Setting out, explained

Eleven short pieces on how each figure on a cut schedule is arrived at, worked through one real roof throughout.

• How a common rafter is set out
  Run, rise and true length, and where each one comes from on a 4800 span at 35 degrees.
• The ridge deduction, and why your length already includes it
  Half the ridge thickness comes off the run before the length is worked out, and the working shows it as its own step.
• Birdsmouths: the third-depth rule, seat and plumb cuts
  How deep the notch goes, why two rules disagree by three per cent, and the three different lengths that one notch produces.
• HAP: the number that keeps your roof planes true
  What the height above plate is, where it comes from, and why it is the figure to check before the rest of the roof goes on.
• Hips: why the pitch is shallower and the length is longer
  A hip climbs the same rise over a longer run, which makes it shallower than the roof it sits in and longer than any common.
• Backed or dropped? The hip decision explained
  Two ways to get a hip's top edges into the roof planes, what each costs in work, and which one CutRoof uses by default.
• Jack rafters: the common difference, the hip-face deduction and handing
  Jacks step by a fixed amount, come cut to the hip face rather than its centre, and go on in handed pairs.
• Reading plumb and seat cuts off a square
  Why an angle on a roof has two right answers, which reference CutRoof states them from, and how to tell the two apart before you set the saw.
• Pitch three ways: degrees, rise:run, total rise
  Three ways of stating the same angle, how CutRoof converts between them, and the one measurement people get the wrong way round.
• What CutRoof calculates and what stays your judgement
  The geometry the app is responsible for, the decisions it hands back to you, and the roofs it will not attempt.
• How the hips meet the ridge, and why the hip is shortened
  The crown rafter arrangement CutRoof uses at each end of the ridge, and the two numbers it changes.