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.

Two rafters meet at the ridge with a board between them. That board takes up space, and the space has to come out of the rafters.

On a 4800 span with a 32 ridge, the sum runs like this. Half the span is 2400. Half the ridge is 16. The run is 2384.

Then the length is worked out from the deducted run:

TL = 2384 ÷ cos 35° = 2910 mm

Why 16 becomes 20

The deduction is 16 on plan. The rafter climbs, so along its top edge that same 16 is worth more.

Set the same roof with no ridge board and the run stays at 2400, which gives a true length of 2930. The difference is 20mm, not 16. That is half the ridge thickness divided by the cosine of the pitch. The steeper the roof, the more a given ridge board costs you in rafter length.

Where it shows up

CutRoof takes the deduction before it works out any length, so every common rafter figure on the schedule already has it in. Cut to 2910 and the rafter fits a 32 ridge. There is nothing further to take off at the bench.

The working still shows all three steps separately: the half span, the deduction, then the cut length. A length that quietly contains a deduction is impossible to check against a square, so the app never gives you one.

When there is no ridge board

Vaulted work and some cut jobs have no ridge board at all. The rafters meet each other, or they meet a beam that sits below the apex.

That is convention C1 in the app. Set the run to the plain half span and the deduction disappears: run 2400, length 2930 on the same roof. The app forces the ridge thickness to zero when you pick it, so you cannot leave a 32 in the field and wonder where the 20mm went.

The crown rafter is different

On a hipped roof, the two end commons are a special case. They butt the end face of the ridge board rather than its side, so nothing is in their way.

On the same roof those crown rafters run 2400 and measure 2930, while every other common runs 2384 and measures 2910. Twenty millimetres of difference between two rafters on the same roof looks like an error until you know why it is there, so the schedule lists them as their own member with their own working.

Checking it on site

Hold a rafter against the ridge before you cut the rest of the set. If the plumb cut sits flush on the ridge face and the birdsmouth drops onto the plate without daylight under the seat, the deduction is right. If the birdsmouth lands short of the plate face by roughly half a ridge, the run was set out on the full half span.

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