Expert Installing Roof Lanterns on Flat Roofs in Nassau County

Can you cut a huge opening for a roof lantern into your flat roof in Nassau County-and still sleep at night when a Nor’easter hits? Yes, if the framing, curb, and waterproofing are treated like a serious roof detail, not just a window install. Installing roof lanterns on flat roofs costs $4,200-$9,800 in Nassau County depending on lantern size, structural needs, and membrane type, but the critical difference between a stunning light feature and a chronic leak sits in those four inches of vertical curb where your flat roof membrane meets the lantern frame-get that detail wrong, and no amount of beautiful glass will save you.

Most online guides fixate on the lantern unit itself: frame materials, vent options, glass coatings. They skip the harder question: how to install a roof lantern on a flat roof without turning your ceiling into a water feature every time it rains sideways. After 18 years doing these installs across Nassau County-from Manhasset kitchen extensions to Merrick family room additions-I’ve learned that roof lantern installation on flat roofs is 70% roof detail, 30% window work. The structure has to carry glass and snow loads without sagging. The curb has to keep wind-driven rain out. The membrane tie-in has to flex through freeze-thaw cycles without splitting. Only then do you bolt down the pretty part.

Here’s the step-by-step process that keeps lanterns bright and dry through Long Island’s wet springs and heavy snow winters.

Confirm Your Structure Can Carry the Opening

On a Garden City kitchen extension last year, the homeowner wanted a 6′ × 10′ lantern centered over an island. Beautiful idea-except the existing joists ran the wrong direction and the architect’s plans showed zero header detail. Before you price lanterns or pick glass, you need to answer: can your roof frame lose 60 square feet of material and still support itself, the lantern weight, snow loads, and your next roofer walking around it?

Flat roof joists in Nassau County are typically engineered I-joists, 2×10s, or TJIs on 16″ or 24″ centers. When you cut an opening, you’re removing structural support, so you must install doubled or tripled headers perpendicular to the joists-essentially creating a reinforced box frame that carries the load around the opening. For a typical 4′ × 6′ lantern, that means:

• Two joists on each long side (or one LVL beam if span exceeds 8 feet)
• Doubled 2×10 or LVL headers on the short ends
• Joist hangers rated for the actual load, not just nailed straps
• Structural screws or through-bolts at every connection

Nassau County building code follows NYS Residential Code R802 for roof framing. Your headers must be sized for the clear span plus half the joist depth on each side, and the inspector will check hanger nails and bearing. If your lantern is larger than 50 square feet or sits on a living space below, you’ll need an engineer’s stamp showing load paths. Budget $850-$1,400 for the structural engineering letter, which also gives your roofer the header dimensions and fastener schedule so there’s no guessing.

One detail that matters more on flat roofs than pitched: deflection. Even if your framing can carry the weight, if it sags more than L/360 under load, the lantern frame will rack, seals will gap, and you’ll get leaks. Use the engineer’s specs, not what “looks strong enough.”

Design the Curb with Pitch, Height, and Drainage

Here’s what separates lantern installs that leak from those that don’t: the curb. A roof lantern curb is a short vertical frame-usually 6″ to 12″ tall-that lifts the glass unit above the flat roof surface. It does three critical jobs: sheds water away from the frame, gives the membrane something vertical to flash against, and creates an insulated thermal break so the cold glass doesn’t sit directly on warm interior air (which causes condensation).

Most homeowners don’t realize the curb isn’t level. It has a very slight outward pitch-about 1/8″ per foot-so any water that hits the lantern frame runs off the curb, not into the seal. On a Bellmore sunroom project where we replaced a leaking lantern, the original installer built a dead-flat curb; every rain event left a 1/4″ puddle along the uphill side of the frame, and within two years the sealant failed. The fix wasn’t reglazing-it was rebuilding the curb with pitch.

