snow sliding onto your neighbor’s roof.
• Nearby taller buildings that might look down into your skylight-consider privacy glass, interior shades, or shifting location if overlooked by occupied windows.
• Roof access restrictions: No bulkhead or exterior stair can mean smaller skylight units (nothing over 50 pounds) and hand-carrying everything up interior stairs.
• Rooftop mechanicals: HVAC condensers, exhaust fans, and vent stacks create shade, noise, and exhaust patterns that affect skylight location and comfort.

Common Mistakes With Deck-Mounted Skylights on Flat Roofs

Most skylight failures I’m called to fix aren’t caused by defective units-they’re caused by installation mistakes that ignore the realities of flat-roof water management and structural framing. Here are the red flags that lead to leaks, condensation, and unhappy owners:

1. Treating a flat roof like a pitched roof: Relying solely on step flashing and shingle-style details without building a proper curb or integrating with the membrane system. Water doesn’t shed away on a flat roof-it ponds and finds any gap.
2. Cutting joists without structural support: Removing two or three joists to fit a skylight and never installing headers and trimmers, leaving the roof sagging and the skylight racking out of square within a year.
3. Placing skylights in ponding areas: Installing where water already sits for days after rain, guaranteeing that the skylight will be submerged or surrounded by standing water every storm.
4. Using incompatible flashing materials: Mixing EPDM and asphalt, or TPO and PVC, or relying on generic silicone caulk instead of system-specific tapes and sealants that fail within one season.
5. Skipping interior air sealing and insulation: Leaving the shaft uninsulated or failing to stop air leaks at the deck and ceiling, leading to winter condensation that drips and stains within weeks of the first cold snap.
6. Installing after a roof warranty is in place: Cutting into a newly warranted roof without involving the original installer or membrane manufacturer, voiding coverage for the entire roof-not just the skylight area.

I once saw a Gowanus warehouse conversion where a general contractor installed six deck-mounted skylights directly onto an EPDM roof with no curbs, just big beads of polyurethane caulk around the flanges. Every single unit leaked within three months. The fix required building curbs under each skylight, cutting back and re-terminating all the EPDM flashing, and re-fastening the units to the new curbs-basically doing the entire job over. The owner paid twice.

FAQ: Deck-Mounted Skylights on Flat Roofs in Brooklyn, NY

Can I install a deck-mounted skylight on a completely flat roof?
True zero-slope roofs aren’t ideal for any skylight because water and snow have nowhere to go. Most installations on “flat” Brooklyn roofs include a small built-up curb or slope box (even just three or four inches tall) to create positive drainage away from the unit. This curb must be carefully integrated with your roof membrane and structural framing. If your roof has significant ponding, you’ll need tapered insulation or a full re-slope before adding a skylight.

Will adding a skylight void my flat roof warranty?
It can, unless the skylight installation is done or approved by a contractor certified by your membrane manufacturer and follows their published flashing details. Most roof warranties exclude damage caused by “unauthorized penetrations or alterations,” and a skylight is a major penetration. Before you cut, contact the company that installed your roof or the membrane manufacturer and get written approval of your skylight plan. Some will inspect and re-certify the warranty after the work; others will exclude only the immediate skylight area.

Is a deck-mounted skylight better than a dome skylight on a curb?
It depends on your roof type, aesthetic goals, and performance needs. Deck-mounted glass units offer better insulation, cleaner modern looks, and options for venting or electronic shades-but they require more precise flashing and are less forgiving of ponding water. Dome skylights on tall curbs shed water and snow easily, are simpler to flash on very low-slope roofs, and cost less-but they scratch, yellow over time, and have poorer thermal performance. On a Brooklyn rowhouse with a modern TPO roof and good drainage, I’d choose deck-mounted glass. On an old built-up roof with marginal slope and ponding issues, a curb-mounted dome is safer and more reliable.

How long does a typical deck-mounted skylight install take?
For one skylight on a structurally sound roof with straightforward access, plan on two to three days: one day for framing and rough opening, one day for skylight setting and exterior flashing, and a half to full day for interior shaft framing, insulation, and drywall prep. Complex roofs with challenging joist layouts, landmark approvals, or coordination with other trades can stretch that to a week or more. Weather delays and material lead times (some high-end skylights take weeks to ship) also affect the schedule.

Do I need a permit to add a skylight in Brooklyn?
Yes. New roof openings and structural framing changes require a DOB permit and, in most cases, stamped structural drawings from a licensed PE. If your building is in a historic district, you’ll also need Landmarks Preservation Commission approval before filing with DOB. Unpermitted skylight installs can complicate future sales, refinancing, and insurance claims-and if the installation causes a structural issue or leak, you’ll have no recourse because the work was never inspected or approved.

Plan a Deck-Mounted Skylight Installation for Your Brooklyn Flat Roof

A successful deck-mounted skylight on a flat roof in Brooklyn requires structural planning, thoughtful placement, careful membrane integration, and interior condensation control. Miss any one of those and you’ll trade your dark hallway for chronic leaks, stained ceilings, or drafty cold spots. When all the pieces are coordinated-framing that respects load paths, curbs that create positive drainage, flashing that ties seamlessly into your roof system, and insulated shafts that stay dry-you get bright, comfortable rooms without sacrificing the weathertight integrity of your flat roof. I’ve installed dozens of these systems across Brooklyn, and the ones that last twenty years without a service call are the ones where we treated the skylight as a complete roof-and-structure project, not just a hole with glass.

Request a flat roof skylight assessment in Brooklyn: Share photos of your roof, building type, and the rooms you want to brighten. We’ll review structural feasibility, confirm your roof system compatibility, and recommend skylight options-size, glazing, venting, and curb height-that fit your building and budget. Whether you need engineering coordination, landmark approvals, or just a second opinion on a contractor’s proposal, we work with local structural engineers, certified roofers, and DOB expediters to deliver skylights that are bright, dry, and code-compliant. Let’s plan an installation that brings natural light into your Brooklyn home without creating the leaks and headaches that come from shortcuts.