Specifications & Testing Data
| Frame material | Wood core sealed in white polyurethane (GPU/GGU/GXU) or lacquered wood (GDL), maintenance-free metal exterior cladding |
|---|---|
| Design pressure (DP) rating | Tested to NAFS (AAMA/WDMA/CSA 101/I.S.2/A440); verify structural rating per size at quote time |
| AAMA performance class | NFRC-certified under skylight criteria (rated at 20° slope) — not comparable to wall-window U-factors |
| Glass options | Laminated Low-E3 insulating glass with ThermoTechnology seal, Manual light-filtering, room-darkening, Venetian and blackout shades, Insect screen |
| Available styles | GPU top-hinged, opens to 45°, 15–55° pitch, 8 sizes, GGU center-pivot, rotates 180° for cleaning, 15–90° pitch, 17 sizes, GXU roof-access with locking stay, 15–85° pitch, GDL Cabrio balcony, 35–53° pitch |
What the Roof Window Actually Is
A roof window is what Europeans have put in attics for eighty years and what most Idaho homeowners have never been offered: a real operable window, with a handle you turn by hand, installed in the slope of the roof at a height you can reach. VELUX sells four in the US, all built on a wood core sealed in white polyurethane with metal cladding outside, all flashed to the deck like a skylight, and all manual — no motor, no battery, no remote.
GPU Top-Hinged. Hinged at the head, it swings outward to 45° so the sash never enters the room. A bottom handle opens it; a top control bar works the ventilation flap. Rated for 15° to 55° pitches in eight sizes from CK04 (22⅛" x 39" rough opening) to UK08 (53¼" x 55½"). This is the egress unit.
GGU Center-Pivot. The original VELUX design. The sash pivots on its horizontal midline, so it can be set lower on the slope, operated from a chair, and rotated a full 180° to clean the outer glass from inside. Rated 15° to 90° — it will go in a near-vertical mansard — in seventeen sizes up to 53¼" x 55½".
GXU Roof-Access. A top-hinged unit that opens wide on a locking stay for roof access, service and escape, rated 15° to 85°.
GDL Cabrio Balcony. Two sashes that unfold into a small standing balcony with rails, rated for 35° to 53° pitches and sized 33 1/16" or 39⅜" wide by 101" tall.
The natural-pine GGL and GPL versions you will see in European literature are not in the current US catalog; the white polyurethane finish is what VELUX ships here, and it suits humid baths and painted trim.
The Numbers That Matter
Roof windows are certified as skylights under NFRC procedures — rated at a 20° slope facing the sky, on the skylight scale, so their U-factors read higher than a wall window's and should only be compared with other overhead glass. VELUX glazes them with laminated Low-E insulating glass and its ThermoTechnology seal; we quote the NFRC label values for the exact size and glass you select rather than publish a range here, because roof-window ratings vary by size more than skylight ratings do.
The dimension that matters more than U-factor on a roof window is egress. VELUX marks the GPU in MK06, MK08, PK10, SK06 and UK08, the GXU, and the GDL as satisfying most emergency escape requirements, with the standing caveat that local code governs. The IRC's clear-opening minimum and 44-inch maximum sill height apply, and on a sloped installation the effective opening depends on pitch and hinge geometry — which is why a bedroom roof window is measured and permitted, not eyeballed.
| Model | Operation | Pitch range | Sizes | Egress-marked sizes |
|---|---|---|---|---|
| GPU | Top-hinged, 45° | 15–55° | 8 | MK06, MK08, PK10, SK06, UK08 |
| GGU | Center-pivot, 180° rotation | 15–90° | 17 | None |
| GXU | Top-hinged access, locking stay | 15–85° | 1 | Yes |
| GDL | Fold-out balcony | 35–53° | 2 | Yes |
All four take VELUX manual shades — light-filtering, room-darkening, Venetian and blackout — and an insect screen, and all are tested to the NAFS structural standard.
