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.