Video Transcript

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

In this episode, we’re looking at a timber frame eaves detail for a ventilated cold roof.

This is a junction where the roof build-up, timber frame wall, insulation, and moisture control layers all meet — so it’s a really useful detail to understand properly.

Free Detail Download

Before we get started, you can download the free Detail Clinic Pack for this episode. It includes the clean annotated detail, a glossary of key terms, the WAHS checklist, and key notes to support your own drawings.

You’ll find the link in the video description or over on First In Architecture.

Welcome to the Detail Clinic

Welcome back to the Detail Clinic, brought to you by First In Architecture.

This series breaks down construction details in a clear, practical way — looking at what each layer does, how it connects to the next, and how to draw details that are both buildable and technically sound.

A quick note: these videos focus on one junction at a time, so you can understand how the parts work. In real projects, the final detail will always depend on your regulations, exposure, specification, fire strategy, and manufacturer guidance, — so treat this as a way to build understanding, not a universal solution.

Glossary of Terms

Before we walk through the detail, here are the key terms you’ll see in this junction:

• Cold roof (ventilated roof space)
  A roof where the main insulation sits at ceiling level, leaving the roof void above ventilated.
• Breathable sarking membrane / roof underlay
  A vapour-permeable, water-resistant layer beneath the roof coverings. It helps protect from wind-driven rain while allowing moisture vapour to escape.
• Battens and counter battens
  Timber battens support roof tiles. Counter battens can create drainage and ventilation space around the membrane, depending on the system.
• Cross-flow ventilator / eaves ventilator
  A proprietary vent that maintains airflow at the eaves where the insulation can block the path of ventilation. Typically the ventilator achieves the required minimum ventilation gap.
• Vapour control layer (VCL)
  A layer on the warm side of insulation that slows or stops vapour movement into the colder roof zone, preventing condensation.
• Wall plate and holding-down strap
  The wall plate spreads roof loads onto the vertical structure. Straps restrain it against uplift and provide lateral stability- to the engineer’s design.
• Insulated cavity barrier
  A fire-resistant and thermally resistant barrier closing cavities at junctions/openings to limit fire and smoke spread, while maintaining insulation continuity.

Walk Through the Detail

Starting from the outside, the roof is finished with tiles on battens, and beneath that is the roof underlay / breathable sarking membrane. This layer is part of the weathering strategy — it backs up the tiles and helps manage any wind-driven rain that gets past the roof tiles.

At the eaves, the roof build-up meets the wall and the gutter line. This junction is also where the ventilation path is formed. In a cold roof, we want a continuous ventilated void above the insulation, so air can move through the roof space and reduce condensation risk.

Inside the building envelope, the insulation is located at ceiling level — between the ceiling joists and often with an additional layer over the joists. On the warm side of that insulation is the vapour control layer, which helps limit moisture vapour moving into the cold roof void.

At the wall head, the roof structure bears onto a wall plate, with restraint straps shown to suit the engineer’s requirements. Around the junction, an insulated cavity barrier closes off cavities and maintains both fire performance and thermal continuity.

Internally, the lining is finished with plasterboard and skim, completing the internal airtight and finished layer (with airtightness usually being managed by the VCL and continuity at junctions).

Student Questions

1. Is the ventilator attached to the rafter or the battens?

Typically, the proprietary eaves ventilator is fixed to a stable timber element at the eaves — commonly the rafters, depending on the product system. The key is that it maintains a clear ventilation path into the roof void without being blocked by insulation.

2. What are these elements? (holding down strap + connection plate)

This part of the mark-up is referring to the metal connection plate and bracket or tie shown at the rafter/joist zone.

The connection plate and bracket/tie (or connectors) is a metal fixing used to provide a secure connection between timber members — helping transfer loads and provide stability at the junction. The exact connector type and fixing pattern will depend on the roof and engineer’s design and the timber frame manufacturer’s system.

There are a number of different connection, restraint and strapping options that vary according to the roof design, pitch, wall structure, loadings, exposure and structural engineer design.

3. Is this to slow down the water?

The first job of the tilting fillet is to lift and support the bottom edge of the first row of tiles or slates. Its secondary role is less about “slowing” water and more about guiding it. It supports the underlay at the correct angle so any wind driven water on the membrane drains cleanly into the gutter, rather than getting trapped behind the fascia.

4. How is this insulation attached — won’t it fall if it’s a roll? (slim vertical insulation at the edge)

This question is about the slim vertical strip of insulation tight to the exterior line. That piece is usually friction-fitted or installed as a semi-rigid batten/strip, so it stays tight against the adjacent structure without needing plug fixings. The key is that it’s fitted snugly with no gaps, helping maintain thermal continuity and reduce cold bridging at the edge.

Key Points for This Detail

• Cold roofs rely on ventilation above the insulation — keep the ventilation path clear and continuous through the eaves.
• The VCL should be continuous on the warm side and detailed carefully at junctions.
• Roof underlays and eaves detailing should ensure water drains into the gutter, not into the wall.
• Maintain thermal continuity at the wall head — avoid gaps at the insulation line.
• Fire strategy is important: cavity barriers must be coordinated with the rest of the wall and roof build-up.
• Always follow manufacturer guidance for ventilators, membranes, and roof coverings, and coordinate with the structural engineer for wall plate restraint and fixings.

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

There is always more than one way to design a detail.