You are correct in saying that there is no existing/demo for roof framing. This does not mean that it is not possible.
Currently the take-off will not read anything that is locked. So when you tick the 'Existing' box in the wall tool for example, all that happens is that PlusSpec automatically locks the wall and the wall can now not be edited and it will not be picked up on the take-off. So when you have roof framing that is existing, all you have to do is right-click on the roof framing and select 'Lock'. This is the same as ticking an existing tick box.
We will implement an existing tick box for roof framing in the future.