On 19/11/2019 16:51, Steve J wrote:
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

That looks like surface mould rather than anything else, but it is as
well to wear a decent face mask when working in there. The spores of
some damp moulds are potentially nasty lung infections.

https://www.nhs.uk/conditions/Aspergillosis/

And although that source says you have to be immune suppressed or
otherwise vulnerable I known of apparently healthy adults affected.

--
Regards,
Martin Brown