Here’s how to build a proper lantern curb on a flat roof:

1. Frame the curb box from 2×8 or 2×10 pressure-treated lumber (PT resists rot if water ever gets in). The curb sits on top of your roof deck, directly over the framed opening.
2. Shim one edge 1/8″ higher than the opposite edge so the top surface pitches outward, away from the center of the lantern.
3. Wrap the curb exterior in peel-and-stick membrane (Grace Vycor, Protecto-Wrap, or similar) from the deck surface up and over the top edge, creating a waterproof “boot.”
4. Insulate the curb interior with rigid foam (minimum R-10) cut to fit snugly between the curb walls, then seal every gap with spray foam. This keeps the curb warm and prevents condensation on the inside face.
5. Cap the top with treated plywood to create a flat nailing surface for the lantern frame, keeping the pitch built into the curb height.

Minimum curb height in most manufacturer specs is 6 inches, but I build 8-10 inches in Nassau County for two reasons. First, heavy wet snow can pile up 4-5 inches on a flat roof during a single storm; the higher curb keeps meltwater from pooling against the frame. Second, the taller curb gives more vertical surface for flashing and membrane tie-ins, which means more forgiveness if the roof membrane shrinks slightly over time.

Integrate the Curb into Your Flat Roof Membrane

This is the step where most DIY attempts-and some contractor installs-fall apart. Your flat roof membrane (EPDM rubber, TPO, modified bitumen, or built-up) must transition from horizontal (the roof deck) to vertical (the curb face) to horizontal again (over the curb top) without gaps, wrinkles, or fastener penetrations that let water in. Each membrane type has a different detail:

EPDM: Use EPDM inside and outside corners, lap adhesive, and a termination bar across the top of the curb. The membrane comes up the curb exterior, folds over the top edge, and laps down the interior face at least 3 inches. Seal every seam with lap sealant, not just adhesive.

TPO or PVC: Heat-weld the field membrane to a prefabricated TPO curb boot, or hand-weld custom patches at the corners. TPO shrinks more than EPDM in cold weather, so leave 1/2″ of relaxed material at the curb base-don’t pull it drum-tight.

Modified bitumen: Torch or cold-apply the base sheet up the curb, then the cap sheet, overlapping at least 6 inches at every seam. Install a metal counterflashing or Wakaflex lead-alternative over the top edge to protect the exposed membrane from UV.

On a Westbury flat-roof addition three years ago, the homeowner’s original roofer installed the lantern directly onto the EPDM with a bead of caulk-no curb, no flashing boot, just sealant. It leaked within six months. We had to remove the lantern, build a proper curb, re-membrane around it, and reinstall. The lesson: your lantern sits on the curb, and the curb is integrated into the roof membrane, not sitting on top of it like an afterthought.

Set and Seal the Lantern Frame

Once your curb is built, insulated, and flashed, the actual lantern installation is straightforward-but only if you follow the manufacturer’s fastener schedule and use the right sealants. Roof lanterns are aluminum or uPVC frames with double- or triple-glazed glass panels. They arrive as a complete unit with hinged vents (or fixed glass), gaskets pre-installed, and a perimeter flange that bolts down to your curb.

Here’s the installation sequence:

1. Run a continuous bead of polyurethane or silicone sealant (Tremco Spectrem, Sikaflex 221, or manufacturer-spec product) around the top of the curb where the lantern flange will land. This is your primary water seal.

2. Set the lantern onto the curb with at least two people-most 4′ × 6′ units weigh 180-250 pounds. Check that it’s square by measuring diagonals; if they’re off more than 1/4″, shim one corner.

3. Fasten the frame using stainless steel screws every 12-16 inches through the flange into the curb cap. Do not overdrive-if you compress the gasket too much, it can’t move with thermal expansion and you’ll crack seals.

4. Tool the sealant bead smooth on both the interior and exterior edges. Some installers skip the interior bead, but I always run one; it stops drafts and gives a second line of defense if wind-driven rain gets past the exterior seal.

