Video Transcript

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

In this episode, we’re looking at a timber frame window head detail with brick cladding.

This is a junction where structure, moisture control, fire safety, insulation and airtightness all overlap — and it’s very easy to misunderstand what runs where, particularly when it comes to lintels, cavity trays, weep holes and cavity barriers. There’s quite a lot going on.

The markup highlights some common areas of confusion, so we’ll work through the detail carefully and explain what each layer is doing and why it’s there.

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.

Welcome to the Detail Clinic

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

In this series, we break down construction details step by step — explaining how layers connect, what their roles are, and how to draw details that make sense technically and on site.

Before we get into the detail, it’s worth saying that what you’re looking at here is one way of resolving this junction. We’re deliberately focusing on a single detail and explaining the role of each element in isolation.

In practice, details always sit within a wider context. It’s not possible to cover every variable in one short video, so think of this as a way to understand how the parts work, not a “one size fits all” answer.

Glossary of Terms

Before we dive in, let’s clarify a few key elements you’ll see in this detail:

• Steel lintel
  In this scenario, it is a structural element that spans the window opening and supports the brickwork above.
• Cavity tray
  A damp-proof barrier installed above openings to collect moisture in the cavity and direct it outwards via weep holes.
• Weep holes
  Small drainage openings in the external brickwork that allow water collected by the cavity tray to escape.
• Breather membrane
  A vapour-open, water-resistant membrane fixed to the external face of the timber frame.
• Cavity barrier / closer
  A fire-resistant element installed around openings to slow the spread of fire through the cavity.
• Insulated plasterboard to reveal
  A rigid insulation and plasterboard lining used to improve thermal performance at the window reveal.
• Compressible filler
  A flexible, sponge-like strip used to accommodate movement and provide a thermal break between the window and structure.

Walk Through the Detail

Starting from the outside:

The facing brickwork forms the external weathering layer. This is tied back to the timber frame using timber-frame-specific wall ties, which fix into the structural sheathing or timber structure.

Above the window opening sits the steel lintel, which spans the full width of the window and bears into the masonry on either side — typically with a minimum bearing of around 150 mm each side, subject to the engineer’s specification.

Behind the brickwork is a drained cavity and then the breather membrane, which is lapped over the cavity tray at the window head. Running over the full length of the lintel is the cavity tray. This tray is continuous across the opening and directs moisture outwards via the weep holes. Stop ends are usually formed at each end of the tray, depending on the manufacturer’s system.

The weep holes are positioned in the outer leaf directly above the window opening, typically at maximum 450 mm centres, with at least two over each opening. Within the cavity at the head of the opening is a 30-minute fire-resistant cavity barrier. This is required around windows to slow the spread of fire within the cavity and is a standard requirement under Approved Document B.

Moving internally, the timber stud wall is insulated between studs, with a vapour control layer on the warm side. A service void formed with battens allows services to run without penetrating the VCL.

At the window reveal, insulated plasterboard is used in this example. This helps reduce thermal bridging at the window head. There are different types of insulation that could be used in this position, such as wood fibre, wood wool boards and more. 

Between the window frame and the structure is a compressible fill. This is a sponge-like strip often used with timber windows instead of expanding foam. It allows for movement, improves airtightness, and creates a thermal break between the frame and surrounding structure.

Student Questions

1. Do the steel lintel and cavity tray run the full width of the window?

Yes.
The lintel spans the full width of the opening and bears into the wall on either side, usually a minimum of 150mm either side of the window opening
The cavity tray runs over the full length of the lintel with stop ends at each end, and weep holes are provided at intervals.

2. Are weep holes just gaps where there is no mortar?

No.
Weep holes are formed using plastic spacers or proprietary weep formers that allow water to discharge while keeping insects and debris out. Mortar is deliberately omitted in this location so water collected by the cavity tray can drain out safely.

3. What is the cavity barrier made of?

Cavity barriers are typically formed from fire-resistant mineral wool, sometimes with an intumescent facing. Their role is to slow fire spread within the cavity around openings.

4. Is the insulation at the internal reveal always rigid?

No.
A range of insulation types can be used. In this detail, rigid insulated plasterboard is shown so that additional timber supports are not required.
If mineral wool or spray foam were used instead, timber framing would be needed to support the internal finish.

5. What is the compressible fill made of?

The compressible fill is usually a foam or sponge-like strip, designed to compress slightly while maintaining a thermal and airtight seal. It’s commonly used with timber windows instead of expanding foam.

Key Points for This Detail

• Lintels span the full opening with adequate bearing onto masonry.
• Cavity trays run the full length of the lintel, with stop ends as required.
• Weep holes at max 450 mm centres, minimum two per opening.
• Breather membrane must lap over the cavity tray.
• Fire-resistant cavity barriers required around window openings.
• Insulation continuity at the head reduces cold bridging.
• Sealant to plasterboard edges provides airtightness and fire protection.
• Compressible fillers allow movement and improve thermal performance.

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.