What size skylight should I get for my roof?
Short answer. Pick a size that fits between your existing roof framing whenever you can, usually about 14.5 inches wide for framing at 16 inches on center or 22.5 inches for 24 inches on center, because wider units mean cutting rafters and adding headers, and trusses should never be cut without an engineer's design. For a replacement, match the existing rough opening or curb so the ceiling and shaft do not need rework.
Check your attic first
- Rafters: individual sloped boards, common in older homes and vaulted ceilings. A carpenter can cut one and add doubled rafters and headers for a wider unit.
- Trusses: factory-built triangles with web members, common in Georgia homes built since the 1970s. Never cut a truss without a repair design from an engineer or the truss maker; stick to sizes that fit between them.
- Spacing: measure center to center. Most trusses are 24 inches apart, so the 22.5 inch width fits without cutting.
Think about the shaft
Under a flat ceiling with an attic above, light travels down a shaft from the roof to the ceiling. Long shafts dim the light, so a slightly larger skylight or a flared shaft helps. If the shaft would be long and narrow, a tubular skylight may be the better tool.
Other fit questions
- Pitch: confirm the unit and its flashing kit are rated for your roof slope, especially on low-pitch roofs.
- Curb or deck mount: deck-mounted units sit lower and look sleeker; curb-mounted units suit low slopes and replacements on existing curbs.
- Placement: keep skylights out of valleys and away from spots where two roof planes dump water toward them.
- Glazing: skylights must meet the building code's safety glazing rules, which quality residential units are made to.
For replacements
Measure the rough opening or the outside of the curb, not the glass, and order a unit and flashing kit to match. A reroof is the most efficient time to replace one. See replacing skylights with a new roof and roof anatomy explained.