What that price covers, and what it does not
The estimate prices a common job: a 30 by 46 inch fixed skylight with laminated low-E glass, set into an existing roof over a room with a finished ceiling close underneath, such as a vaulted living room or a top-floor bedroom with a sloped ceiling. The crew lays out the opening, cuts the deck, frames it, sets the unit, flashes it into the shingles and trims the inside.
The framing line surprises people. A 30-inch skylight is wider than the gap between rafters on a house framed at 24 inches on center, so one rafter is cut and doubled headers carry its load to the rafters on either side. That is structural work, and it is why the job needs a permit.
What is not in the figure: a light shaft through an attic, moving wiring or ductwork that sits where the opening wants to go, and repairs to a roof that turns out to be soft around the cut. Each of those is quoted on site, and the light shaft is the one most likely to change the total, so it gets its own section below.
Fixed, venting or tubular
Fixed skylights do not open. They are the cheapest framed unit and the least likely to leak, because there is no sash to seal. They suit living rooms, hallways and anywhere the goal is daylight rather than air.
Venting skylights open, by hand crank, by a wall switch or by a small solar panel. In a bathroom or kitchen they let heat and steam out at the highest point of the room, which a window lower down cannot do as well. The unit costs more than a fixed one of the same size, the motorized versions more again, and the better ones close themselves when a rain sensor gets wet. Installation is much the same as for a fixed unit, so the difference is mostly in the price of the skylight.
Tubular skylights are a different product. A small dome on the roof feeds a reflective tube, usually 10 to 14 inches across, down to a diffuser in the ceiling. Nothing structural is cut and there is no shaft to build, so they install in a few hours and cost well under a framed skylight. They light a hallway, closet or small bathroom well. They do not give a view of the sky, and they will not light a large room.
The light shaft is the biggest variable
If the ceiling under the roof is vaulted, the skylight sits in the ceiling and there is no shaft. If there is an attic between the roof and a flat ceiling, the light has to be carried down through it in a framed and drywalled shaft, and that is a carpentry and drywall job on top of the roofing one.
A shaft adds framing, drywall, insulation on its outer faces so it does not become a cold chimney in the attic, taping, sanding and paint, usually over two or three visits while the compound dries. A flared shaft, wider at the ceiling than at the roof, spreads the light across more of the room and costs a little more to frame. Ask for the shaft to be priced as its own line, because it is where two quotes for the same skylight most often differ.
Replacing a skylight or cutting a new one
Replacing an old skylight in the same opening skips the layout and the framing, which is most of the labor above. It is usually a half-day job if the new unit fits the existing rough opening. Measure the old frame before choosing a replacement, because modern units come in standard sizes and an opening that has to be enlarged or reduced puts some framing back into the job.
Always replace the flashing with the unit. The flashing kit is matched to the skylight, and reusing old step flashing around a new frame is a common source of the leaks people blame on the skylight.
If the roof itself is near the end of its life, do both at once. A skylight flashed into a new roof by the same crew avoids paying twice for the shingles around it, and it removes the most likely leak point from your next re-roof.
Is a skylight worth it?
For a dark room that gets used in daytime, usually yes. A skylight lets in far more light than a wall window of the same size, because it faces the open sky rather than a neighbor or a fence.
The trade-offs are heat and maintenance. A skylight on a south or west slope adds summer heat, which low-E glass and a shade reduce but do not remove. Glass overhead also has to meet the building code: IRC section R308.6 limits skylight glazing to types such as laminated glass, which holds together if it breaks. The unit priced here is laminated for that reason.
Leaks are less of a risk than their reputation suggests. Most skylight leaks are flashing failures at installation, and water running down the inside of the glass on a cold morning is usually condensation rather than a leak. A properly flashed deck-mount unit on a roof steep enough for it should stay dry for the life of the roof.