oversite

An oversite is a continuous concrete slab laid directly on the ground beneath a building's ground floor. It serves as a moisture and radon barrier, preventing water, soil gas, and other ground contaminants from rising through the foundation into the structure above. On a typical domestic build, it sits 100 to 150 mm thick, poured over compacted hardcore or sand blinding, and acts as the working platform for the ground floor construction itself.

The term comes from the old practice of surveying or 'overseeing' the site before building, though its modern meaning has crystallized around this specific concrete layer. In some regions and older texts, you will see it called a 'concrete bed' or 'ground beam', though these terms carry slightly different technical meanings. The oversite is distinct from a full concrete raft foundation; it is simpler, cheaper, and suitable where ground conditions are stable and loadings are modest.

Materials and Installation

A standard oversite uses ready-mix concrete, typically C10 or C15 grade, laid over a prepared base. That base might be 100 mm of compacted stone or a thinner blinding layer of sand to level irregularities in the soil. The concrete is left unfinished, as it will be covered by insulation, a damp-proof membrane, and then floor finishes. The DPM (damp-proof membrane) is critical: it may be laid beneath or above the concrete depending on building code and design, but the pairing of oversite plus DPM is what actually stops rising damp.

Common defects occur when the base is poorly compacted, allowing settlement and cracking of the slab above. If the DPM tears during installation or sits exposed during construction, its barrier function fails. Radon resistance requires a complete seal, meaning all joints, pipe penetrations, and edges must be carefully sealed; a standard oversite without extra measures will not stop radon effectively.

In modern building practice, oversites are standard on house building and light commercial structures built on solid ground. On soft clay, thick peat, or fill, a structural engineer may specify a deeper slab or raft instead. The oversite sits between the architect's brief (keep damp out) and the structural engineer's design (keep the load off the soil evenly), making it one of the least glamorous but most necessary parts of the foundation package.