Laying carpet on a magnacite floor

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Hi all - I'm a new member here, so hello from me and thanks in advance for any help that you may be able to offer.

I live in a 1950's built ex-council house which has cavity walls and solid floors throughout the ground floor. These are or were all covered in a red composite which I believe to be magnacite. It is a layer about 13mm thick, which looks to be laid on top of a lower solid layer of another type of hard material which I can't tell the composition of.

There is evidence externally of an original DPC on the outside of the house below a brick course which has several air bricks in it. However through different building works over the years we have had floors completely dug up in several rooms and there has been no evidence of a DPC internally. In these rooms we have had new solid floors laid with DPC included.

However we are finally (after 20 years) getting round to decorating the last room, which until now has been a general junk room and therefore not been looked at. It had a good quality Axminster carpet laid on top of underlay by the previous owners and has caused no problems for nearly 30 years, with no evidence of damp and no bad odours.

Now we have lifted the carpet there is evidence that the magnacite floor in this room has had problems with damp in the past. There is a large patch of screed in place of the magnacite in one corner and when I tap on the magnacite in other places it sounds hollow and has obviously lifted away from the under layer.

There was a patch where the suface was wet to the touch once we lifted the underlay and where the gripper rod was nailed down the naisl were rusted and magnacite has broekn up as the gripper has been removed.

I'm trying to avoid having to have the whole floor dug up and relaid but I'm not sure whether I can patch the damage with a screed. I'm also concerned by the wetness described but I'm worried that if I seal the surface with something like Synthaprufe I will trap in the moisture and cause more problems. I'm aware that magnacite is hygroscopic and therefore holds and retains moisture but guess that over the years the carpet has allowed it to generally evaporate. I'm also aware that it is electrically conductive so damp meter readings will always give a full scale damp reading even if the floor is not wet.

Given that the previous carpet had happily survived nearly 30 years despite the dampness underneath what I would like advice on is:

1) Can I just patch the magnacite with a screed and leave it to absorb and evaporate moisture as it always has?

2) Could I seal it safely to stop the dampness possibly affecting a new carpet and if so, am I likely to just push a damp problem to the edges of the room where it may come up the walls?

3) Should I be getting it all dug up and starting again?

I've attached some pictures of the magnacite layer and the general room showing the existing repair.

Many thanks
 

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Looks like magnesite to me.
Take it up mate. You can't screed over it. You could lay a carpet over it with a breathable underlay.
But I would take it up and lay a liquid DPM then screeding compound.
 

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