Skylight Rough Opening Calculator — IRC R802.6

Calculate the rough opening, header length, sill length, and framing lumber for a skylight from its nominal outside-frame dimensions. Adds 1/4" per side clearance per IRC R802.6 and manufacturer rough-opening specs.

By TradeCalc, Roofer Calculators — Code-Referenced — IRC R802.6 (openings in roof framing), IRC R802.6.1 (header and sill framing), Velux Deck-Mount & Curb-Mount Rough Opening Specs, Andersen Skylight Installation Manual

Related Calculators

How to Frame a Skylight Rough Opening

Rough Opening vs Nominal Size

A skylight rough opening (RO) is the framed hole in the roof deck, slightly larger than the skylight's outside frame to leave room for leveling, flashing, and shimming. The skylight's nominal size (e.g. 2246 = 22" × 46") refers to the outside-frame dimensions, not the visible glass. RO is always bigger: typically nominal + 1/4" per side (0.5" total per dimension), per manufacturer rough-opening tables.

The 1/4"-per-Side Clearance

RO width = Nominal width + 2 × clearance

RO length = Nominal length + 2 × clearance

Clearance = 1/4" per side is the Velux / Andersen default
Header cut length = RO width + 3" (1.5" bearing on each trimmer jack)
Sill cut length = RO width + 3" (same bearing)
Trimmer jacks = 2 per opening, full rafter depth, toenailed to header and sill

Common Velux deck-mount ROs: 2230 → 22.5" × 30.5"; 2234 → 22.5" × 34.5"; 2246 → 22.5" × 46.5"; 3030 → 30.5" × 30.5"; 3046 → 30.5" × 46.5". Always confirm against the spec sheet for your exact model — certain flushing kits add 1/2" instead of 1/4" per side.

Framing per IRC R802.6

IRC R802.6 requires that any opening in the roof framing be reinforced with a double header above and a single sill below, each bearing at least 1.5" on full-depth trimmer jack rafters on both sides. The two original rafters that get cut become "cripples" above the header and below the sill, restoring the rafter bay continuity. The header and sill carry the load that the cut rafters used to carry.

Worked Example

Scenario: A Velux 2246 deck-mount skylight (nominal 22" × 46") goes into a roof framed 16" o.c. Find the rough opening and header length.

RO width = 22 + 2 × 0.25 = 22.5"
RO length = 46 + 2 × 0.25 = 46.5"
Header cut length = 22.5 + 3 = 25.5" (1.5" bearing each side)
Sill cut length = 25.5" (same)
Opening area = 22.5 × 46.5 / 144 = 7.27 sq ft

Order: 1 piece of 2× the rafter depth, ~26" long, for the doubled header (cut 2 pieces); 1 piece for the sill; and 2 trimmer jacks the full rafter length, ripped to fit the 22.5" RO between existing rafters (a 22.5" RO between 16" o.c. rafters needs one extra "sistered" rafter splitting the bay).

Practical Tips

Confirm the RO on the spec sheet, not the box label. Box labels sometimes print nominal size; the rough-opening dimension is on the installation manual. A 1/4" error means the skylight won't seat in the curb or the flashing won't lap.
22" skylights fit between 24" o.c. rafters; 30" needs a trimmer. A 22" nominal skylight drops into a 24" o.c. bay with 1.5" of bearing each side. A 30" skylight straddles two bays and needs a trimmer jack to carry the cut rafter. Plan the location around the framing.
Curb-mount skylights have a built-in curb. Deck-mount sits flat on the roof deck (RO = nominal + clearance). Curb-mount sits on a 2×6 or 2×8 curb you build up around the RO — add the curb height (typically 4") to the RO area for flashing material, and order the curb-mount model specifically.
Double the header on spans over 4 ft. IRC R802.6 requires a built-up header (2-ply) on any skylight wider than 4 ft, or any skylight in a high-snow zone. Nail the two plies with 16d common nails @ 16" o.c. staggered.
Flashing kit > roll-your-own. Manufacturer flashing kits (Velux EBW, Andersen EZ) are engineered to lap correctly and are required for the skylight warranty. Order the kit matched to your roofing material (shingle vs tile vs membrane).

Code References

IRC R802.6 (openings in roof framing), IRC R802.6.1 (header and sill framing), Velux Deck-Mount & Curb-Mount Rough Opening Specs, Andersen Skylight Installation Manual

Frequently Asked Questions

What is the rough opening for a 2246 Velux skylight?

A Velux 2246 (22" × 46" nominal outside frame) needs a 22.5" × 46.5" rough opening — the nominal size plus 1/4" per side for clearance and flashing. The header cut length is 22.5 + 3 = 25.5" to bear 1.5" on each trimmer jack rafter. Always verify against the model-specific installation manual before cutting.

How much bigger than the skylight should the rough opening be?

Add 1/4" per side (0.5" total per dimension) for deck-mount skylights per the Velux/Andersen standard. So a 22"×46" skylight gets a 22.5"×46.5" RO. Curb-mount skylights may need 1/2" per side for the curb to seat — check the spec sheet. Never make the RO exactly nominal size; you need room to level the frame and lap the flashing.

How do I frame a skylight header?

Cut the two rafters in the skylight bay, then install a doubled header above and a single sill below, each bearing at least 1.5" on full-depth trimmer jack rafters sistered to the adjacent rafters. Header and sill cut length = RO width + 3" (1.5" bearing each side). The cut rafter pieces become "cripples" extending from header up to the ridge and from sill down to the wall plate, restoring bay continuity per IRC R802.6.

What is the difference between deck-mount and curb-mount skylights?

Deck-mount skylights sit flat on the roof deck with the flashing lapped under the shingles — RO = nominal + 1/4" per side. Curb-mount skylights sit on top of a built-up wood curb (typically 2×6 or 2×8 frame, 4" tall) that you frame around the RO — the curb raises the skylight above the roof plane and is required for low-slope roofs (< 3:12) and flat roofs. Curb-mount RO is the same nominal + clearance, but you also build the curb.