Video Transcript

What This Detail Is – And What You’ll Learn Today

This is a cold roof eaves detail, meaning the insulation sits at ceiling level between and over the joists, leaving the roof space above ventilated.

By the end of this session, you’ll understand:

• What a breathable sarking membrane is and where it goes
• Why ventilation is needed at the eaves, and how it’s provided
• What role the insulated cavity barrier plays
• How the wall plate is fixed and restrained
• What all those internal finishes and layers are doing

Glossary of Terms

• Breathable sarking membrane — A water-resistant but vapour-permeable sheet on the cold side of the insulation. It keeps rain out but allows internal vapour to escape.
• Cross-flow ventilator / soffit vent — A manufactured component providing a continuous airflow path at the eaves to maintain a clear 25 mm ventilation gap.
• Insulated cavity barrier — Fire- and thermal-stop at the wall head. May not be required with full-fill mineral wool or other A1 fire-rated insulation.
• Wall plate — A horizontal timber fixed to the top of the wall, distributing loads from the roof structure. It’s held in place with straps or fixings.
• Holding down strap — A metal strap securing the wall plate to the masonry, preventing uplift in high winds.
• Parge coat — A thin coat to the inner masonry leaf improving airtightness.
• Vapour control layer (VCL) — A layer on the warm side of insulation, reducing vapour movement into the structure.This is sometimes integrated into some rigid insulation boards.
• Insulated plasterboard — Plasterboard bonded to insulation, improving the wall’s thermal performance. 

Walk Through the Detail

Starting outside, the roof tiles are fixed to horizontal battens on counter-battens, over a breathable sarking membrane. This membrane is key: it allows vapour inside the roof build-up to escape, but blocks water getting in from the outside. There are different types of roof membranes, for example in some refurbishment projects a non breathable membrane may be used (but more on that in another video).

Below the tiles, a cross-flow ventilator ensures a minimum 25 mm ventilation gap at the eaves. This keeps the roof void ventilated, reducing condensation risk. Air enters through the soffit vent and passes into the roof space.

At the edge of the wall, an insulated cavity barrier closes the cavity. Depending on the insulation type, this may be mineral wool or another A1-rated insulation. 

The wall plate sits on the blockwork inner leaf, bedded level and fixed with holding down straps for stability. The rafter bears onto the wall plate, transferring roof loads to the masonry wall below.

Inside, the build-up continues with 100 mm insulation between ceiling joists and 150 mm laid over joists, achieving the roof’s thermal performance in this case a u-value of 0.11 w/m2k. A vapour control layer lies below the insulation to prevent moisture from entering the cold roof space from the warm rooms below which could condense and cause moisture issues in the roof.

For finishes, we see:

• 12.5 mm plasterboard ceiling lining to the roof
• 6 mm parge coat to the inner leaf of the wall
• 52.5 mm insulated plasterboard fixed internally to the wall
• 3 mm skim coat to complete the finish

These layers improve airtightness, thermal performance, and provide the internal finish.

Student Questions

1. What is a breathable sarking membrane?
   It’s a sheet that keeps out wind and rain but allows vapour from inside the roof build-up to escape. It reduces the risk of condensation while still protecting against moisture ingress.
2. Can’t quite picture this / how do the materials connect?
   Think of the eaves as a meeting point of roof and wall. The membrane runs down over the rafters, the ventilator bridges the gap between soffit and roof void, and the cavity barrier closes off the insulation at the wall head. Each layer connects to maintain continuity of moisture, thermal and fire lines.
3. What is the insulated cavity barrier — is it different insulation, does it act like a stopper?
   Yes, it’s often denser, non-combustible insulation that closes the cavity at junctions. It prevents fire spread, blocks air leakage, and keeps the insulation line continuous. If you are using a non combustible A1 insulation fully filling the cavity a cavity barrier may not be required. 
4. What is the wall plate fixing with strap, can’t quite picture how this comes together?
   The wall plate is the timber that the rafters sit on. It’s secured to the masonry with metal holding down straps, resisting uplift forces from wind.
5. What are these extra layers — parge coat, VCL, insulated plasterboard and skim?
   These work together as the airtightness and thermal layers on the inside. The parge coat reduces air leakage through the blockwork wall, the VCL slows vapour movement, the insulated plasterboard improves thermal performance, and each board is fixed to the wall with dabs of adhesive, with the joints then closed off with a specific tape. The 3mm skim is a thin finishing coat of plaster, creating a uniform finish to the wall. 

Key Points for This Detail

• Ventilation – Cold roofs must be ventilated to reduce condensation. Provide a continuous 25 mm ventilation gap at the eaves and in some cases ridge level dependent on roof size, membranes and other requirements. Ventilation requirements can be tricky, and will depend on the type of membrane specified – breathable or non breathable. 
• Moisture management – Ensure the sarking membrane is lapped correctly and the VCL is continuous on the warm side.
• Thermal continuity – Ensure the insulation is continuous from the wall cavity to the roof to avoid any areas of cold bridging. 
• Fire safety – Use appropriate cavity closers or barriers to meet fire performance requirements according to Building Regulations Approved Document Part B.
• Structural stability – Wall plates must be fixed level and restrained with straps to resist uplift.
• Internal performance – Airtightness relies on the parge coat and VCL, while insulated plasterboard boosts thermal performance.

Project specifics matter. Substrate, moisture, exposure, finishes, acoustic and fire requirements, warranties/insurer requirements and manufacturer guidance can all shift the final arrangement. 

Don’t forget, there is always more than one way to design a detail.