Skylights for Flat Roof Extensions
If you’re adding a rear extension to your Brooklyn home, how much more will it actually cost to fill it with daylight instead of just more drywall and light bulbs? The answer depends on what you buy and how you build the curb and flashing-but the real question is whether you want a long, dim hallway behind those French doors or a genuinely bright space that feels twice as large. One well-placed flat roof skylight on an extension typically adds $3,200-$6,800 per unit installed in Brooklyn, covering the skylight itself, structural framing, curb fabrication, membrane integration, and interior finishes. That number rises if you’re using walk-on glass, motorized venting units, or complex multi-light configurations.
Most homeowners think of extension flat roof skylights as a last-minute upgrade. That approach leads to leaks, structural headaches, and heat problems. Instead, treat skylights like what they are: a structural and waterproofing decision that needs to be baked into the extension design from day one, alongside your roof deck, insulation layers, and membrane system.
Why Skylights Transform a Flat Roof Extension in Brooklyn
Brooklyn rear and side extensions solve space problems but create a predictable lighting one. When you push a kitchen or living room 12, 15, or 20 feet deeper into the lot on a rowhouse or brownstone, you leave the natural light behind. Even if you install huge sliding doors at the back, the middle and front sections of that new space stay dark, especially when neighboring buildings shadow your yard. On narrow Park Slope or Carroll Gardens lots, the problem is worse: your extension becomes a tunnel between your old house and the rear wall of glass.
Flat roof skylights solve this by dropping light straight down into the core of the extension. You’re not relying on light bouncing 15 feet across the ceiling from a single source. The skylight brings it directly to where you prep food, eat, or sit-brightening the entire depth of the room without giving up wall space or privacy. A pair of smaller skylights spaced along the length often feels better than one giant one at the far end, because they distribute light and reduce the cave-like feeling in the front half.
The two fears I hear constantly: leaks and summer heat. Both are real if you use bad products, poor flashing details, or no solar control. The rest of this guide shows you how to get skylight benefits-brightness, spatial drama, and higher resale value-without those failures.
Start With the Extension: What Kind of Flat Roof Are You Building Onto?
Before you pick skylights, understand the extension roof you’re building. Brooklyn flat roof extensions fall into three main categories, each with different structural and skylight considerations.
Single-story rear extensions are the most common: a 10-20 foot deep kitchen or family room bump that extends from the back of a brownstone, brick rowhouse, or wood-frame building. The flat roof above is usually wood-framed with plywood decking, sometimes topped with a small terrace or mechanical equipment. This is the easiest scenario for skylights-you can frame openings between joists or add doubled headers for larger units, and you’re working with a relatively lightweight, flexible roof system.
Two-story rear additions extend living spaces on both floors and often use steel beams or concrete decks on the lower roof to support upper-level loads. Skylight installations here require more engineering-cutting through steel or coring concrete isn’t a weekend carpentry project. But the payoff is significant: if that first-floor space is deep and surrounded by tall neighboring buildings, skylights may be your only source of consistent daylight.
Garden-level or ground-floor extensions under an upper terrace are increasingly popular in Bed-Stuy, Clinton Hill, and Crown Heights multi-families. The “roof” is someone’s outdoor floor, so any skylight must be walk-on rated and integrated into the terrace deck system. These are the most complex from a waterproofing and load standpoint, but they let you bring light into below-grade or semi-subterranean spaces that would otherwise be caves.
New construction versus retrofit also matters. On a new extension, you design the skylight openings, curbs, and flashing into the roof build from the start-ideal. Retrofitting into an existing flat roof means checking deck condition, insulation thickness, membrane age, and whether cutting holes will void any warranties. I always recommend doing it right during the original build rather than coming back later.
Skylight Options for Flat Roof Extensions: What You Can Actually Use
Pitched-roof skylights won’t work on your flat extension. You need products specifically designed for low-slope or flat applications-curb-mounted systems that sit on an upstand and shed water correctly. Here’s what homeowners and architects actually specify in Brooklyn:
- Fixed flat roof windows / rooflights: These are sleek, frameless-looking glass units that sit nearly flush on a curb. Brands like Velux, FAKRO, and European imports dominate this category. They’re perfect for pure daylighting-over kitchen islands, dining tables, or living areas where you don’t need ventilation because you have operable doors or windows elsewhere. Typical sizes range from 2×2 feet up to 4×6 feet or larger custom units. Expect $1,800-$4,500 per unit installed for quality double-glazed models in Brooklyn.
- Opening / venting skylights: These units can tilt or slide open, either manually with a crank or via electric motor and remote control. Motorized versions add $600-$1,200 to the cost but are worth it on high ceilings or in cooking-heavy kitchens where you want to exhaust heat and odors upward. Rain sensors automatically close the skylight if a storm hits while you’re out. Venting skylights work best when placed where rising warm air naturally gathers-near stove zones or at the back of a deep extension.