5. Install the interior trim-a plaster bead, drywall return, or wood casing that covers the curb and creates the finished light well. This step is cosmetic but critical: the trim hides your insulation, framing, and membrane work, and if it’s not caulked to the lantern frame, you’ll get air leaks that waste energy and cause condensation.

One technical point: glass type matters more on flat roofs than you’d expect. In Nassau County, your lantern glass must meet NYS Building Code for overhead glazing-fully tempered or laminated, minimum 1/4″ thick. Most quality lanterns use double-glazed units with one tempered pane and one laminated pane; if the glass breaks, the laminated layer holds the shards so you’re not showered with glass. Also check the U-factor (aim for 0.30 or lower) and Solar Heat Gain Coefficient; too much SHGC and your kitchen will cook in summer sun, but too little and you lose the passive warmth that makes lanterns feel so good in March.

Finishing the Interior Light Well

Your roof lantern doesn’t just sit on the roof-it drops light through a vertical shaft (the light well) into the room below. That shaft needs insulation, an air barrier, and a finish surface, or you’ll get cold drafts, condensation, and an ugly view of raw framing every time you look up.

Standard light-well construction in Nassau County:

• Frame the vertical walls of the well with 2×4s between the roof opening and ceiling opening, keeping the well plumb or slightly splayed (wider at the top than bottom) to spread light.
• Insulate the wall cavities with fiberglass batts or spray foam-minimum R-19, but R-30 is better for energy efficiency and condensation control.
• Install a vapor barrier on the warm (interior) side of the insulation-6-mil poly or foil-faced foam board-sealed at all edges with canned foam or acoustic sealant.
• Finish with drywall or beadboard painted white or a light color to maximize light reflection. Some homeowners go with angled soffits that “bounce” daylight deeper into the room; others keep it simple with straight vertical walls.

On a Long Beach bungalow kitchen last fall, we built the light well with a 15-degree splay-10 inches wider at the roof than at the ceiling. That angle, combined with semi-gloss white paint, doubled the effective light spread compared to a straight vertical shaft. The homeowner said it felt like adding two windows, not one skylight.

Maintenance and Common Failure Points

Roof lanterns on flat roofs need annual inspections, not because they’re fragile, but because flat roofs hide problems longer than pitched roofs. Water doesn’t run off-it pools, evaporates, freezes, and eventually finds any weak spot. Here’s what to check each spring and fall:

Sealant beads around the frame: UV, thermal cycles, and weather degrade polyurethane and silicone over 5-8 years. If you see cracks, gaps, or sections that have pulled away, re-seal them immediately. Cost to re-caulk a lantern perimeter: $180-$320 for a contractor visit.

Membrane tie-ins at the curb: Look for splits, lifted edges, or areas where the membrane has pulled away from the curb. EPDM can shrink up to 2% over its life; TPO shrinks even more in cold snaps. If you catch a small separation early, a patch and lap sealant will fix it. Wait too long and you’re re-flashing the whole curb.

Vent operation and gaskets: Hinged vents on roof lanterns use concealed hinges and gas struts. Check that they open smoothly and close fully; if a vent sags or won’t latch, the gasket isn’t compressed and you’ll get drafts or leaks. Replacement gas struts cost $45-$75 per pair.

Condensation inside the glass: If you see fog or moisture between the glass panes, the sealed unit has failed. This is a glass replacement, not a cleaning job-expect $600-$1,200 per panel depending on size and glass type. Most quality lanterns have 10-year sealed-unit warranties; keep your paperwork.

Interior light-well cracks or stains: Small hairline cracks in drywall are normal settling. Brown or yellow stains mean water intrusion-usually from the curb-to-membrane detail or a missing interior air seal causing condensation.

The most common failure I see in Nassau County is water entry at the uphill side of the frame during heavy, wind-driven rain. The fix is almost always the same: the original curb wasn’t pitched, or the sealant bead wasn’t continuous. Both are preventable if the install is done right the first time.

