When constructing a hip roof, one of your tasks is to cut the rafters that act as supports to the roof cover. As a roof specialist knows, there are three kinds of cuts that can be done to the roof timber. You have a “seat cut”, that is at a horizontally-level angle, a “plumb cut”, that is at a vertically-plumb angle and a “bird’s mouth cut” which is a combination of both cuts. It is good practice to cut two rafters and fit them. If the two rafters can fit well, then you must use of the rafters as a reference to cut other rafters.
Step 1 – Check Timber Camber
Check the timber length and observe its inclines and bows. The bows and inclines of the timber will determine the overall difference in the length of the timber. To offset the difference, you should straighten the timber bows by placing weights on the upsides of the timber bows.
Step 2 – Cut the Plumb
Make the plumb-cut, beginning from the left side. As an illustration, for a 5 to 10 pitch on the lower edge of the timber rafter, you should measure 5 inches on the external part of the tongue. Next, measure 10 inches on the internal part of the body. Shift the upper part of the tongue on the timber rafter to the top left-hand corner. On the frame of the square tongue, mark a line on the external edge. Make a cut on the line.
Step 3 – Cut the Seat
Make a seat-cut on the right hand side of the timber board by following the steps outlined above. Cut the framed square on the long side.
Step 4 – Make a Bird Mouth Joint
Take a hand saw and cut a triangular section on the timber rafter. This cut, known as a ‘bird’s cut’, should be made on the 1/3 position of the rafter’s breadth. This cut allows the roof rafter to sit on the wall plate.
Step 5 – Cut Rafter Tail
Cut the rafter tail. Make a plumb cut as well as a seat cut on the lower part of the timber rafter. If this is difficult, attempt to make the cut by placing the rafters together and checking how they fit.
Step 6 – Test the Two Rafters
Test the hip roof rafters. Nail a 4 x 2 wood block to the upper cut of a single rafter. This serves as a temporary ridge board. Tightly put the back side of the bird mouth on the external wall and allow the top parts to join. When they appear open at the top, adjustments should be made to shorten their lengths.
Step 7 – Cut Other Roof Rafters
Take one rafter and use it as a calibration for measuring other rafters.The same rafter should be used repeatedly for precise measurements.The reference rafter should have its top part set level with rafters below for correctly marking other rafters against the reference.