Many pre 1930s properties were built with solid floors, particularly in the kitchen and under stairs cupboard. These often comprise of either red quarry tiles or bricks.Normally these floors were laid without any form of damp proof course or “membrane”.
Over time the floors will start to “lift” and especially when covered by something such as linoleum or cushion floor will “sweat” and become damp.
They then need either sealing with mastic asphalt which will provide a level surface and effective damp proof membrane, or where they have become too uneven, they will have to be taken up and completely replaced with concrete which should incorporate a damp proof barrier and thermal insulation in accordance with the Building Regulations.