Installing a skylight or roof lantern in an existing roof isn't just fitting the glazed unit: it means opening a hole in a surface that until then was watertight, and each stage of that work has its cost.
Opening and structural reinforcement of the opening
Opening the hole in the roof requires cutting the existing structure (purlins, rafters) and reinforcing the perimeter so the opening is properly supported, redistributing the load that the cut elements used to carry. This reinforcement is essential and shouldn't be left out even if the opening looks small.
Perimeter sealing: the critical watertightness point
The perimeter between the skylight frame and the roof is the point where leaks most often appear if it isn't sealed properly, with a flashing or a watertightness kit specific to the skylight manufacturer, adapted to the roof type (tile, sheet, flat).
Skylight type: fixed, opening or motorised
A fixed skylight is the cheapest option; an opening one (for ventilation) adds fittings and an opening mechanism; and a motorised one, with electric opening and sometimes a rain sensor, is the most expensive but most convenient option, especially on hard-to-reach roofs.
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