What's this under the vinyl?



I removed some nasty boxing in in the bathroom to find where the musty smell was coming from. Sure enough there was mould on the wall which I have wiped off and sprayed.
I pulled back some vinyl, due to be replaced, and there is a black surface on top of the chipboard floor.
What is this? It doesn't wash off so I don't suppose it is mould and covers the entire floor anyway. Is it some sort of waterproofing?
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Why would I do that then mate.

as i said it is chipboard flooring
6 Apr 2012
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United Kingdom
If it's hardboard it's about 3.6mm thick which looks like it's had its day.
Take that up then put 6mm ply down.
Just going off the picture I could be wrong mate.
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It's not wet, it is bone dry.
It doesn't smell musty.
It is all the same under the entire floor right up to the skirting apart from in the airing cupboard which is in the corner of the bathroom. If it was caused by water then I would have expected stains at least in there.
There is no softening, damaged areas, bulges etc.
The mould that was on the wall washed off easily, this doesn't.


I've taken the vinyl up now. The brown is almost like a varnish. In general it goes right up to the skirtings but stops just under the bath panel. I'm cleaning it up with a poly disc and it will go back down to chipboard, I'm not being that thorough. You can see the outline of foot square presumably lino tiles which were probably the original finish.

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