Skylights are available in a variety of shapes and sizes. Some open for ventilation, others are fixed. Larger skylights that can be used as doors are sometimes called "roof windows." Roof windows are always located within a few feet of the floor.

The most common shape for a skylight is rectangular, but they are also available in circular, oval, diamond, triangular, and multi-sided configurations. Non-rectangular units usually use plastic glazing, though higher quality units use glass. The glazing can be flat, arched, domed, pyramidal, and "warped plane" (the glazing is flat on the low side and concave in section on the high side). Of these, the pyramidal, arched, and domed shapes offer flexibility in placement. Their raised design allows light to enter from more extreme angles than flat or warped plane units. This allows more positioning options. The slope or curvature of the glazing also helps to shed moisture and leaves. These skylight designs also do not require the additional framing needed to slope a flat skylight for proper drainage on flat or low-slope roofs.

Skylights can provide ventilation as well as light. Ventilating a building through a skylight opening releases the hot air that naturally accumulates near the ceiling. Ventilating skylights usually open outward at the bottom, some more than others. Some units vent through a small, hinged panel. One design uses a swing-down, inner sash with a protected vent strip above. This can reduce the potential for rain or snow entering the room if the vents are open. Skylights may be opened manually with a pole, chain, or crank. Automated units with electric motors or pneumatic devices are also available, and some models incorporate moisture sensors to automatically close the skylight when it rains.

The physical size of the skylight greatly affects the illumination level and temperature of the space below. Use the following "rule of thumb" for sizing a skylight: the skylight size should never be more than 5% of the floor area in rooms with many windows and no more than 15% of the room’s total floor area for spaces with few windows.

In very cold weather, skylights are often prone to water vapor condensing on the glazing. The accumulation of water may then drip into the room. Better skylights usually have an interior channel to collect the condensate so it can evaporate later. The most thermally efficient skylights (R-4 or greater) are less prone to condensation problems.

Recent, "high tech" developments maximize skylights for daylighting. An "element" on the roof becomes an aperture for collecting sunlight. It may be a sun-tracking open-sided cylinder, a large lens-like element, or merely a conventional skylight with a mirrored reflector mounted adjacent to it. This aperture may then connect to a mirrored pipe, or "light pipe," which has a diffusing lens that mounts on or is recessed into the ceiling of the room below. These designs, relative to equivalent traditional skylights, effectively reduce daytime overheating and nighttime heat loss, but do not provide views or ventilation.

Fabrication and Materials Quality
The materials used in a skylight can impact energy savings, occupant safety, and unit reliability. Consider the choice of glazing carefully. The two types of glazing commonly used in residential skylights are plastic and glass.

While, plastic glazing are usually inexpensive and less liable to break than most other glazing materials, their surfaces scratch easily and they may become brittle and discolored over time. Many plastics also allow most of the ultraviolet (UV) rays in (unless the glazing is coated with a special film). This increases fading damage to furnishings. Acrylics and polycarbonates are the most commonly used plastic glazing. Acrylics are weaker than polycarbonates, but cost less. Although polycarbonates offer high impact resistance, some yellow with age.

Glass is usually found on the more expensive skylights. Glass is much harder and durable than plastics and does not discolor. All glass used for skylights must be made of "safety glazing." This is a generic term for both tempered and laminated glass. Tempered glass is the most impact resistant. Laminated glass is fabricated with a thin layer of plastic embedded near the center of the glass. Both types keep the glass from breaking into large, sharp pieces. Skylights are often made with a tempered glass on the exterior side and a laminated pane on the interior side. This arrangement gives maximum impact resistance while protecting occupants from falling shards of glass.

Because skylights are located on the roof of a building, they can result in large amounts of unwanted summer time solar heat gains and winter time heat losses. Manufacturers use various methods to reduce these impacts. The most common methods are multiple layers of glazing, gas fills (of inert argon or krypton gas) between multiple layers, and/or low-e (low-emissivity) glazing or films. Some manufacturers even install a translucent insulation material between several glazing layers to create a more thermally efficient assembly. Limiting summer heat gain from skylights facing west and south can be difficult. There are skylights with solar control tints or coatings that reduce solar heat gain. They can, however, reduce the level of light transmission and the potentially beneficial solar gain from your skylight in the winter.

Positioning and Slope
The location of a skylight is the primary consideration if you want to maximize natural lighting and passive solar heating potential. Skylights on roofs that face north provide fairly constant but cool illumination. Those on east-facing roofs provide maximum light and solar heat gain in the morning. West-facing skylights provide afternoon sunlight and heat gain. South-facing skylights provide the greatest potential for desirable winter passive solar heat gain than any other location, but often allow unwanted heat gain in the summer. You can prevent unwanted solar heat gain by installing the skylight in the shade of deciduous (leaf-shedding) trees, or add a movable shading device, such as louvers, shades, or awnings on the inside or outside of the skylight. Some units have a solar control film, with a solar heat gain rating.

The slope, or tilt, of the skylight also has a great effect on solar heat gain. A low-slope will admit relatively more solar heat in the summer and less in the winter, exactly the opposite of what is desirable. A general rule of thumb is to achieve a slope equal to your geographical latitude plus 5 to 15 degrees. For example, the optimum slope for a south-facing skylight in Columbus, OH, at 40° North latitude, is 45° to 55°. At least one skylight manufacturer makes a prefabricated, tilted base that increases the angle of a skylight above the roof.

Preventing Leaks
Leaks are a common problem with skylights. To avoid leaks, you should mount the skylight above the roof surface, install a curb (a raised, watertight lip that helps to deflect water away from the skylight) and flashing, thoroughly seal joints, and follow the manufacturer’s guidelines. It is also prudent to apply a layer of sheet waterproofing over the flanges/flashing of the skylight. This is generally installed under the finish roofing material as an aid in protecting against ice dams. Avoid water diversion devices such as roof crickets or diverter strips, as they often create more problems than they solve.

Buying Your Skylight
Skylight prices vary widely. A small skylight can cost $300 or more. Expect to pay more for special glazing materials, venting, shading devices, or other options. Installation costs depend on the complexity of the job. If you choose to have a professional install your skylight, be sure to obtain several estimates and a list of references. Contractor skill levels and fees can vary tremendously. Be wary of any bid that seems too low.

Credit: U.S. Department of Energy