- Lanterns and curb-mounted glass boxes: Glazed lanterns rise 12-20 inches above the roof deck, creating a more traditional conservatory aesthetic. They’re framed with aluminum or powder-coated steel and offer more visual drama inside-higher ceilings, more glass surface area, and often better ventilation. The tradeoff is cost ($4,000-$8,000+ installed) and visibility: if you’re in a historic district or have neighbors looking down, lanterns are more obvious than low-profile rooflights.
- Walk-on skylights: Structural glass panels designed to handle foot traffic, furniture, planters, and even snow loads as part of your terrace surface. These use laminated tempered glass (often triple-layer) in heavy-duty aluminum frames, with slip-resistant coatings or textured glass. They’re essential when you want a usable roof deck and light below. Budget $8,000-$15,000+ per walk-on unit depending on size and load rating. Common on Windsor Terrace and Ditmas Park single-family extensions where the family wants both a backyard terrace and a bright lower-level space.
Glazing matters as much as the frame. In Brooklyn’s climate, double-glazed units with low-E coatings and argon gas fill are the minimum. Triple glazing adds cost and weight but dramatically reduces winter heat loss and condensation-worth it if your extension has high ceilings or minimal wall insulation. For south- or west-facing skylights, solar-control glass (low solar heat gain coefficient) prevents the greenhouse effect in July and August. Internal blinds, either manual or motorized, give you flexibility to block direct sun or glare during peak hours.
Light Strategy: Where to Put Skylights in a Flat Roof Extension
Placement is more important than size. I’ve seen huge skylights fail to brighten a room because they’re positioned wrong, and smaller ones that completely transform a space because they drop light exactly where it’s needed.
Start by mapping your extension in zones. Where do you cook, eat, work, and move through the space? On a Cobble Hill kitchen extension I worked on last year, the homeowner initially wanted one big 4×6 rooflight centered over the island. After walking the space, we shifted to two 3×4 units-one over the island and prep area, the other 8 feet back toward the dining zone. That simple change eliminated the dark pocket between the island and the old house wall, and the space felt balanced instead of backlit.
Skylights closer to the existing rear wall pull light deeper into your home and counterbalance the strong light from patio doors. In narrow Brooklyn extensions shadowed by neighboring buildings, this inward placement is critical. If your extension is only 12 feet deep, a single skylight centered at 6 feet works. If you’re pushing 18-25 feet, two skylights spaced every 8-10 feet give much better coverage.
Avoid structural and service clashes early. Coordinate with your architect or structural engineer to locate skylights between roof joists where possible. If a skylight must cut across joists, plan doubled-up framing (headers and trimmers) to carry the loads around the opening. Also check ductwork, electrical conduit, sprinkler lines, and any rooftop mechanicals planned for the extension. I’ve seen projects where HVAC ducts were roughed in before skylight locations were finalized-then the skylight had to move or shrink to fit around the duct chase, ruining the original light plan.
Privacy and neighbor sightlines matter in dense Brooklyn blocks. Skylights typically look straight up at sky, but if you’re surrounded by taller buildings, they may also look toward upper-floor windows. Frosted or obscure glass, internal shades, or careful angling can maintain privacy without losing much light. On a recent Boerum Hill project, we used clear glass on the front skylight (facing the owner’s own upper floor) and lightly frosted glass on the rear unit (facing a neighbor’s bedroom windows three stories up).
Leak-Free Detailing: How Skylights Tie Into a Flat Roof System
Most skylight leaks aren’t skylight failures-they’re flashing and curb detailing mistakes. Understanding the anatomy of a watertight skylight detail will help you evaluate proposals and spot red flags.
Here’s the correct layer sequence from inside out: roof deck (plywood or concrete), air/vapor control layer if required, rigid insulation boards, roofing membrane (EPDM, TPO, modified bitumen, or built-up), skylight curb, curb flashing integrated into the membrane, and finally the skylight frame bolted or clipped to the curb. Water must flow under the skylight frame, down the membrane on all four sides of the curb, and toward drains or scuppers-never into joints or pockets.
