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.

A jack rafter is a common rafter that runs out of roof before it reaches the ridge. It has the same pitch, the same birdsmouth and the same seat cut as the commons beside it, and it dies into the hip.

The common difference

Jacks sit at the same centres as the commons, so each one is a fixed step longer than the one before it. That step is the common difference:

ΔTL = c ÷ cos θ = 400 ÷ cos 35° = 488 mm

At 400 centres on a 35 degree roof, every jack is 488 longer than its neighbour towards the corner. The ladder on the worked example runs 448, 936, 1424, 1913, 2401. Goss prints the same figure as CENTRES DECREASE.

It depends on the centres and the pitch, and nothing else. Change the span or the plate length and it stays 488.

Cut to the face, not the centre

A jack does not reach the hip's centreline. It stops at the hip's face, and the difference is worth having.

bₕ ÷ √2 = 47 ÷ √2 = 33 mm on plan

Taken up the slope, that is 41mm off every jack. CutRoof always applies it and shows it as its own step in the working. This is not a convention you can set: a length to the hip centreline is a length no one can cut to.

The edge bevel

The jack's end is cut across the face as well as plumb. It is the one cut a common rafter never has, and the one most often taken off the wrong reference.

Edge bevel to the hip: set bevel to 39.3° against the rafter edge

Stock along the rafter's top edge, 39.3 degrees. That is what Goss prints in his JACK RAFTER - EDGE column.

tan β = cos θ = cos 35° ⇒ β = 39.3°

Measure it from a line squared across the timber instead and you get 50.7, because the two references sit 90 degrees apart. Both are right; only one matches the book in your van. Convention C6 switches CutRoof to the other, and the schedule says which is in force.

The plumb and seat cuts are the common's, unchanged at 55 and 35 against the edge.

Handing

Jacks come in pairs. For every jack running up the left side of a hip there is a mirror image running up the right, with its edge bevel cut the other way.

Cut eight identical jacks and half of them are scrap, which is why the schedule lists left and right separately. On the worked example there are eight at each step: four left-hand, four right-hand, one pair at each corner.

Mark the hand on each stick as it comes off the saw. A pile of jacks all cut to 936 looks identical until you take one up the roof.

Where the first one goes

CutRoof's default puts the first jack one full centre from the corner. That is convention C8.

The other option centres the whole layout on the plate run. It changes every jack length in the set and never changes the common difference, so the step stays 488 either way. The schedule states which origin it used.

Watch one thing on that setting. If it lands a position hard against the corner, the hip-face deduction can eat most of the rafter and leave a stick shorter than the birdsmouth it carries. CutRoof says so rather than putting it on the list.

Every figure above is worked out by the same engine that produces CutRoof's cut schedules, and is checked against it on every build.