Where It Fits in Idaho — and Where It Doesn't
The roof-window house in the Treasure Valley is the half-story: the 1910s–1940s bungalows of Boise's North End and East End with a finished attic under a steep roof, knee walls at four or five feet, and one gable window per end. Those rooms are dark and hot in equal measure. A pair of GGU MK06 units set low over the knee wall doubles the daylight and, opened a few inches on a summer night, drains the heat that gable windows cannot. In a historic district the low-profile metal cladding and absence of a dome tend to pass design review where a bubble skylight would not — we bring the spec sheet to the hearing.
The second fit is the bonus room over the garage in 2000s two-stories from Meridian to Nampa — a room the builder gave one small window and a lot of ceiling slope. If that room is being finished as a bedroom, it needs egress, and an egress-marked GPU in the roof slope is often the only way to get it without a dormer. A dormer costs several times more.
Attic conversions in general — the lofted office, the teen suite — follow the same logic: roof windows only make sense where the glass is within reach. If the ceiling is flat and the roof deck is six feet above it, a roof window becomes an awkward skylight; go to the Fresh Air or fixed lines instead. The GPU also stops at 55° pitch, so an 18:12 A-frame wants the GGU. And on any roof under 15°, none of these apply.
Warranty and Service, Locally
VELUX warrants roof windows for 10 years against defects in materials and workmanship, the insulating glass for 20 years against seal failure, and shades and controls for 5 years. VELUX's 10-year installation warranty is described around its deck- and curb-mounted skylight family installed with VELUX flashing; roof windows use the same engineered flashing approach, and we confirm with VELUX in writing whether the installation warranty extends to a specific roof-window order rather than promise it here.
Roof windows involve more interior carpentry than any other VELUX product — headers, jamb extensions, knee-wall tie-ins, sometimes an egress permit — and Wood Windows Inc does all of it with employee installers out of Garden City, as we have for 46 years. The crew that frames the opening is the crew that comes back if a hinge ever needs adjustment.
Have a half-story or bonus room that needs light, air, or a legal way out? Request a quote on this page or call (208) 376-2200.
Frequently Asked Questions
Top-hinged GPU or center-pivot GGU — which roof window should I choose?
It depends on where you stand relative to the glass. The GPU hinges at the top and swings out to 45°, so nothing intrudes into the room and you can lean out for a full view — it is the one that meets egress in its larger sizes and the one we spec for bonus rooms where the window sits above a knee wall you stand at. The GGU pivots at the center, so the top of the sash swings into the room, but you can install it lower, operate it from a seat, put furniture under it and spin it 180° to wash the outside. The GGU also runs to 90° pitch; the GPU stops at 55°.
Can a VELUX roof window serve as an emergency egress window?
The GPU in sizes MK06, MK08, PK10, SK06 and UK08 is marked by VELUX as satisfying most egress requirements, as are the GXU roof-access and GDL balcony units; the GGU center-pivot is not. Idaho jurisdictions apply the IRC's emergency escape rules — a minimum clear opening and a maximum sill height — and the roof pitch changes how a roof window's opening measures, so we confirm the specific unit and sill height with the local building department before we order for a bedroom.
What is the difference between a roof window and a skylight?
Reach. A skylight sits out of reach in a ceiling and is fixed or powered; a roof window is a full-size operable window set in a roof slope where you can put your hand on it, with a handle, a ventilation flap, and glass that goes much larger — the GGU UK08 is 53¼ inches wide. If the room has a knee wall and a sloped ceiling you can touch, it is a roof-window room. If the glass would be eight feet up, look at our fixed or Fresh Air skylight pages.
What does a VELUX roof window cost installed in the Treasure Valley?
More than a comparable skylight, because the units are larger, the framing sits in living space rather than an attic, and finishing the interior jambs and knee wall is trim carpentry. Bedroom egress installs also carry a permit and inspection. We quote after measuring the rafter bay and pitch; our skylight cost guide covers the drivers that move the number.
Sources & Testing Data
Last reviewed 2026-09-10 by Wood Windows Inc — (208) 376-2200