Can I Install Skylights in an Existing Roof?

Structural

The opening framing (the rafters cut — the headers and the trimmers carrying the interrupted rafters; the doubled rafters at the opening), the roof type (stick-framed rafters — the straightforward retrofit; trusses — the engineered modification, never field-cut without the truss designer's approval; the truss's web configuration often prohibiting the opening), and the loads (the skylight's weight — modest; the snow — the curb's drifting; the wind — the unit's rating). The structural check ($500-$1,500) precedes the roofer.

Weathertightness

Curb-mounted vs. deck-mounted (the curb's height above the roof — the flashing's working room; the deck-mounted's lower profile — the sleeker, more flashing-critical), the flashing kit (the manufacturer's system — the step flashing woven with the shingles; the aftermarket flashing is the leak source), and the roof's remaining life (the skylight outliving the roof — coordinate the reroof; the skylight younger than the roof — the flashing's integration with the aged roofing). The leak statistics (the poorly flashed skylight is the #1 retrofit leak — the flashing, not the unit, fails).

Energy and code

The glazing (the low-E, argon-filled — the U-factor and the SHGC; the south-facing solar gain — the summer overheating; the diffusing glazing — the glare control), the energy code (the skylight area limits — the 3-5% of roof area prescriptive caps; the U-factor requirements), and the permit (the structural modification — the framing inspection; the energy compliance). Cost: $1,500-$4,000+ per installed skylight (the structural work, the unit, the flashing, the interior finish). The permitted, flashed skylight transforms the room; the unpermitted one stains the ceiling.

The bottom line

Skylight retrofits need structural headers (never field-cut trusses), manufacturer flashing kits, and energy-code glazing. Budget $1,500-$4,000+ per skylight; coordinate with the roof's remaining life.

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