Video Transcript

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

In this episode, we’re looking at a timber frame flat roof warm deck detail, where the insulation sits above the roof deck, with a single ply membrane as the main waterproofing layer.

This junction is all about keeping water out, keeping the thermal layer continuous, and making sure the roof edge is buildable and durable.

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 before we start: these videos focus on one junction at a time, so you can learn 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.

Free Detail Download

Before we get into the detail, 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. The link is in the video description or on First In Architecture.

Glossary of Terms

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

• Warm deck flat roof — A flat roof build-up where the insulation is placed above the structural roof deck, keeping the deck and structure warmer and reducing condensation risk.
• Single ply membrane — A waterproof roof covering that forms the primary weatherproof layer.
• Rigid insulation (warm roof insulation) — Insulation boards laid above the roof deck to achieve the required U-value and maintain a continuous thermal layer.
• Roof deck — The structural decking (commonly plywood/OSB) fixed to the roof structure, providing the substrate for the vapour control layer and insulation.
• Vapour control layer (VCL) — A layer that limits moisture vapour movement from the warm interior into the roof build-up, reducing condensation risk.
• Insulated cavity barrier — A fire-resistant barrier closing cavities at junctions, helping limit fire and smoke spread while maintaining insulation continuity. 

Walk Through the Detail

Starting at the top surface, this roof is finished with a single ply membrane laid over the insulation. In a warm deck build-up, the membrane is doing the main waterproofing job, so the edge detailing and laps are critical. 

Below the membrane are two layers of rigid insulation, shown here as a built-up thickness (for example, 110mm over 120mm in the drawing). The insulation thickness and type are always project-specific, based on thermal performance targets, fire, embodied carbon and the roof build-up amongst many other factors. Other insulations can be specified here, like wood fibre, or mineral wool, but be mindful that the detail might need to change — either to accommodate different fixing methods, achieve the same U-value, or address fire and moisture performance.

Under the insulation sits the vapour control layer, which is positioned over the roof deck. 

The deck is the structural substrate fixed to the roof structure beneath. This warm-deck sequence helps manage condensation risk by keeping the deck warmer and controlling vapour movement from inside. 

At the roof edge, the insulation and membrane terminate into an edge flashing / trim detail that protects the roof build-up and forms a clean junction to the fascia/soffit zone. You’ll notice there is not a gutter at the edge, this is because the roof would have a fall away, directing water to the gutter on another side of the roof or perpendicular to this side. 

Additional insulation is positioned at the edge of the roof build up to provide continuity to the thermal line and avoid cold bridging. 

The timber frame wall below is shown with internal lining and vapour control layer, and an insulated cavity barrier is noted at the junction. The barrier is a non-combustible insulation that closes the cavity at junctions. It prevents fire spread within the cavity.

To learn more about the timber frame wall build up – check out Episode 8. 

Student Questions

1. Is there a reason for this not being flush?

Yes. Roof edges are often not flush because the build-up needs space for the edge trim, membrane termination, drips, and weathering geometry. The roof will have a fall perpendicular to this edge and the step helps ensure water sheds in the correct direction towards the gutter side. It also protects the membrane termination from the weather and thermal movement.

2. Are these timber elements too?

In most warm deck flat roof edges, the fascia/soffit zone and edge upstand supports are typically timber components (or timber-backed trims), but the exact elements depend on the chosen roof system. The key is that these components provide a stable fixing zone for trims and finishes, and form the edge geometry.

3. Is this a void or a batten?

This is typically a battened zone / timber trim build-up rather than a “void” left empty on purpose. It’s there to form the soffit/fascia junction and allows the roof to overhang,, and help with edge alignment.

4. Is this the ply deck?

Yes — the deck shown beneath the insulation and vapour control layer is the roof deck (often plywood or OSB). It provides the structural substrate for the VCL and insulation layer above.

5. What is this timber element for?

That timber element is generally doing one (or more) of these jobs:

• providing fixing for the overhang roof edge trim and fascia/soffit
• forming the edge upstand/geometry needed for membrane termination whilst providing some voids to fit insulation and reduce thermal bridging
• helping support the build-up so the insulation and trim don’t end up with weak or unsupported edges

Exact size and placement should follow the roof manufacturer’s standard detail and the engineer’s design.

Key Points for This Detail

• Roof edge detailing is where warm roofs often fail: keep the termination and flashings clear, robust, and manufacturer-approved.
• Thermal continuity matters most at the perimeter: the edge is a common cold-bridge zone if insulation thins or becomes discontinuous.
• VCL position and continuity should be explicit on drawings, particularly at transitions from roof to wall.
• Fire strategy needs coordinating, especially where cavities and voids occur at roof edges and behind cladding.
• Buildability: ensure there’s a realistic fixing zone for trims and finishes, and that sequencing is achievable on site.
• Specification drives the detail: membrane system, insulation type, and edge trim manufacturer requirements will change the exact arrangement. 

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.