On Tuesday, 19 November 2019 15:26:10 UTC, Andrew wrote:
On 18/11/2019 12:34, Steve J wrote:
There are underfloor vents at the side but the patio has blocked the vents at the back of the house.

And that could well be the cause of the problem.

Whoever built the patio should have buried 4 inch drainage
pipes to connect the existing vents back to the outside ,
allowing crossflow of air.

The mold is white and fury and round and does not seem to have damaged the floor yet see https://inspectapedia.com/mold/Subfl...d135-DJFss.jpg