Building components · Floor Build-Up
Slab Edge
Explains the slab edge as a junction rather than a boundary, because it is where two insulation layers must meet.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a slab edge is
The slab edge is where a floor slab stops and the external wall begins. It is a small area with a disproportionate effect, because it is where the floor insulation and the wall insulation have to meet, and where a dense structural element reaches the outside face of the building.
At upper floors it is also where the floor structure passes through the facade zone, and where fire separation between storeys must be maintained at the external wall.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not the wall. It is the junction between floor and wall, and it belongs to both.
- It is not a floor finish edge. This is the structural perimeter, well beneath any finish.
- It is not a perimeter isolation strip. That handles acoustic separation of a floating layer; the slab edge is a thermal and structural junction.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- floor edge
- slab perimeter
- floor perimeter
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- How the floor and wall insulation layers are brought together, since a gap here is a continuous line around the building.
- Whether the slab is in contact with the outside, which determines how much of a thermal path it provides.
- At upper floors, how fire separation is maintained where the floor meets the facade.
- How the damp-proof course and damp-proof membrane connect at the ground floor edge.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A slab edge without insulation continuity is a continuous linear thermal bridge around the whole building perimeter, which is why it is one of the standard junction details.
At ground floor, the slab edge is where the floor damp-proof membrane and the wall damp-proof course must connect, and a break there is a path for moisture.
At upper floors, the junction between floor and external wall is where fire separation between storeys is maintained or lost.
The slab edge is where the floor bears on or connects to the wall, so the junction is structural as well as environmental.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Junctions
Interfaces this element takes part in
Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.
Slab Edge to External Wall
Where a floor meets the external wall, forming a line that runs around the whole building at every level.
Raises: Thermal continuity · Moisture · Fire continuity · Structural transfer
Wall Base Damp-Proofing Junction
Where a wall's damp-proof course and a floor's damp-proof membrane must be joined so neither has an open end.
Raises: Moisture · Thermal continuity · Buildability · Structural transfer
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Basement
- Building Envelope
- Ground Floor
- Intermediate Floor
Clarifications
Commonly misunderstood
- That a well-insulated floor and a well-insulated wall make a well-insulated building. The junction between them is where performance is commonly lost.
- That the slab edge is a detail. It runs continuously around every floor of the building.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- How do the floor and wall insulation layers meet at this edge?
- Is the slab in contact with the outside anywhere along its perimeter?
- At ground floor, how do the damp-proof membrane and damp-proof course connect?
- At upper floors, how is fire separation maintained where the floor meets the facade?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No insulation arrangement, thermal value, fire stopping or damp-proofing detail is stated here.
- Junction detailing at slab edges is a design matter subject to thermal and fire regulation.
Floor Build-Up Elements
The deck, toppings, floating layers, isolation strips and voids that sit between a floor structure and its finish.
Browse all floor build-up entries →