Once the first few courses of block are out of the ground, the different layers that form the oversight are ready to be established. Additionally, the space within the trenches surrounding the laid masonry can be backfilled with hardcore and soil. The level of the oversight at this point has been reduced to an appropriate height to receive all of the required layers involved in the ground floor buildup. The outside area of the trenches can be filled back in with good soil or hardcore up to a level appropriate to receive new turf, in relation to the common ground level. The inside of the trench can also be backfilled in the same fashion, up to the height of the bottom of the oversight.  

Hardcore is a stoney mix that contains broken up rubble and finer particulates. It is commonly used as a backfilling material, due to its low cost and ability to be easily compacted. Once the trench is filled and the masonry under the ground is all covered up, attention can be turned to the oversight. 

The oversight is the name given to the inside area found within the foundations. Whilst no structural load is dispersed across this oversight, a solid buildup must be created in order to stabilise the finished floor slab. In modern residential construction, the most common style of oversight is a simple insulated concrete slab. This style is reliable, cost effective, and works well within the flow of the groundworks process.  

Working backwards down from the finished floor for the purpose of explanation can help us to understand the different layers and where they sit within the oversight. Bear in mind that the specific buildups and their thicknesses can vary between jobs and will always be specified on the drawings. Here we will look at a typical assembly, common in UK residential construction as of the time of writing.  

The ground floor finished floor level describes the top of the screed/floorboards, before the installation of any floor covering such as carpets, tiles, vinyl etc. The common thickness of screed is around 70mm in most small-scale residential applications. The screed is laid on top of a layer of rigid foam insulation, with a vapour barrier between them. 25mm of foam insulation, or a layer of foam roll is run around the perimeter of the area that is being screeded, to allow for expansion in the slab of screed, and prevent cold temperatures from entering the sides of the slab. The insulation prevents rising cold from affecting the indoor temperature and is commonly around 100mm thick. The screed is installed later on in the job, once the roof is weathered in and the windows and doors are installed. Underneath the insulation layer is a slab of concrete, typically reinforced at a thickness of 100mm, with a layer of DPM between the two. It's not uncommon for these two layers to be switched, with the concrete sitting on top of the insulation, and the screed being laid on top. This decision is left to preference, and requirements of the job. 

The concrete slab is poured during the groundworks stage, and provides a hard, flat surface that can be effectively worked from moving forwards in the project. Regardless of the arrangement of the insulation and concrete, another layer of DPM is installed underneath the bottom layer of the slab, on top of the prepared substrate. This DPM layer laps up the side of the laid masonry, preventing moisture from penetrating the slab from below or the sides.

A thin 30mm layer of blinding sand is thrown around onto the oversight under the DPM to flatten out the oversight and prevent any sharp stones from puncturing the DPM during installation and over time. The blinding sand is thrown over the bottom layer of the buildup of the oversight. This layer is made up from hardcore that has been compacted with the use of a whacker plate. A common thickness of around 150mm of hardcore is called for here, that is placed into the oversight directly on top of the reduced dig. As we are restricted in thickness by the set intended sizes of the other more critical components, such as the insulation, concrete, and screed, if adjustments in the height of the overall assembly need to be made, we can make it here by increasing or decreasing the amount of hardcore by a small amount.