Hi - I have a large roof area which is welsh slate straight to sarkin boards, which when slates go missing causes water penetration to my lounge area - received quotes to de-slate/fit membrane/re-slate which is ~£26,000.
As a cheaper alternative I was thinking of doing following since I have easy access to internal roof space :-
1) Fit insulation blocks between rafters eg Reticel Eurothane GP insulation board
2) Seal area between rafters with a non breathable waterproof membrane
3) Fit additional fibre insulation
4) Fit plasterboard
Is this a workable solution ? What are potential issues ?