Plasterboarding over new insulation with sloping roof

[HandyAndy]

Morning. I've been struggling with this for a while and not found the solution here, so posting my query.

 

I've been insulating the previously uninsulated sloping roofs in my 1960s house, taking existing plasterboard off and adding 75mm PIR insulation between the rafters. It's already had a satisfying 3-5'C improvement in the surface temperature of the roof! I'm thinking of adding 25mm insulation underneath the rafters, followed by a VCL (+ tape) and then plasterboard finish. I'll have it all professionally skimmed once done.

 

I'm not clear how best to finish off the join with the existing plasterboard. I've knocked up a possible cross-section in the attached photo and would welcome feedback. While I'd taper the insulation, I was planning to leave the plasterboard butt-jointed (screwed into the existing horizontal plasterboard), leaving the finer details of taping, filling and finishing to the plasterer. Does this sound like a reasonable approach - or would I just be better leaving the boarding to a plasterer who can then do the whole job from start to finish?!

[Carrerahill]

I had this detail in my extension on a sloped section and I just scribed the PB with my knife at an angle several times in quite deep and it came off with a roughly mitred edge. 

 

The issue is the edge may be prone to cracking, there are options here.

 

You could put a bit of dot & dab adhesive along the edge to get a good ridged joint.

 

 

[Redbeard]

Just taper the joint as @carrerahill says and the plasterer or joint-filler can tape and fill/plaster over it. I did dozens of jobs like this and it was never a problem. If you thought it might be then perhaps do the mitre, paint the 'raw' end with PVA and bed the joint between that and the flat ceiling in something like Sticks Like Sh*t.

[nod]

Do as you are doing and buy some skim stop beads Your plaster will fix them to the slope and skim a straight line down to them Ceiling first Then the slopes on a separate gauge 

[HandyAndy]

All - you're heroic - MANY thanks for the fast input ... I'd got myself into a tailspin about what to do - great to have your input to move me forward!