Pattern Rafters & Calculations

Pattern Rafters & Calculations Gable & Hipped Roof M.S.Martin Nov. 2005, Revised Feb. 2006 Reviewed March 2010

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.

First some simple Terms SPAN ½ SPAN

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) 266mm 500mm

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

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. True length per/m and Centre Line Length

The Gable Roof

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)

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 L L This line is still the same position (back of birdsmouth)

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.

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.

Your pattern rafter and the Gable Roof. Now look at the roof and your pattern above, any questions?

The Hipped Roof

Lets first look at the pattern we know, then add to it.

For the hip lets look at it in solid form, for there is marks we need to place on the edge as well C Cut for eaves overhang Cut for top end of rafter Centre line Birdsmouth

With a hipped roof, we need to add creeper cuts to this pattern L Now this becomes the centre line of first creeper C We can do this with your square or by measurement The two distances you need are either (1) rafter spacings at base of square, or (2) true length at the top.

Measurements and calculations • Before we go any further, lets note down some more figures. • (1) Rafter spacings, for our exercise, lets say they are at 450mm centres. This means your centre to centre measurement on base of square would be 450mm. • (2) Now for the true measurement at top, if you choose to do it this way, we need to do a simple calculation. Remember our ‘True length per / m’? • True length per / m x rafter centres gives you measurement (2) above. • 1.133 x .450 = .509mm • Lets look at the rafter again

Steel Square centre to centre measurement L C 450mm measured at base of square

True length centre to centre measurement C True length of 509mm is measured at top of rafter.

1st Creeper adjustment Centre line of first creeper C From this centre line point we need to mark the ‘long point for the first creeper' and we can do this with two simple measurements. 1) half mitre thickness of the hip 2) Half the thickness of a rafter. First lets look at what this means

Adjustment measurements Half the mitre thickness of a hip Half the thickness of a rafter

Adjustments on the rafter Forward half thickness of rafter Centre line of first creeper C Gives you long point of first creeper Back half mitre thickness of hip

The Geometry as to Why Now lets look at the rafter again

Pattern Rafter with first Creeper Centre line of first creeper C Long point of first creeper The last step is to mark remaining creepers, which you do with centre to centre measurement, either by square or by measurement of true length.

Marking remaining creepersNote: this will be there long point L Long point of next creeper, and continue down the length of the rafter for longer lengths C Start from long point of first creeper Simply mark remaining long points from here, the simplest way is with a steel square

One more item – the Crown end cut Crown end cut C Simply measure ½ the thickness of a rafter from original centre mark, this is your crown end cut.

Your finished Pattern for a hipped roof Second creeper long point Crown end cut C Common Rafter cut Birdsmouth First creeper long point Centre line of ridge Remember: longer rafter, more creepers marked Plumb cut for eave overhang

Have another look at the Hipped Roof

Pattern Rafters & Calculations