Cutting Jack Rafters

JACK RAFTERS: One which does not extend from plate to ridge. Hip Jack, one running from plate to hip at 90º to plate. Valley Jack, one running from ridge to valley at 90º to ridge. Cripple Jack, one which neither touches the ridge nor plate, but runs from a hip rafter to a valley rafter at 90º to the ridge.

The rise and run of a jack rafter are the same as that of a common rafter. When marking jacks use the common rafter scale and same number (inch rise). Where rafter rests against hip or valley, mark plumb cut, then cut at 45º angle along this mark. This will give both plumb cut and side cut. When resting on ridge or plate, lay out the same as for the common rafters. For cripple jacks, mark plumb cuts on both ends and saw at 45º as above.

When measuring the length of the jack rafter, measure from longest corner (plumb cut on 45º) to other plumb cut mark, along Top Side (same for hip rafter). Cripples are measured from long point to long point diagonally along top edge. Measuring to the long point will compensate for 1/2 of the ridge thickness (or for jacks, 1/2 of valley or hip thickness). There is no problem in laying out these angles on the rafters as long as you keep in mind which side of the hip (or whatever) you want the rafter to fit against. Usually a carpenter will space the ceiling joist from an outside wall and working to a 48" center. This gives proper spacing for dry wall or panelling or whatever is used. Proper spacing of ceiling joist will aid in roof

Construction. Measure shortest jack first (usually running next to a ceiling joist), from plate to hip rafter. The difference in length of the rest of the jacks is taken from chart. Set each rafter along side ceiling joist and spike well. The ceiling joist then ties the roof together.

Figure the rafter material lengths so you can cut a long and short jack rafter from each piece. When you have cut your shortest jack, the angle of the long end will then fit on the other side of the hip. Do this all the way up the hip, always leaving the cut off end for the other side. If lumber has crown in it, put crown up on longest cut off piece.

In some cases a carpenter will build the valley on top of the main roof, not using a valley rafter. This of course would be the easiest way on any remodeling job, room addition, etc. It saves cutting into and weakening the main roof. Mark location of valley on roof boards, 45º to common raftersormer,). Set long point of bottom end of rafter even with this line ("G" of). The top cut of the rafter is the same as top cut of common. Bottom end is a horizontal cut that extends all the way across rafter. Then tilt the base of your saw to the same angle as the roof on which the bottom end of the rafter will rest. I.E., if rafter end is to fit on a roof with 6" rise, you would tilt the base of saw to an angle of 26-1/2º (6" rise) and cut along horizontal line. With saw set at this angle you willhat it fits over the pointed end of top of common rafter, because this would also be a 26-1/2º (6" rise) angle. Save the cut off ends for the other side.