| Curb Detail Element | Purpose | Common Brooklyn Mistake |
|---|---|---|
| Curb height | Lifts skylight frame above insulation, pavers, and standing water zone | Using 4″ curbs on roofs with 3″ insulation + 2″ pavers-barely any clearance for drainage |
| Corner reinforcement | Prevents membrane tears at stress points during thermal expansion | Skipping pre-formed corner patches on TPO/EPDM, leading to splits after one winter |
| Base flashing | Seals curb to main membrane; must be continuous and lapped correctly | Using caulk or mastic instead of heat-welded or adhered membrane flashing |
| Counterflashing | Covers top edge of base flashing, creating double barrier | Omitting counterflashing entirely, exposing base flashing to UV and foot traffic |
| Slope | Even “flat” skylights need 2-5° slope to shed water and debris | Installing units dead flat, causing dirt staining and algae growth that blocks light |
Never place skylights in low spots, ponding areas, or directly adjacent to roof drains. If a drain clogs during a snowmelt event, water surrounds the skylight curb and finds any weak point in the flashing. I always map drainage flow on extension roofs before finalizing skylight locations-even a 1/4″ per foot slope matters.
Walk-on skylights add another layer of complexity. The membrane runs under the skylight frame and up the curb that supports the structural glass. Pavers or decking finish tight to the glass edge, with small drainage gaps or channels to let water escape rather than trapping it at the joint. Loads from foot traffic, grills, and furniture must transfer into structural supports-pedestals or steel framing-not into roof insulation, which will compress and create low spots over time.
This is why your roofer, not just a carpenter or skylight vendor, must handle the curb and flashing integration. A TPO roofer knows how to heat-weld TPO flashing to TPO membrane. An EPDM specialist knows the correct primers and lap adhesives for EPDM curb boots. Mixing incompatible materials or using generic caulk voids warranties and creates failures within two to three years. On every extension skylight project I oversee, the roofing contractor is responsible for the entire curb assembly, flashing, and final skylight mounting-one point of accountability.
Planning Around Brooklyn Codes, Landmarks, and Neighbors
New extensions almost always go through full DOB permitting, and skylights will be shown on your architectural and roofing drawings. The building department checks egress, fire ratings (if the extension abuts a property line), energy code compliance (U-factor and solar heat gain), and structural adequacy of the roof framing around openings. If you’re only replacing an existing skylight on a one- or two-family home, the paperwork may be lighter-sometimes just a roofing permit-but you still need to meet current energy code.
In historic districts-and Brooklyn has many, from Brooklyn Heights to Prospect-Lefferts Gardens-skylights on rear extensions may be acceptable if they’re not visible from the street or from specific public viewing corridors. Landmarks Preservation Commission typically allows low-profile rooflights set back from parapets on rear yard extensions, but tall lantern-style skylights or glass boxes that rise above the roofline can get more scrutiny. Your architect will handle the LPC application, but expect to provide sightline studies showing the skylight isn’t visible from key public vantage points.
In multi-family buildings, cutting a skylight into a shared or common roof structure requires co-op or condo board approval, coordination with the building’s engineer, and clear delineation of maintenance responsibilities. Who fixes a leak-the unit owner or the building? Who pays to replace the skylight in 20 years? These questions must be answered in writing before you start cutting.
Project Flow: How Skylights Fit Into Your Flat Roof Extension Timeline
Skylight decisions happen in phases, not all at once. During schematic design, you and your architect decide how many skylights you want, rough sizes, and approximate locations on the roof plan. By design development, you lock in specific skylight models, exact dimensions, curb heights, and glazing specs so the structural engineer can calculate framing loads and the energy consultant can model heat gain and loss.
During construction, the sequence matters. After the extension roof deck is framed and sheathed, openings for skylights are cut and the perimeter is framed with headers and trimmers. Curbs are built and installed before insulation and membranes go down-this allows the roofer to integrate flashing in one continuous step with the main roof system. Skylight units themselves are often installed after most of the heavy work (mechanical rough-in, deck pavers or finishes) is complete to reduce the risk of damage from foot traffic or dropped tools.
Before interior finishes close up the ceiling around skylights, do a water test. A garden hose on the roof for 15 minutes will reveal bad flashing or frame seals while you can still fix them easily. Once the HVAC is running, check for condensation on glass or around the interior trim-a sign of thermal bridging or air leaks that need spray foam or additional insulation.
Common Mistakes With Skylights on Flat Roof Extensions
These are the errors I see repeatedly on Brooklyn projects, often discovered only after the first big rainstorm or summer heat wave:
- Placing skylights in low spots or near drains: Water naturally collects in these areas. One clogged drain during a snowmelt leaves your skylight curb surrounded by standing water for days. Even good flashing can fail under prolonged ponding.
- Tiny curbs on thick, insulated roofs: Your energy consultant specifies 4 inches of polyiso insulation, you add 2-inch paver pedestals, and suddenly the skylight curb is barely visible above the roof surface. Splashback from rain and wind-driven water hits the skylight frame directly, overwhelming seals and gaskets.
