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Size skylights from the room area and the daylight target, then estimate flashing lengths, ice-and-water membrane and the installed cost including framing and interior finishing. The unit itself is often the smaller part of a skylight quote — the light shaft, flashing kit and interior work usually cost more. Use the result alongside the flashing calculator and the skylight installation cost guide before requesting quotes.
Skylight sizing is a balance between daylight and heat. Too little and the room is no brighter; too much and it overheats in summer and loses heat in winter. The usual guideline is a percentage of the floor area of the room below.
| Room floor area | 5% (rooms with other windows) | 10% (rooms with few windows) | 15% (maximum) |
|---|---|---|---|
| 100 sq ft | 5 sq ft | 10 sq ft | 15 sq ft |
| 150 sq ft | 7.5 sq ft | 15 sq ft | 22.5 sq ft |
| 200 sq ft | 10 sq ft | 20 sq ft | 30 sq ft |
| 300 sq ft | 15 sq ft | 30 sq ft | 45 sq ft |
| 400 sq ft | 20 sq ft | 40 sq ft | 60 sq ft |
Five percent suits a room that already has windows; ten percent suits a room with few or none. Above about fifteen percent the heat gain and loss usually outweigh the extra light.
Between the roof opening and the ceiling below is a shaft through the attic, and it has to be framed, insulated, drywalled and decorated. On a deep attic that is often more work than the skylight itself. Splaying the shaft outward toward the room spreads the light noticeably better than a straight vertical one.
Around 5% of the room's floor area if the room already has windows, and up to 10% if it has few. Above about 15% the summer heat gain and winter heat loss usually outweigh the extra light.
North gives even light all day with the least heat gain, which suits a workspace. South gives the most light and the most summer heat, so consider low solar heat gain glazing or a blind.
Not if the unit fits between two rafters, which is why standard sizes match common rafter spacings. A wider unit needs headers and is structural work.
The shaft. Between the roof opening and the ceiling below is a framed, insulated, drywalled and decorated shaft through the attic, and on a deep attic that is often more work than the skylight itself.
Budgeting the job? The skylight installation cost guide breaks down the unit, flashing kit and labour.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.