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A roof window is an outward opening window that is incorporated as part of the design of a roof. Often confused with a skylight, a roof window differs in a few basic ways. A roof window is often a good option when there is a desire to allow both light and fresh air into the space.
A roof window tends to be larger than a skylight, making it possible to enjoy a wider view of the sky overhead. In addition, skylights are usually stationary; that is, they cannot be opened and closed. With some designs of a roof window, it is possible to retract a portion of the glazed panes to allow in fresh air, as well as enjoy the natural light. It is common for different manufacturers to have slight variations in the definition of roof windows and skylights. For example, Velux defines skylights as able to be mounted to a curb, while roof windows must be installed in the same orientation and plane as the surrounding roof. On the other hand, Dakea defines skylights as suitable for unoccupied spaces such as lofts or garages, while roof windows offer more insulative properties.
A roof window is also different from a tubular skylight, in that the light is not directed through any type of channel or tube in order to provide lighting for the interior of a building. This type of light tube design is often employed with buildings where the installation of a skylight or roof window is not practical.
While a roof window is normally included in the original construction of the building, it is possible to add the design feature to an existing structure. As long as the framework and the slope of the roof allow for the inclusion of this type of window, it can be installed with relative ease. Many manufacturers offer prefabricated window inserts of this type that can be installed by a professional in a matter of hours. 
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