Hip jack rafters are shortened rafters that fall against the side of the hip rafter, seated with a common rafter birdsmouth over the wall plate. The jack rafters continue the on-centre layout of the common rafters, following on from the last common rafter to provide adequate support for the roof covering system. An edge bevel is cut along the full diagonal thickness of the jack rafter, in combination with a common rafter plumb cut, to produce a compound angle that fits perfectly against the side of a properly installed 45-degree hip rafter. The top edge of the jack rafter sits against the top edge of a non backed hip rafter, or the low edge of a backed hip rafter. Assuming that all of the previous components are installed accurately, and that the jack rafters are installed on a consistent layout, each jack rafter is shorter than the last by a specific length in relation to the pitch of the roof. This length is known as the jack rafter diminish. For example, on any 45-degree pitch roof, each jack rafter is 566mm shorter than the last jack rafter, provided that they remain on a 400mm centre layout. The specific jack rafter diminish for any pitch roof and any layout can be calculated by a roofing calculator, as well as being listed in most roofing books, as well as the roofing tables on a framing square. The ready reckoner itself features the diminish of all pitches for most common centres. The diminish can also be easily worked out on site through the use of a framing square off of the last common rafter.  

If we pull a 400mm parallel mark off of the last common rafter onto the edge of the hip, we can measure the length of the jack rafter from the point of intersection to the back of the birdsmouth. A common framing square is 450mm long on the tongue. This enables the framing square to be hooked over the last common rafter, providing a perfectly perpendicular reference at 400mm away from the last common rafter. This ensures that layout marks follow the correct layout of the roof. Marking this point on the rafter as well will allow us to find the diminish. The distance from the top of the common rafter to the line will be the diminish of the jack rafters, and so we can incrementally cut each pair of jack rafters with this set diminish in mind. We can measure the length of the first jack rafter by measuring the distance between the line on the last common rafter and the back of the birdsmouth of the last common rafter. When measuring the actual length of the jack rafter in place from the 400mm centre mark and a backing block, we must make sure to specify our measurements to the operator cutting through the use of some specific language. “Shortest point”, and “longest point” are common phrases that occur in any task where angled cuts are made. When shouting our measurements down to the cutter on the ground, we must always follow up our measurement with a descriptor. In most instances, especially if were laying out the hip by measuring from the far side of the last common rafter, all of our measurements will be to the longest point, specifying that the measurement we shouted out is from the back of the birdsmouth of the rafter to the longest point of the edge cut.

The compound angle cut made at the top of the jack rafter is known as the jack edge cut. As is the case with the hip rafter edge bevel, this jack top edge cut can trip newcomers up easily. On plan, the jack rafters receive into the side of the hip at a 45-degree angle, perpendicular to the wall plates. In reality, the exact angle of cut changes with the pitch of the roof. Again, when performing this cut using a power tool with a flat reference surface, such as mitre saw or circular saw, the compound cut produces the correct bevel angle along the top surface of the jack rafter in combination with the correct angle of plumb cut when the plumb cut is made at 45 degrees. When marking this out by hand, the layout markings are not 45 degrees. The method of marking out the exact angle is the same procedure as that of marking out the hip edge bevel. A plumb line is drawn on the top of the jack rafter. A perpendicular line is made from this plumb cut the thickness of the jack rafter timber. Another plumb line is made at this point, and both lines are taken across the top thickness of the timber. The two lines are connected in relation to the desired angle of the jack rafter to produce the correct angle of the cut.   

This method applies to both hip rafters and jack rafters due to the nature of a right-angled triangle. As the pitch of the roof increases, the plumb line becomes steeper in presentation to the top edge of the hip or jack rafter. When the perpendicular thickness line is made from any plumb line, the resulting length of the top edge of the timber is directly affected by the steepness of the pitch. If we performed this procedure on a perfectly flat timber with 0 degrees of pitch, the perpendicular thickness line would create a perfect square on top of the timber, resulting in a true 45-degree diagonal cut. As the pitch rises, the diagonal thickness of the timber relative to the pitch increases, directly impacting the exact angle presented in relation to the pitch of the roof. This can be simplified further in explanation by imaging this procedure as a right-angled triangle. The thickness line is always perpendicular to the plumb line, with the hypotenuse of the triangle representing the distance between the two layout marks. As the pitch of the roof increases, the angle between the hypotenuse and the adjacent lessens, and the length of the hypotenuse (distance between the layout lines) increases in length. Therefore, regardless of the pitch, this method provides the correct layout angles for cutting a hip or jack edge cut by hand. Note that this does not mean that the angle for both the hip and jack diagonal cuts is the same in one roof. The pitch of a hip rafter is different from the pitch of the common rafters/jack rafters in the same roof assembly, and so the exact angle will be different depending on the pitch of the intended component. That being said, the procedure is the same.  

On a regular 45-degree hip end, the jack rafters land on either side of the hip rafter in pairs, meeting each other in the same location on the hip provided that the corner of the wall plates are truly 90 degrees. When cutting the jack rafter edge cut on a timber, the resulting offcut also features the same angle as the length that we cut. Using this principle to our advantage, we can typically get two jack rafter edge bevels out of one length of timber. Starting with the longest jack rafter, the offcut once the edge bevel is made produces a shorter length of timber with the required edge bevel cut that is most likely suitable in length for use as one of the shorter jack rafters. As we work our way down and decrease the length of the jack rafters by the diminish, the length of the offcut with the correct edge cut increases. These offcuts grow in length by the diminish also, making them suitable in length to be used in ascending order.  

As the pairs of jack rafters are installed, care should be taken to ensure that the hip rafter remains straight. Nailing a jack rafter into the side of a hip rafter once the birdsmouth is nailed to the wall plate can have a tendency to bow the hip rafter outwards, especially if the hip rafter is relatively long. As each pair is installed, care should be taken to straighten the hip rafter by sighting along its length. If the hip rafter is bowing in a particular direction, then the jack rafters can be tapped on their face to push them in or out.  

When fixing the jack rafters, the birdsmouth is nailed through the back corner and into the plate with 4” nails. The top edge cut is nailed to the side of the hip rafter through its face using 3” nails.