1930’s Concrete Floor Slab explained

This is the floor of a house that was built in the late 1930’s.

The concrete floor-slab is around 120mm thick and was cast directly over levelled and compacted ground soil. Back then there was no damp-proof membrane between the slab and the ground to prevent damp rising up through the slab. They used alternative methods.

In the hall and lounges, they installed wooden ‘parquet’ blocks on a 3mm bed of hot bitumen. It cooled to bond the blocks in place but also to form a continuous damp-proof barrier across the slab surface.

Wooden block is not appropriate in wet areas like the kitchen so they used hard clay ‘Quarry’ tiles instead. The tiles were bed and grouted with lime mortar which allowed trapped moisture to escape as vapour through the joints.

If the tiles are securely bonded you should absolutely not break them up. Leave them alone as they are holding back so much moisture in the slab.

But, after nearly 100 years the tiles often settle so if you wanted to refurbish you would have to level the floor with a self-levelling compound. However, vapour will still escape through the tile joints so what you have to do first is stop this by sealing the surface with a roll-on or brushed on epoxy-resin damp-proof membrane. Then you can lay the self-levelling compound.