Hello group
Apologies in advance for a wordy and rambling post, but I'm not sure what's important and what isn't. So, I'll just tell it like I found it.
I've removed a cheap floating laminate floor and underlay from a ground floor room in a house built around 1950. I want to replace it with a floating wood floor. There are several visible large patches of moisture under the damp proof membrane and the membrane is wet to the touch on the underside. One area of moisture abuts an external wall for about 4 feet, but others are located away from walls, so this may be a coincidence.
There is no moisture above the membrane and no signs of damp in the walls. A DPC is present in the external walls.
The original floor finish appears to be asphalt over concrete, but it's distorted and cracked in places. In some of the damp patches the finish appears to be a couple of mm of fine grey brittle concrete with about 10mm of brown crumbly sandy material underneath, with concrete under that. It looks a lot like some cheap self levelling compound I used once which tended to separate like this. The material is damp enough to scrape away with fingers down to the concrete. I guess that whoever put down the floor has removed some broken areas of asphalt and put this poor thin screed onto the bare concrete.
My questions are:
Could this just be condensation or something else which will just go away? Do I just patch the holes and put the new floor down? Do I need to do something scary and expensive like excavate the floor?