I am fully renovating an attic flat that was possibly installed in 1985. That's when the house was converted, but this flat may have been done a little later on.

Anyway i have taken out all the plasterboard ceiling and removed the 50mm PIR insulation between the rafters. The rafter depth is 85-90mm, so there was a decent breathing gap behind, but I think they may have pushed come insulation a bit too far into the slate area. There is no felt/membrane under the slates.

I have damp rafters in most of the roof, some bits worse than others. Even some of the roof lats are damp in places.

There was no vapour barrier and the insulation was not foil taped.

My plan is to extend the rafter with a 50x25mm roof baton (done today) and then fit 75mm between the rafters and underboard with 50mm. Then i will foil tape the 50mm sheets up.

There is a section of the flat where I cannot baton out so much, but rather can only add 10mm. In these areas i will have to use 70mm insulation between and underboard by 50mm. This mean in these two areas (bathroom and kitchen) the breathing gap will be 25-30mm. The rest of the flat will have 40-45mm.

Will i have the same issue as before ? Should i be looking at introducing some breathing slates in the roof to get more air flow in there ?

This photo shows the water staining on the rafter.

Isnt it going to always be a problem without any roof membrane?...especially if its in a windy area.

My extension/ build has a slate roof, roofing membrane battened and slated. There are no slate vents we are in quite a windy area and the gaps between the slates allow sufficient ventilation BCO was happy and architect did not specify.

I have insulated between rafters with 170 mm rock wool (have 225mm trusses )then 100mm PIR boarded over and taped. It is difficult to get good sealing between rafter and insulation so if you can overboard with rigid insulation with rockwool in between you will be able to get a well sealed ceiling. Duplex plasterboard to finish off.
I am surprised that you have condensation as slates have lots off gaps between them allowing ventilation but having a kitchen and bathroom up there will add considerably to the humidity level I guess. Have you enough headroom to overboard? Don't know the feasibility of adding roofing felt between the rafters, could be fixed to the battens and down the edges of the rafters. Needs someone more knowledgeable than me on that one.

Better off with no sarking breather, make sure that the vapour control layer whatever and wherever it is is all joined up to the one in walls

I would add soffit vents and ridge vents both sides of the ridge board, ideally a bit of insulated flat ceiling that allows cross and through ventilation.

You can’t stop loft condensation by managing the humidity in the house