Backing A Hip Rafter For Square Cornered Building. First Method: Since the line of measurement of a hip rafter is along the center of the top edge, if the rafter is framed with the same plumb distance as was given the common rafters, 1-3, Fig. 52, it stands to reason that the roof boards will not fit the top edge of the hip properly until the arrises of the hip have been removed as in the cross-section of Fig. 64. The laying out and removal of these arrises is known as backing the hip.
The amount of backing for a hip rafter will depend upon the rafter thickness, the pitch of the roof, and the number of sides to the plate, and is indicated by gage lines on either side and one on the top edge of the rafter. To determine the location of these gage lines on the sides of the rafter, (1) place the square on the hip as in laying out the seat cut for the hip on which the backing is to be placed, the constant, 17", on the tongue and the rise on the blade, if the house is rectangular, Fig. 64. (2) Measure from the edge of the hip back along the tongue a distance equal to ½ the thickness of the rafter, and mark. This point gives the setting for the gage. (3) Gage both sides of the rafter and then remove the arrises as shown in the cross-section. Carpenters more frequently frame a hip without backing, allowing the roof boards to rest upon the arrises of the hip, forming a small triangular space between the roof boards and the top edge of the hip. In order to keep these arrises in the same planes as the tops of the common rafters, they must reduce the plumb height 1-3, Fig. 52, of the hip. The amount of reduction, that is, the amount of drop the hip must make is equal to the plumb height of the backing, Fig. 64.
Fig. 64. Backing the Hip Rafter.
Framing Valley Rafter at Plate
Second Method: Take the rise in inches per foot of run of common rafter on the tongue, and the length of hip or valley per foot of run of common rafter on the blade; scribe along the tongue to get the angle of backing.