Ok... this may seem harsh but I would never cut a roof in the UK using untreated timbers.
Reason? very simple, the untreated rafters and roof members will have a short lifespan in comparison to treated timbers.
Bare minimum would be to install the wallplate using treated timber. I also sit my wallplates on DPC and then bed them in mortar. Years ago natural slate was used with the old lime mortar.
Wallplates if not sat on DPC will rot and decay first.
If you were to ask an Architect they would say use treated but Building control would give you the option not too (Then again they pass modern trusses!).
I have lots of experience with cut roofing and I do not claim to be an expert! but...I would strongly suggest treated timbers for cut roofing.
Some would argue about breathable felt being used would eradicate the problem of condensation etc but it does'nt solve it entirely. You could use untreated and spray them after with something like clear protim or just use tanalised form the start. Obviously C16 and C24 timbers depending on spans etc.
I suppose everyone has there own opinion on it, but in this part of the world we have a very damp climate and it takes it toll on timber whether its exposed to the elements or not.
So if you want your cut roof to be around for the lifespan of the house its sat on TREAT IT!
