VELUX Flat Roof Skylights: Specs, Testing Data & Idaho Fit

Specifications & Testing Data

What the Flat Roof Skylight Actually Is

Every pitched-roof skylight relies on gravity to move water off the glass and down the flashing. Take the pitch away and the rules change: water sits, snow sits, and any joint that is merely lapped will eventually wick. VELUX's flat-roof products solve that with one of two geometries — a curved glass cap that sheds water on its own, or a raised curb that lifts the glass above whatever is pooling on the membrane.

The CFU/ISU is the signature piece. The CFU is a fixed base unit: an insulated PVC frame with a polystyrene core and a laminated glass inner pane. Over it clips the ISU CurveTech cover, a curved sheet of tempered glass that drains rain even at 0°. The two ship separately and assemble on the roof. VELUX rates the assembly for 0° to 15° slopes and offers eleven sizes, from a 23⅝" square (060060) up to 78¾" x 39⁷⁄₁₆" (200100) and a 59⅛" square (150150). The interior has a drywall groove for a clean finish with no trim.

The CXP shares the CFU frame but adds a manually operated sash that opens to 60° under a domed polycarbonate cover — a roof-access hatch for flat-roof decks and mechanical service, sold in three sizes.

The FCM is VELUX's standard curb-mount fixed skylight (covered in depth on our fixed skylight page), and it belongs here because its 0° to 60° rating and site-built curb make it the most-specified skylight on low-slope commercial roofs. For openings the FCM cannot reach, the GSM SkyMax is a custom powder-coated aluminum fixed unit up to 10 feet on the long side and 35 square feet, and the FXG double-dome acrylic is the budget curb-mount option.

The Numbers That Matter

Pitch is the first number. Below the 14° floor of a deck-mount FS, the choices are the CFU family (0–15°), the FCM or FXG on a curb (0–60°), or the GSM (1–60°). At exactly 15° to 20° the FCM and CFU overlap and we pick based on appearance and budget.

Glass is the second. The CFU's inner unit is laminated Low-E over a captive air layer under the CurveTech cap; the extra layer keeps the inner pane warmer, quiets rain, and reduces condensation on the room side — a real problem on old single-dome flat-roof units in an Idaho January. The FCM carries the same laminated Low-E options as the rest of the residential line, with impact, white-diffusing and special-order triple-pane upgrades, and its NFRC-certified U-factors under the skylight criteria run roughly 0.47–0.53 depending on glass. Those are skylight numbers rated at a 20° test slope, not wall-window numbers; do not compare across categories. For the CFU/ISU assembly and the GSM, we pull the specific NFRC label for the size and glass you choose at quote time rather than quote a range here.

Structurally, VELUX tests these units to NAFS (AAMA/WDMA/CSA 101/I.S.2/A440). Snow load on a flat roof is a design-load question the engineer answers; we size and glaze to it, and on commercial work we hand the submittal package to the architect.

Where It Fits in Idaho — and Where It Doesn't

Two kinds of Treasure Valley projects bring us flat-roof skylights. The first is the modern custom home — the flat- and shed-roofed builds going up in the foothills, on infill lots, and around the Eagle and Star river corridors, with TPO membranes, parapets and interior courtyards. On those, the CFU CurveTech is usually what the architect drew: a 120120 or 150150 over a stairwell or bath, seen from a roof deck, with no dome to date the house. Because Wood Windows Inc does commercial glass as well as residential windows, coordinating the curb detail with the roofer and the glazing schedule with the builder is normal work for us.

The second is commercial: retail, office and light-industrial roofs where daylight cuts lighting load and the owner wants a product a roofer can flash to a membrane. That is FCM-on-a-curb territory, often several openings on one roof, and often at re-roof time when the membrane is off anyway.

Where a flat-roof skylight is the wrong call: any pitched roof over 15°, where an FS deck-mount costs less and seals tighter; a room with no way to shade it, since flat glass faces the noon sun directly and a west-valley flat roof in July will overheat the room below without a blind; and a low-slope roof with poor drainage. If water already ponds near the proposed opening, the roof needs correcting before anyone cuts it.

Warranty and Service, Locally

The FCM installed on a curb with its VELUX ECL or ECW flashing kit qualifies for VELUX's 10-year installation warranty, which is written around the deck- and curb-mounted skylight family, plus 20 years on the glass seal, 10 on the product and 5 on shades and controls. The CFU/ISU carries VELUX's product and glass warranties; because its insulated curb is integral rather than flashed with a separate VELUX kit, we confirm current installation-warranty applicability with VELUX in writing before the order is placed.

On a flat roof the warranty conversation includes the roofer, and we run it. Wood Windows Inc has been in Garden City for 46 years with employee installers, and on a membrane roof we schedule alongside the roofing contractor so the curb, the tie-in and the skylight are one sequence with one point of accountability.

Have a flat or low-slope roof and a dark room under it? Request a quote on this page or call (208) 376-2200.

Last reviewed 2026-09-10 by Wood Windows Inc — (208) 376-2200