- Oversized glass with no solar control: A 6×8 foot clear-glass rooflight over a south-facing Sunset Park extension turns the kitchen into a sauna by 3 p.m. in July. Low-E coatings and internal blinds aren’t optional-they’re essential for comfort and to avoid running your AC nonstop.
- Cutting structure without engineering: Removing two or three roof joists to fit one large skylight without properly sized headers causes roof sag, cracked interior finishes, and even membrane tearing as the deck deflects. Always involve a structural engineer when skylight openings affect multiple framing members.
- Mixing contractors with no single point of responsibility: A carpenter builds the curb, a general roofer flashes it, and a skylight vendor installs the unit. When a leak appears, each blames the other and you’re stuck mediating and paying for repairs. I always recommend one contractor-ideally the roofer-owns the entire skylight assembly from curb to glass.
Watch for warning signs in drawings or proposals: skylights shown very close to parapets, drains, or party walls without clear drainage paths; contracts that mention skylights but say nothing about flashing materials or compatibility with the roof system; installers who propose standard residential Velux kits meant for sloped shingle roofs without custom curbs or flat-roof flashing.
Maintenance: Keeping Extension Skylights Tight, Clear, and Comfortable
Skylights aren’t set-and-forget. Plan on checking them twice a year-spring and fall-to catch small problems before they become expensive failures.
From inside, look for condensation between glass panes (a sign the sealed unit has failed and needs replacement) and around the interior trim. Staining, bubbling paint, or mold growth on drywall near the skylight well indicates water intrusion or air leakage. If you see these signs after a heavy storm, schedule a roof inspection immediately.
On the roof, check seals around the skylight frame and visually confirm there’s no cracked flashing, lifted membrane edges, or debris trapped against the curb. On walk-on skylights, make sure paver joints and drainage channels around the glass aren’t clogged with leaves or dirt. Even a small blockage can divert water toward the frame instead of away from it.
Clean exterior glass once or twice a year using manufacturer-recommended cleaners-avoid abrasive tools or harsh chemicals that scratch low-E coatings. Dirty glass blocks 10-15% of light and looks dingy from inside, undermining the whole point of the skylight. Adjust internal blinds seasonally: open in winter to capture passive solar heat, partially closed in summer to reduce glare and cooling loads.
Choosing a Brooklyn Team for Skylights on Flat Roof Extensions
Skylights on extension flat roofs require coordination between architect, roofer, and skylight supplier. The best projects happen when one experienced contractor-usually the roofer-takes full responsibility for the skylight assembly and its integration into the roof system. Your architect designs the size and location, the structural engineer frames the opening, and the roofer builds the curb, flashes it, and installs the skylight unit.
When interviewing contractors, ask how many flat roof extension skylights they’ve installed in Brooklyn, and request photos or addresses of completed projects. Ask which skylight brands and roof membrane systems they work with most often, and why. The answer should reference specific manufacturers and compatibility-not vague promises. Clarify who is responsible for waterproofing and warranty at the curb and flashing interfaces; if you hear “that’s the skylight guy’s problem” or “the roofer handles that separately,” you’re looking at future finger-pointing.
A solid proposal should spell out exact skylight models, sizes, glazing specifications (U-factor, SHGC, visible transmittance), and curb heights. It should describe the roof build-up around the skylights: insulation thickness and type, membrane system, flashing method (heat-welded, adhered, or mechanical), and any structural framing modifications. If DOB filings or Landmarks review are required, the proposal should clearly state who handles them and whether those costs are included or separate.
Is a Skylight Right for Your Flat Roof Extension in Brooklyn?
Skylights make sense when your extension is deep, relies heavily on artificial light, or feels tunnel-like despite good rear glazing. They’re worth the investment-$3,200-$6,800 per unit for quality fixed skylights, more for venting or walk-on models-if you’re still in design or early construction and can integrate them cleanly. The return is a brighter, more valuable space that feels larger and connects to the sky, not just the yard.
Reconsider if your roof structure or DOB constraints make cutting openings prohibitively complex, if you’re extremely sensitive to summer heat and unwilling to use blinds or mechanical cooling, or if budget is so tight that simpler clerestory windows or larger rear doors would give enough light for less cost. Skylights aren’t magical-they’re a tool that works brilliantly when planned correctly and fails expensively when rushed or detailed poorly.
If you’re planning an extension in Brooklyn, gather any existing plans or sketches and mark where you imagine skylights might go. Schedule a consultation with a Brooklyn-based roofer or architect experienced in extension flat roof skylights-bring this guide as a checklist so your final design balances daylight, comfort, and long-term durability on your specific roof and in your specific neighborhood. The difference between a dark extension and a bright one is often just 12 square feet of well-placed glass and a curb that’s flashed correctly.