German floor (German flooring, oversite concrete)

After the foundation is cast and the foundation walls are built up to the planned floor level, the space inside is filled with earth or sand, compacted, and often blinded with a thin layer. A concrete slab is then cast over it. Nigerian builders call this slab the German floor; the engineering term is oversite concrete.

The German floor:

• gives the ground floor a solid, level base for screeding and tiles;
• helps keep ground moisture out, together with the damp-proof membrane beneath it and the DPC in the walls;
• in raft foundations, is tied into the foundation itself.

No one has a firm record of where the name came from. It is widely used on Nigerian sites and in builders' cost breakdowns, as in "fill, compact and cast German floor".

Skipping it. Some builders lay the floor later, or use a thinner slab, to save money. That can lead to cracked floors, settlement and damp. Ask your engineer what the design needs.

On the building timeline. The usual order on a Nigerian site is: setting out, foundation excavation and casting, foundation walls, filling and compaction, German floor, then the damp-proof course and the first course of walls. Together these make up the "DPC stage", after which walls rise towards lintel level.

Cost and timing. The German floor uses a large volume of concrete and the fill under it can be costly on low-lying sites that need sand-filling, so it is one of the bigger costs of the early stages. Builders' cost diaries often price it as a separate line from the foundation.

Inspecting it. Before walls go up is the best time to check the slab. Look for a level, even surface without large cracks, and ask the builder how the fill was compacted.

• Low-lying, wet areas of Lagos: the slab and membrane are especially important against rising damp; sites are often sand-filled first.
• Everywhere: the thickness and reinforcement depend on the structural design, not on custom.