What does skylight replacement cost, and when should a leaking skylight be replaced?

Often, yes, and it depends on which part failed. A leak at the head or sides is normally a flashing failure and is repaired by re-flashing, not by replacing the unit. A cracked acrylic dome can be swapped if the model is still available. A failed operator motor or handle on a venting unit is a serviceable part. What cannot be repaired is a sealed glass unit that has fogged between the panes, because the seal is the product, and a kerb that has rotted, because that is carpentry rather than glazing.

The part we publish is the flashing work: $450 per skylight to strip, re-flash and weather it into a new roof, which is the labour that decides whether it leaks again. The unit price depends entirely on which unit — fixed, curb-mounted, manual venting, solar venting or tubular — and on the size of the opening, so it is quoted from the actual model after measurement and put on the estimate as its own line rather than published as an average we would then have to explain.

Three clear triggers. When the sealed unit has fogged, because the glazing has failed and no repair restores it. When the frame, kerb or deck around it has rotted, because the opening has been letting water in for years. And when the roof is being replaced and the skylight is more than about fifteen years old, because the flashing has to be stripped and remade anyway, the opening is exposed, and doing it later means cutting into a new roof. That last one is the case where replacing early is genuinely cheaper.

A roofing contractor, not a window company, because the failure is almost always in the flashing or in the roof above rather than in the glass. Book an inspection that includes the attic if it is accessible: water entering above a skylight runs down the underside of the deck and appears at the top of the shaft, which is why so many skylights are blamed for leaks that started three feet uphill. If the glazing itself has failed, the roofer will tell you and the manufacturer service network handles the unit.

The manufacturer kit for that exact model, installed in the right order with ice-and-water membrane run up around the opening, is the correct way to flash a skylight and it works. Generic kits and site-formed metal can work in skilled hands and frequently do not, because the head flashing detail and the way the step flashing laps into the side channels are model-specific. What never works long-term is sealant used instead of a kit.

Yes, and that is by far the cheapest moment to do it, because the deck is exposed, the framing can be seen and the flashing is being formed anyway. It needs the rafters checked and usually a header framed, the shaft built and finished inside, and it is permitted work. It is not a decision to make on the day the tear-off starts — the unit has to be ordered and the interior work planned before the roof opens.