Floor below DPC, dampness.

[timetoretire]

Our utility area was built on side of house with doors to the front and rear. Approx 11 meters in length. Concrete floor with DPM. Floor height is 1.5 bricks below house damp proof course. Floor is sealed and painted. The house wall is also painted with masonry paint and below the DPC it needs re painting every 12 months or so as it can get damp as expected. To protect the floor from wear and tear I laid interlocking plastic tiles and now every few months I find I have to lift a metre or so of tiles back from each door to dry up dampness that is starting to go black. It is worse close to the house walls, so I suspect either it is the damp brickwork spreading under the tiles, which are not sealed to the walls, but being close to the doors and colder could this be condensation? If I leave the areas uncovered by the tiles they are dry as all the wet evaporates. All other tiled areas of the floor are dry. I have tried the polythene test on the floor, it is dry. Any advice or solution would be great. Thanks

[Barry_Convex]

Hmm, if polythene is dry then my first hunch is condensation.

Do you know how the slab is insulated? Cold bridging may be happening where slab meets wall.

And you say it's a utility room. Is it humid? Tumble dryer? Is it well ventilated? Get a humistat fan fitted...

Could you raise the floor a little? Layer of insulation then another DPM then your flooring?

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