Warm Flat Roof Retrofit

You’ve got the right instincts here—trying to drop and wrap a VCL into individual masonry joist pockets is a complete nightmare in practice and is practically guaranteed to fail or tear during installation. Here is how you cleanly resolve this detail to keep the condensation line exactly where you want it: 1. The VCL stays flat on top: Keep your main vapour control layer flat across the top of your new timber deck. It does not drop into the joist pockets. Instead, you stop internal moisture from ever reaching those pockets from inside the room. Before you plasterboard the ceiling, apply a robust parge coat (a sand-and-cement or specialist airtightness paint/render) to the raw solid brickwork, sealing tightly all the way around the four sides of every joist penetration. Use a foil-backed plasterboard for the ceiling and tape the perimeter to form your internal vapour barrier. 2. No air gap at the joist ends: Do not leave an air gap before the fascias. In a warm roof system, the entire timber structure—including the joist ends—must sit on the warm side of the insulation. If cold outside air circulates around the joist ends under the fascia, those timber ends will hit dew point and rot out. 3. Enveloping the new extended overhang: Pack the gaps between the joists tightly over the wall plate with rigid PIR or dense mineral wool to stop lateral heat loss through the masonry. Then, run your main roof deck PIR insulation right out to the very edge of your new extended fascia line. 4. Future-proofing for the EWI: To stop cold air from getting up under your new extended overhang, line the underside of the new soffit box with a layer of rigid insulation (like 50mm PIR). When you eventually install your External Wall Insulation (EWI) in the future, the wall insulation will run straight up the solid brickwork and butt up perfectly tight against this insulated soffit. This keeps your entire timber envelope entirely within the 'warm zone,' avoids complex VCL origami, and creates a continuous thermal barrier ready for your future EWI."

Alawys leave airgaps if its possible.