Roof lights generally will require much smaller openings within the main roof structure than a dormer described above, therefore a similar method to that illustrated in Fig. 3.18 will be more than adequate. However, as the roof lights may not extend up the roof slope the full distance between two purlins, separate secondary purlins or trimmers may have to be introduced. If this is the case the rafters onto which the trimmers are fixed must be reinforced by attaching an additional rafter to each side of the opening, these additional rafters extending from the lower to the upper purlin (see Fig. 8.26).
The term 'roof window' is a term recently introduced to the building industry to describe what is in effect an opening roof light. The roof light is normally fixed, of course, and provides only light to the building. Roof windows are most commonly of proprietary manufacture with the manufacturers providing detailed guidance on the method of fitting the roof window to both existing and new roof structures. Reference should be made to the manufacturer's instructions if such a roof window is to be fitted.
The advantage of the roof window over the dormer, if additional floor space is not the criterion, is that because the glaze area is angled directly at the sky, significantly more light is admitted to the room. The manufacturers of the proprietary roof windows also claim up to 70% saving in cost over a comparable dormer construction.
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