The 30-second version
- Velux's dedicated flat-roof family is engineered for 0°–15° roof pitches — pitches where standard skylights physically can't shed water reliably.
- GSM Fixed SkyMax: up to 35 sqft, 10' at the longest dimension. The flagship flat-roof skylight. Custom sizes 17.5" to 120".
- FXG Double Dome Acrylic: classic curb-mounted domed skylight for flat-roof residential applications.
- CFU / ISU CurveTech Glass: fixed skylight with a curved tempered-glass top cover that sheds rainwater cleanly on flat roofs.
- CXP / ISD Hatch: a manually-opening polycarbonate dome hatch — for roof access on flat roofs.
- Custom finish colors available on GSM: Bone White, Quaker Bronze, Black, Dark Bronze Matte, Silver Matte, Copper Clad.
Why flat roofs need dedicated skylights
Standard curb-mounted skylights (like the FCM) work down to 0° pitch on paper, but truly flat roofs — 0° to about 3° — present real problems that a flat glass surface can't solve. Rainwater doesn't drain off it. Debris accumulates. Standing water eventually finds a way through even the best flashing seal.
Velux's dedicated flat-roof family solves this with two design innovations: domed tops (FXG, CXP) that shed water by geometry, or curved / raised glass tops (CFU/ISU CurveTech, GSM SkyMax) that create just enough slope on the top surface to move water off before it can pool.
GSM Fixed SkyMax — the flagship
The GSM is Velux's largest single-panel flat-roof skylight. It covers up to 35 square feet with a maximum single dimension of 10 feet, making it the go-to specification for large-format daylighting on modern LA architect-designed homes, additions, and lofts.
Key features:
- Laminated LowE³ Glass (standard); optional glass upgrades including Hurricane Resistant
- Standard 2604 powdered-coat finish in VELUX Grey; optional custom colors and copper cladding
- Custom sizes available: minimum 17.5" to maximum 120" (10 feet)
- Roof pitch: 1°–60° (works from just above flat all the way to steep-pitched)
- No shade or flashing kit compatible with GSM — the frame is designed for direct membrane integration
Standard sizes: 4896 (48"×96") and 5199 (51"×99") — both very large.
Where GSM belongs: single dramatic skylight on modern home, penthouse or top-floor addition, gallery-style lightwell over a stairwell, dramatic entry foyer, or any project where a 10' skylight is the design centerpiece.
GSM custom finish colors
Beyond standard VELUX Grey, custom finishes include Bone White, Quaker Bronze, Black, Dark Bronze Matte, Silver Matte, and Copper Clad. The Copper Clad option is particularly popular on estate projects where the frame patinas naturally over time to match copper gutter and flashing details.
FXG Double Dome with Acrylic
The FXG is the traditional flat-roof skylight — curb-mounted with a fully-encapsulated double-dome acrylic top. Common on mid-century modern LA homes as an original or replacement skylight.
- Curb-mounted with VELUX Neutral Grey exterior
- Encapsulated inner frame is 100% thermally broken (reduces condensation)
- Standard glazing: clear over clear or clear over white acrylic
- Bronze over clear acrylic and custom sizes available on special order
- Roof pitch: 0°–60°
- Standard sizes: 225225, 225465, 305305, 345345, 465465 (in curb-outside dimensions × 10)
Where FXG belongs: replacing an existing acrylic dome skylight on a mid-century home where matching the original aesthetic matters. FXG is also the value option — it costs less than glass-topped alternatives while still delivering good daylight.
CFU / ISU CurveTech Glass
The CFU is a fixed base unit paired with an ISU top cover featuring Velux's CurveTech glass — a subtly curved tempered-glass panel that sheds rainwater off cleanly. This gives you the look of glass (much cleaner than acrylic dome, higher-end appearance) on a flat roof.
- Tempered glass top cover — virtually scratch-proof, durable protection layer
- Inner laminated glass for safety, sound control, and thermal protection
- Insulated PVC frame
- Roof pitch: 0°–15°
- Wide range of standard sizes from 060060 (23.7"×23.7") to 200100 (78.75"×39.4")
Where CFU/ISU belongs: modern architect-designed homes where the client wants glass (not acrylic) on a flat roof. Also excellent for premium coastal properties in Marina Del Rey, Manhattan Beach, and Malibu where UV degradation on acrylic domes would be a concern.
CXP / ISD Polycarbonate Dome Hatch
The CXP is the specialty item — a manually-opening flat-roof hatch with a highly-impact-resistant domed polycarbonate outer cover. Opens up to 60° for roof access.
- Roof pitch: 0°–15°
- Ideal for: roof-mounted solar array access, HVAC equipment access on flat-roof mid-century homes, occasional maintenance access, or emergency egress from top-floor spaces
- Standard sizes: 090120, 100100, 120120
- The polycarbonate dome outer cover survives hail and impact where glass or acrylic domes would crack
LA-installed cost ranges (2026)
- FXG small (225225, 27.5"×25.5"): $2,000–$3,600 installed
- FXG large (465465, 49.5"×49.5"): $3,400–$6,200 installed
- CFU/ISU CurveTech (small, 060060): $2,400–$4,200 installed
- CFU/ISU CurveTech (large, 150150): $4,000–$7,200 installed
- GSM SkyMax (4896, 4'×8'): $7,500–$14,000 installed
- GSM SkyMax (5199, 4.25'×8.25'): $8,500–$16,000 installed
- GSM Custom sizes up to 10': quote-based, typically $12,000–$28,000+ installed
- CXP roof-access hatch (090120): $2,800–$5,200 installed
- Add copper cladding (GSM): +$1,200–$3,500
- Add custom finish color (GSM): +$400–$1,200
- Add impact glass (FXG or CFU): +$400–$900
Best pairings with LA architecture
- Mid-century modern flat-roof (Cal Heights, Silver Lake, Culver City): FXG or CFU/ISU depending on budget
- Contemporary architect-designed (Brentwood, Bel Air moderns): GSM SkyMax as centerpiece
- Marina Del Rey / Manhattan Beach waterfront condos: CFU/ISU with impact glass upgrade
- Additions or ADUs with flat roofs: FCM (for smaller spans) or FXG for standard sizes; GSM for larger dramatic moments
Get a free Velux quote for your LA home
Halo Build Co is a Velux-authorized dealer serving the Greater Los Angeles area. Every quote includes site inspection, skylight sizing recommendations, cutout and flashing details, and total installed cost — no hidden fees. Text, call, or fill out the form.