Building components · Substructure & Ground
Oversite Covering
Explains the oversite as a covering over ground inside a building, distinct from a floor and from a damp-proof membrane in a slab.
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 oversite is
An oversite covering is laid over the ground within a building footprint where that ground is not itself built on — most commonly under a suspended floor, where a void separates the structure from the earth beneath.
Its purpose is to deal with what the exposed ground would otherwise contribute to the void above it: moisture, vegetation and, in some settings, ground gases. It is a covering over earth rather than a layer within a structure.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a damp-proof membrane within a slab. That membrane sits inside a floor build-up; an oversite covers open ground beneath a void.
- It is not a floor. Nothing walks on an oversite covering, and it carries no load.
- It is not a gas barrier in the design sense. Where ground gas is a concern, the response is a designed protection system, and this covering may or may not form part of 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.
- oversite
- site covering
- ground covering
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Whether the void above it is ventilated, since the covering and the ventilation act together on the same problem.
- The ground conditions beneath, including moisture and any contamination or gas regime established by investigation.
- Whether it stays continuous, as a covering broken by later work stops doing its job at that point.
- Access arrangements, because a void that is never entered is one where the condition of the covering is never known.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Moisture rising from open ground into a floor void is the problem the covering most commonly addresses, alongside ventilation of the void.
Anything that later enters the void — services in particular — can break the covering, and whether it is made good is rarely visible afterwards.
A covering that is intact at construction and disturbed a decade later is a different element from the one that was installed.
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.
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.
- Ground Floor
- Substructure
Clarifications
Commonly misunderstood
- That it replaces ventilation of the void. Covering the ground and ventilating the void address the same problem from different directions and are commonly both required.
- That it is the same thing as a floor damp-proof membrane. The position, and therefore the duty, is different.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What is the oversite covering here intended to control, and what else is addressing the same issue?
- Has ground gas been assessed, and does that change what the covering has to be part of?
- How is the void ventilated, and does the covering interact with that?
- How will the covering be inspected or reinstated if the void is entered later?
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
- This entry does not describe a ground-gas protection design, which is specialist work informed by site assessment.
- No material, thickness, lap or ventilation provision is stated here.
Substructure & Ground Elements
The prepared surfaces, layers and buried members between undisturbed ground and the first structure above it.
Browse all substructure & ground entries →