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Collar tie roof is similar to the couple close roof. A collar tie is a horizontal member between two rafters and is very common in domestic roof construction. Often collar ties are structural members but they may be used simply to frame a ceiling.
In collar tie roof, the horizontal tie is raised up from the feet of the rafters to the almost middle of the rafters. This raised up horizontal tie is known as the collar tie. Collar ties should not be fixed more than one-third or one- half of the rise of the roof up from the wall plate.
A collar tie is used to prevent spreading the foot of the rafter. When the span increases, or when the load is more, the rafters of the couple close roof have the tendency to bend. So in this condition, the raising-up of collar ties checks the tendency of sagging due to further increase in span or excessive loading conditions. The design of collar tie roof provides greater ceiling height if it is required.
This collar tie roof is suitable for the span varying from the 4 to 5.5.This roof is used for a maximum span of 4.8 m.
The purpose of the collar tie roof is to extend first floor rooms into the roof space and so limit the largely unused roof space. In this collar tie Roof, Storage space provided is less due to collar tie and limited head room for access to water storage cisterns.
The disadvantage of collar tie roof is that the head of the windows formed in a wall will be some distance below the ceiling and will give less penetration of light. So in this collar tie roof, dormer window is provided which is partly built into the wall and party in the roof.