Pattern Rafters

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Pattern Rafters Calculations

• Gable Hipped Roof

M.S.Martin Nov. 2005, Revised Feb. 2006
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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.

3
First some simple Terms
SPAN
½ SPAN
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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.

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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)

266mm
500mm
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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.

266mm
500mm
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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.
• v .532² 1.0²
• v 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.

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The Gable Roof
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Start with a simple Plumb Bevel
Use your steel square for this bevel, or other
means if you prefer.
This is the centre line for the top of the rafter
(centre of ridge position)
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Measure the centre line length of rafter and
place another plumb bevel
This line is the back of the birdsmouth position
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Alternatively, step out your length with a steel
square
L
L
This line is still the same position (back of
birdsmouth)
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Now we mark the level cut on the birdsmouth (Max.
1/3 Rafter depth)
Maximum 1/3
This is easiest with a builders square, or you
can simply mark square off the plumb cut.
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Two more things left, 1. Reduction at the ridge,
2. Eaves overhang
Now measure eaves overhang square off bottom
plumb cut
Measure half ridge thickness square off plumb cut
and place another plumb cut
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.
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Your pattern rafter and the Gable Roof.
Now look at the roof and your pattern above, any
questions?