Explanatory Note The Rafters (Timber sections) shown in this slide show will not appear full size in relation to the dimensions given, however The overall principle and methods used are precise and have been purposely made larger in scale to aid explanation.
Now lets look at some simple Math's Lets use 28° as an example for the pitch of our roof. Find the tan button on your calculator. Now press tan, 28, = and your answer should be.532mm You have just given yourself the rise per metre run for the common rafter. Lets relate this to a square on the next slide.
Rise per metre run of rafter This means for every 1.0m you go along the bottom or span of the roof, you rise up 532mm. (for our example of 28º) If you simulate a steel square as the triangle of a roof, you can divide these measurements by two to use on here. See opposite 1.0m = 500mm (goes on the 2 blade of the square) 532 = 266mm (goes on the 2 tongue of the square) 500mm 266mm
You have also now created the two bevels you need Plumb bevel common rafter. Level bevel common rafter. We can use the square set up like this to set out our pattern rafter. 500mm 266mm
True length per/m and Centre Line Length True length per / m Use your rise per /m run measurements for this and square root them..532² + 1.0² = 1.283 = 1.133mm Centre line length Use the true length per/m and half span for this. T/L per/m x ½ span of roof As an example we will use 3.6m for the ½ span of our roof. 1.133 x 3.6 = 4.079m This is the measurement from the centre line of the ridge, to the back of the birdsmouth, measured along the top of the rafter.
Start with a simple Plumb Bevel This is the centre line for the top of the rafter (centre of ridge position) Use your steel square for this bevel, or other means if you prefer.
Measure the centre line length of rafter and place another plumb bevel This line is the back of the birdsmouth position
Alternatively, step out your length with a steel square This line is still the same position (back of birdsmouth)
Now we mark the level cut on the birdsmouth (Max. 1/3 Rafter depth) This is easiest with a builders square, or you can simply mark square off the plumb cut. Maximum 1/3
Two more things left, 1. Reduction at the ridge, 2. Eaves overhang This pattern is now ready to cut, so other rafters can be marked off it.The lines shown blue are where you cut the rafter. Measure half ridge thickness square off plumb cut and place another plumb cut Now measure eaves overhang square off bottom plumb cut
Your pattern rafter and the Gable Roof. Now look at the roof and your pattern above, any questions?