Building components · Floor Build-Up
Floor Topping
Explains that a topping can be structural or not, and that the difference changes everything about it.
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 topping is
A topping is a layer placed on a structural deck. It may be doing any of three quite different jobs: providing a level, regular surface; adding mass for acoustic or thermal reasons; or tying separate structural units together so they act as one.
The third case makes it structural. A topping over precast units that is designed to make them act compositely is part of the structure, and it cannot be treated as a levelling layer to be reduced or omitted.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a screed. Screeds are published material entries and are a way of forming a topping; the topping is the position and the function.
- It is not a structural deck. The deck spans; a topping sits on the deck and is supported continuously by it.
- It is not a floating floor layer. A floating layer is deliberately isolated from what is beneath; a topping is commonly bonded or at least continuous with it.
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.
- toppings
- structural topping
- concrete topping
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Which of its three functions applies, since a structural topping and a levelling topping have entirely different requirements.
- Its bond to, or separation from, the deck beneath — which follows from that function.
- Drying time where it is a wet layer, since finishes laid over a topping that has not dried will fail.
- Its own shrinkage and how that is accommodated.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Wet toppings introduce a large amount of water into a building and take time to dry; laying impermeable finishes too early is a recognised cause of failure.
Where a topping is designed to make units act compositely, it is structure and its thickness and reinforcement are not adjustable on site.
Added mass is one of the mechanisms by which a floor resists airborne sound, and a topping is a common way of providing it.
A topping shrinks as it cures, and whether it is bonded or unbonded determines how that shrinkage is expressed.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
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.
Topping to Structural Deck
Where a topping meets the deck beneath, and where being bonded or unbonded changes everything about both.
Raises: Structural transfer · Movement · Moisture · Acoustic continuity
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.
- Building Interior
- Ground Floor
- Intermediate Floor
- Separating Construction
- Wet Area
Clarifications
Commonly misunderstood
- That a topping is a levelling layer that can be thinned to suit. Where it is structural, it cannot.
- That a topping is dry when it is walkable. Drying to a state that will accept an impermeable finish takes far longer.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this topping structural, or is it providing level and mass only?
- Is it bonded to the deck, or deliberately separated?
- How long must it dry before the finish is laid, and how will that be established?
- How is shrinkage accommodated?
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 thickness, mix, reinforcement, drying time or acoustic value is stated here.
- Structural toppings are designed elements; altering their thickness or reinforcement is structural alteration.
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 →