Measure curb-mounted skylights at the outside curb and deck-mounted skylights at the finished interior opening — mixing the two is what orders the wrong unit.
A skylight replacement goes wrong at the tape measure, not the roof. The whole measurement splits on one question — is the existing skylight sitting on a raised curb, or is it a self-flashing unit that seals straight to the roof deck? Everything else follows from that answer.
Is the Existing Skylight Curb-Mounted or Deck-Mounted?
Curb-mounted skylights sit on a raised wooden or metal curb box above the roof deck — on the roof, you see flashing wrapping up the sides of that frame. Deck-mounted (self-flashing) skylights mount flush and rely on integrated flashing to shed water, so nothing sits proud of the roof. Stand back and look at the profile before you touch a tape. The two types are measured in completely different places, and a replacement sized off the wrong one will not align with the existing opening.
Measuring a Curb-Mounted Skylight: Outside Curb Dimensions
For a curb-mounted replacement, VELUX’s homeowner guidance says to measure the width of the existing skylight’s outside curb — including the flashing and roofing material on that curb. That exterior curb measurement is the number a standard replacement is matched against, not anything inside the house.
Take width and length across the roof, along the sides where shingles meet flashing, and note the height of the skylight or curb. Skip the interior trim and drywall finish entirely — those have nothing to do with curb size, and folding them in is a common way to end up oversize. You can also note the outside glass size taken from the inside edge of the skylight cladding if the manufacturer asks.
Measuring a Deck-Mounted Skylight: Finished Interior Opening
A deck-mounted or self-flashing replacement is sized off the finished interior drywall-to-drywall opening, measured as close to the light shaft as you can reach. VELUX’s replacement sizing adds 1 inch or 1.25 inches overall to that finished drywall measurement, so the interior opening is the controlling number.
Also record the outside frame around the interior drywall opening, the exterior visible glass area, and the overall size including cladding. Non-VELUX deck-mounted units want additional exterior numbers — the glass length and width inside the cladding, and overall size with cladding — because brands differ in how they size the finished unit. Relying on a single number is where wrong-size orders come from.
If your rough opening lands near 14 inches, our roundup of tested 14-inch skylight options covers the models that fit that footprint.
| Mount Type | Number That Sizes the Replacement | Also Record |
|---|---|---|
| Curb-mounted | Outside curb width and length, flashing included | Curb or skylight height; roof pitch |
| Curb-mounted (alternate) | Outside glass, from inside edge of cladding | Never use interior trim or drywall |
| Deck-mounted (VELUX) | Finished interior drywall opening, plus 1 to 1.25 inches | Outside frame around interior drywall; exterior visible glass area |
| Deck-mounted (other brands) | Interior drywall opening length and width | Exterior glass inside cladding; overall size including cladding |
| Any type | Width and height taken in several spots | Note out-of-square openings before ordering |
One more roof-side number belongs in the mix: pitch. VELUX’s replacement process says to set a pitch gauge directly on the roof where the skylight will be installed, because the replacement unit has to match the roof’s slope.
The measurement discipline that matters most — measure the curb or opening that stays, not the skylight you are throwing away. A discarded unit’s dimensions describe a product about to be in a dumpster, not the hole it has to fill. Measure width and height in several spots too; older roofs go out of square, and a single tape reading can hide a half-inch of drift that shows up as a leak later.
When the numbers are in, photograph the tape against each measurement — VELUX recommends sharing those photos with a contractor or roofer to confirm sizing before ordering. Their replacement resource pages are worth reading in full, including the VELUX homeowner measurement walkthrough, because the correct method differs by mount type and brand. One useful note from their replacement tools: a single calculator built on curb-base dimensions covers nearly every VELUX skylight made in the last thirty years, while other modes work from visible exterior glass or finished drywall figures depending on the unit.
Two final cautions. First, sizing is brand-specific — a deck-mounted skylight from another manufacturer will not match on the VELUX formula alone, so confirm the replacement brand’s own requirements. Second, this is roof work: fall protection and safe conditions are not optional. If the slope or weather is questionable, hand the tape to an installer and use their numbers.
FAQs
Should I measure the old skylight itself?
No. Measure the curb or opening that remains once the old unit comes out. The discarded skylight’s own dimensions are usually smaller than the structural opening it sat in, so sizing a replacement off the old glass or frame produces a unit that does not fit.
Why does VELUX add an inch to the interior measurement?
Deck-mounted sizing starts with the finished drywall opening and adds 1 inch or 1.25 inches overall to arrive at the replacement unit’s dimensions. That allowance accounts for the difference between the finished interior opening and the actual unit size the replacement system needs.
Do I need a pitch gauge as well as a tape measure?
Yes, for most replacements. VELUX’s process calls for measuring roof pitch with a pitch gauge set directly on the roof where the skylight will sit, since the replacement unit has to match the roof’s slope. A tape measure alone does not capture that.
References & Sources
- VELUX. “How to Measure Your Existing Skylight for a Replacement.” Homeowner-facing measurement guidance by mount type.
- VELUX. “Replacement Installation.” Roofer-facing replacement process, including curb and glass measurement modes.
- VELUX. “Replacement Skylights.” Replacement sizing tools, including the thirty-year curb-base calculator coverage.
