How do you measure a skylight for replacement?
Short answer. Start by finding the manufacturer's label, usually inside the frame, since its model and size code is the most reliable way to order a direct replacement. Without a label, measure the outside width and length of the curb for a curb-mounted unit, or the rough opening in the roof framing for a deck-mounted unit, and note the roof pitch, roofing type and whether the unit is fixed or venting.
Step 1: look for the label
Most skylight makers put a sticker or stamped code on the inside of the frame, sometimes behind a trim piece or on the sash of venting units. That code identifies the size, glazing and model, and lets a supplier match both the unit and the right flashing kit.
Step 2: identify the mounting style
- Curb-mounted: the skylight sits on a raised wooden box built on the roof. Measure the outside of the curb, width across the slope and length up the slope, to the nearest eighth of an inch, plus the curb height above the roof.
- Deck-mounted: the frame sits directly on the roof deck. The useful number is the rough opening, the framed hole in the decking, which you can usually measure from the attic or the light shaft.
Step 3: record the rest
- Interior shaft dimensions at the ceiling, for trim.
- Roof pitch, using a level and tape or a phone app on a rafter in the attic.
- Roofing type, asphalt shingle, tile or metal, which determines the flashing kit.
- Fixed or venting, and if venting, manual, electric or solar.
- Glass or acrylic dome, and any tint or built-in blinds.
Safety and accuracy
Measure a curb from the roof only if the roof is low, dry and walkable; otherwise measure from the attic or let the installer do it. Small errors matter, because a unit that does not fit the curb or opening means reframing. If you are upgrading an old curb-mounted dome to a deck-mounted unit, the opening size, not the old dome, is what counts. See replacing skylights with the roof and how roofers measure a roof.