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Most people assume a skylight quote is mostly glass. It rarely is. The unit is often the smaller half of the number, and the difference between a cheap install and an expensive one is what sits between the ceiling and the roof deck. This guide breaks the cost into the unit, the roof opening, the interior light shaft and access, explains vaulted ceilings versus attic shafts, covers tubular daylight devices as the budget route, and details the flashing that keeps skylights from leaking.
A skylight is a hole in the roof, and the cost is driven far less by the unit than by what it takes to frame, flash and finish that hole properly. The shaft through the ceiling below is often the largest part of the job.
| Driver | Effect |
|---|---|
| Fixed vs venting vs powered | Venting and powered units cost substantially more |
| Shaft length | A deep attic means a long shaft — often the biggest line |
| Rafter cutting | A unit wider than one bay needs headers and structural sign-off |
| Roof covering | Tile and metal need specific flashing kits and more labour |
| Doing it during a reroof | Much cheaper — the covering is already off |
| Glazing options | Laminated, low-E, impact-rated and blinds all add |
The manufacturer's flashing kit for that unit on that covering, fitted as instructed. Nearly every leaking skylight is either a site-improvised flashing detail or roof cement used instead of a flashing. If the unit is more than about 30 inches wide across the slope it also needs a cricket on the up-slope side, exactly as a chimney does.
If a reroof is coming in the next few years, do the skylight at the same time. The covering is already off, the flashing goes in as part of the roof rather than as a retrofit into it, and the same crew is already there. Retrofitting into a roof that is otherwise sound costs more and creates a joint between old and new work.
Far less the unit than the work around it: framing the opening, the flashing kit, and building and finishing the light shaft through the attic — the shaft is often the largest single line.
Almost always a site-improvised flashing detail, or roof cement used instead of a flashing. The manufacturer's flashing kit for that unit on that covering, fitted as instructed, is what keeps it dry.
If a reroof is coming within a few years, yes. The covering is already off, the flashing goes in as part of the roof rather than as a retrofit, and the same crew is already there.
If it is more than about 30 inches wide across the slope, yes — the same rule as a chimney. Without one, water ponds against the up-slope side.