Fig. 64. Backing the Hip Rafter
25. Reduction of Hip or Valley Rafter Length Because of Ridge Piece.—If a hip rafter of a square cornered building is to be framed against a ridge piece, [Fig. 40], its length must be reduced correspondingly. To make such allowance, measure square back from the line of plumb cut a distance equal to ½ the diagonal thickness of the ridge, [Fig. 61-A-B].
26. 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 % 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.]
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| Fig. 65-a. | Fig. 65-b. |
| 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.
27. Valley Rafters.—As has been indicated in previous sections of the text, valley rafters have their lengths, plumb cuts, and seat cuts determined like hip rafters.
There is one difference; the valley rafter at its seat must be framed as in [Fig. 65] in order that the plumb line may come directly over the corner of the building. The ends of roof boards will rest upon the valley rafter at its center line, which line is in the same plane as that of the common rafters.
Like the hip rafter, the upper end may be laid out first, after which the rafter length is measured from this, the measurement being made along the middle of the back of the rafter, the top edge.
To lay out the cuts shown in [Fig. 65-a], proceed as in laying out the end of a hip rafter, as described in [Sec. 24], [Fig. 63.] In the case of an octagon the amount would be 5/12 of that used for the square, Fig. 65-b.