Permitting and Building Code for Roof Lanterns

Nassau County requires a building permit for roof lantern installation on flat roofs because you’re altering the roof structure and envelope. The permit process involves:

1. Structural drawings or engineer’s letter showing framing, headers, and load paths.
2. Roofing detail showing curb height, flashing method, and membrane tie-in.
3. Energy compliance (glass U-factor and SHGC must meet NYS Energy Code for your climate zone).
4. Inspection after framing and before closing the curb, plus a final inspection after the lantern is set.

Permit fees in Nassau County run $275-$485 depending on project value. Processing time is 3-6 weeks if plans are complete. Some contractors pull permits; others expect the homeowner to handle it. Either way, don’t skip this step-an unpermitted lantern can complicate refinancing or home sales, and if there’s ever a leak or structural issue, your homeowner’s insurance may deny the claim if they find out the work was unpermitted.

One code detail that surprises people: if your lantern is in a bedroom or accessible space, it may need to meet emergency egress requirements-minimum 5.7 square feet of clear opening, 24″ wide, 20″ tall, and sill no more than 44″ above the floor. Most roof lanterns don’t meet this (they’re in the ceiling, not a wall), so the room must have another code-compliant egress window or door. Your permit reviewer will flag this if it’s an issue.

When DIY Roof Lantern Installation Makes Sense (and When It Doesn’t)

Can you install a roof lantern on a flat roof yourself? Technically yes, if you have framing, roofing, and flashing experience, access to the right tools, and a helper. Realistically, most homeowners should hire this out-not because any single step is impossibly hard, but because the consequences of small mistakes are expensive and hidden until winter.

DIY makes sense if:

• You’ve done structural carpentry before (framing headers, using joist hangers correctly).
• You’re comfortable working with your roof membrane type-heat-welding TPO or torch-applying modified bitumen isn’t beginner-level work.
• You can pull your own permit and schedule inspections.
• The lantern is small (under 4′ × 6′) and the roof structure is straightforward.

Hire a pro if:

• Your roof is EPDM and you’ve never worked with rubber membrane-lap adhesive and sealant technique take practice.
• The lantern is large or the roof span requires engineered headers.
• You’re not confident building a weathertight curb with proper pitch and flashing.
• Your ceiling height is over 10 feet and the interior light-well framing will be complex.

A professional install in Nassau County costs $4,200-$9,800 total, which includes the lantern unit ($1,800-$4,500), structural framing and curb ($800-$1,600), membrane flashing and integration ($650-$1,350), setting and sealing the frame ($400-$950), and interior light-well framing and finishing ($550-$1,400). If you hire out only the roofing and membrane work but do the framing and interior yourself, you can trim costs by $1,200-$2,000-but you still carry the risk if the curb detail isn’t perfect.

Why Roof Lanterns Work Better Than Skylights on Flat Roofs

A quick comparison because homeowners often ask: should I install a roof lantern or a flat skylight? On a flat roof, lanterns win for three reasons. First, the raised curb design naturally sheds water and prevents pooling, while flat skylights sit at roof level and rely entirely on perfect flashing (which is hard to maintain). Second, lanterns bring in more usable light because the glass is angled and multi-sided, catching sun from different angles throughout the day; flat skylights only capture direct overhead light. Third, lanterns add architectural interest-inside and out-where a flat skylight is nearly invisible from below and just looks like a bubble from outside.

The only advantage of flat skylights: lower profile in neighborhoods with height restrictions or if you want the roof to look completely flat. But for light quality, weather performance, and visual impact, roof lanterns are the better choice on flat roofs-if they’re installed correctly.

After nearly two decades installing these in every Nassau County town from Glen Cove to Valley Stream, I’ve learned that homeowners remember the light, but they live with the details. A roof lantern is a beautiful way to transform a dark kitchen or extension into a bright, comfortable space. But that beauty depends entirely on treating the installation as a roof project first-structure, curb, flashing, membrane-and a window project second. Get the foundation right, and the lantern will deliver decades of gorgeous, leak-free daylight.