How does a roof skylight work?
Short answer. A roof skylight is a window set into a framed opening in the roof, with a metal flashing kit woven into the shingles around it to shed water and sealed, insulated glazing that lets light in while limiting heat. Venting models also open for airflow, and tubular skylights pipe daylight through a reflective tube from a small dome on the roof.
The parts
- The opening. A hole cut in the decking between rafters or trusses, with headers framing the top and bottom. Wider units may require cutting framing, which is structural work, and trusses should never be cut without an engineer's design.
- Deck-mounted or curb-mounted. Deck-mounted units sit directly on the roof deck with a low profile. Curb-mounted units sit on a raised wood box, common on older homes and lower slopes.
- The flashing kit. Step flashing along the sides, head flashing at the top and a sill or apron flashing at the bottom, all lapped with the shingles so water flows around the unit. Many kits add an underlayment collar that wraps the frame.
- The glazing. Usually double-pane, low-E coated glass, often tempered outside and laminated inside so broken glass stays in place. Older units may have acrylic domes.
- The light shaft. The drywall tunnel through the attic, straight or flared to spread light.
Types
- Fixed: light only.
- Venting: opens by hand crank, electric motor or solar motor; many include rain sensors that close them automatically.
- Tubular: a small dome and reflective tube for halls, closets and baths, with a much smaller roof opening.
Georgia considerations
In Georgia sun, low-E glass and a light-filtering shade cut heat gain in the room below. Pine needles and leaves pile up on the uphill side of skylights, so that spot needs occasional clearing. And because the flashing kit is made for the unit, skylights are usually reflashed or replaced when the roof is. See replacing skylights with the roof and roof flashing explained.