Rafter spacing with internal dividing walls?

[Digmixfill]

A rough layout of part of our roof is in the attached diagram. A gable at each end with internal dividing walls, with a separate ridge beam for each room.  I'm aiming to place insulation between the rafters across the top of the dividing walls. There will be a service gap and counter battening with more insulation below, so rafter spacing doesn't have an effect on plasterboarding etc.

 

I know that the end rafters need to be spaced from the gables with blocking and restraint straps, but i'm unsure what to do spacing wise around the dividing walls?

 

Also, when starting fixing rafters is the best plan to place one at each gable and lay a string between those to get the best line? My plan is to start with an assumed 50mm birds mouth seat with the assumption that I can adjust in-out from 35mm minimum to 1/3 rafter depth maximum.

[Susie]

As someone currently trimming Rockwool about an inch to fit between the rafters I would consider what insulation is going in, on or above etc, especially if you don’t need rafter spacing for the plasterboard.  Do you not have drawings of roof spec for building control?

[Digmixfill]

I'm still undecided about the insulation between the rafters. It will be either PIR or mineral wool. I don't mind having to trim odd shapes and sizes. It will be a pain, but in the scheme of things a small job. Roof detail in the plans states "existing", and existing is rather rotted and worm ridden. Took the opportunity to change the roof to ridge beam and vault the lot.  Rafter tables suggest we can use any spacing up to 600mm.

[Susie]

It sound like you have plans submitted to BC and have now decided to change the roof. If that’s correct if you are changing more than 50% of the roof you do need BC approval of the work. It sounds like you need to check with your local BC before you go to far. 

[Digmixfill]

Everything has to comply with building control, so I don't think changing a purlin to a beam will make any difference to them as long as everything is done correctly. The external look of the roof will not change, so planners won't give hoot about it.

[Susie]

BC need to know the size of the new rafters and roof design and that it will all hold up the weight of the roof covering, and amount of insulation, warm or cold roof ventilation, getting the roof signed off as completed is one of their checks. 

[Digmixfill]

Building control need the structural engineer's paperwork and they already provide approved rafter span tables. If I where creating some exotic roof style, then it would be prudent to involve them at an early stage, but this is a simple change that results in better roof insulation. Our BC inspectors have so far been really helpful and very accommodating, especially when making improvements.