Skylight and Roof Window

A skylight (in the UK often called a rooflight) is a light-transmitting unit that forms part of a roof, admitting daylight and sometimes ventilation. Open roof apertures date back to antiquity, such as the oculus of the Pantheon in Rome; sealed glazed skylights became practical with improvements in glass manufacture during the Industrial Revolution, and mass production since the mid-20th century made them common in houses.

Types

• Fixed skylight: a frame holding glazing that does not open.
• Venting (operable) skylight: a hinged sash that opens for ventilation. When it is within occupants' reach in a sloping roof, it is generally called a roof window, a term used widely in Europe.
• Tubular daylighting device (sun tunnel): a small roof dome, about 10 in (250 mm) in diameter for homes, connected to a reflective tube that delivers light to a ceiling diffuser.
• Sloped glazing and glazed roof systems: larger multi-panel assemblies.
• Domed rooflights: acrylic or polycarbonate domes, typical on flat roofs.

Installation and flashing

Skylights interrupt the roof covering, so they are flashed much like a chimney: a sill or apron flashing at the bottom, step or side flashings along the sides, and a head flashing at the top, generally using the manufacturer's flashing kit. Self-adhered membrane is commonly used around the opening. Under the US IRC, skylights on roofs with a slope below 3:12 must be mounted on a curb at least 4 in (102 mm) above the roof plane unless the manufacturer specifies otherwise. Sealant alone should not be relied on.

Glazing and energy

Contemporary units usually use sealed double glazing, often with low-emissivity coatings and gas fills; triple glazing is used in the coldest climates. In the US, performance is rated by U-factor (heat loss) and solar heat gain coefficient (SHGC), both expressed from 0 to 1 with lower values meaning less heat transfer.

Building rules

Codes govern the type of glazing used overhead, protection against falling glass, and in some jurisdictions fire separation distances. Large skylights may need structural framing around the opening.