The valleys are cut with the same cuts as the hip, except the tail. Now install the hips or valleys on the roof. This prevents the wider hip bottom from hitting the soffit material later on. Then finally trim the hip tail to fit the 2圆 trim by squaring back off the tail plumb cut to the same length of cut as on the common rafter tail. Cut this end back from this mark 45 degrees each way. For example, a 2' overhang would be 17.69 x 2 = 35.38" less the 45 degree thickness of the rafter trim (backing for the fascia board, usually the same size as the common rafter). This is found by using the rafter table and multiplying the horizontal overhang by the unit run on the hip, which is 17.69". The length of the tail is dependent on the overhang. Mark the plumb cut here with the 5 and 17 configuration. So move this plumb cut toward the tail by a half thickness of the rafter again. We will cut a straight cut instead of a 90 degree notch. The plumb cut for the bird's mouth must also be moved to compensate for the 90 degree angle of the intersection of the outside walls. Layout the seat cut in the usual way, but drop it the extra 1/4", as in our example. It works out about 1/4" for a 1 1/2" thick rafter. The distance between the two lines is the amount of the drop required for the hip. Scribe along the outside the body of the framing square as before. Now shift the framing square along the board, maintaining the 17" and 5" marks along the edge of the board, until the tongue of the framing square is half the thickness (3/4") away from the first tongue mark. Measure from the tongue and mark the distance of half the thickness of the rafter (3/4" in our example). Mark the inside tongue of the framing square at the edge. Scribe along the outside body of the framing square, marking a short line from the edge of the board. With the framing square, align 17" on the body and the 5" mark on the tongue (for our 5/12 roof pitch example) on the bottom edge of the hip rafter (or any other straight edge you can find-we generally use an edge of the rafter because it's right there). To arrive at this drop height follow this simple procedure: To compensate for this, the hip is dropped so the edges will support the sheathing, rather than the center, about 1/4" for a 1 1/2 inch board. You will notice that if the hip or valley was installed now the outer edges of the board is higher than the center of the board where the sheathing will lay. You should have your length marked for the outside of the wall. Okay, now let's go to the wall end of the hip and cut the seat cut (also called bird's mouth). Set the side cut angle on the bevel of the saw and cut along the plumb cut. This is a compound angle best cut with a circular saw or radial arm. The plumb cut for the hip and valley uses 5" and 17" on the framing square, which is marked on the side of the rafter. This is the angle of the side cut which is marked on the top edge of the rafter. Use this number with 12" on the framing square to find the side cut. For a 5/12 roof pitch the side cut is 11.5". The angle of the side cut of the hip and valley rafters are also given on the rafter table. Shorten the hip by half the 45 degree horizontal thickness of the ridge. Multiply 17.69 times the decimal equivalent (.25 instead of 1/4, for example) of the run to get the distance from the center of the ridge to the outside of the wall line. For a 5/12 roof we look across at the 5 and find 17.69. In the third column of the rafter table (see my article: Rafter Tables on the Framing Square) on the framing square is the length of hip or valley rafter. Always make the hip from a board one size larger (in width) than the common rafters. A normal hip rafter, then, would be made out of a 2x8. For every 12" of run on a common rafter we have a hip or valley run of 17".Ī normal common rafter is usually made out of a 2圆. This is what we use on the framing square as our run for the hip, 17". Since the hip forms the diagonal of a square with a 12" run, the diagonal of this square measures 17". The length of the diagonal is the run of the hip rafter. The hip rafter is a diagonal of the square formed from the two common rafters and the two outside wall lines. When everything fits, nail on the rest of the common rafters and adjoining ceiling joists. Nail the common rafters flush on the ends of the ridge board and at 90 degrees to it in the center of the span. This is found by measuring the length of the building and subtracting the span and adding the thickness of the ridge board. The first thing to do is to layout the common rafters, so you need to know the length of the ridge board. If this is a long roof with hips on the ends, there will be common rafters in the center.
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