Floating Walls

I think you'd better find out *exactly* what the walls are made of. Your description sounds to me like timber which has been almost entirely eaten by woodworm. If this is the case, you won't have to worry about *fixing* the walls - you'll need to *replace* them!

It sounds like a thorough expert examination of *all* timber in the house may be in order, if it hasn't already been done.

Looks to me like plaster made with ash rather than woodworm. We've had all the timber checked and treated and will be treating the ptg with vast quantities of boron based treatment.

The black material can be crushed between your fingers in a similar fashion to plaster and produces black gritty powder like charcoal and sand mixed together

Yerrrs.. Somewhere in my memory this rings a bell.

It could simply be some early form of 'cinder block' - lime mortar loaded up with clinker or the like.

Up to you whetther having replaced the floor, you build new walls altgother, or simply strut up to to te walls and infillwith e.g. expanding foam.

If the original walls are 'interesing' i'd patch them - but new studwork and plasterboard - especially if you dry line with paper rather than skim - is pretty cheap and easy.

Period property issues are not uk.d-i-y's forte. Try here:

I don't understand this - do you mean that you replaced the boards and now there's a few mm gap under walls, i.e. that the walls were built up on the floorboards?

Exactly - we have about 10mm between the new boards and the floor now. The walls were built on the old boards. Worst of all, the old boards were only a couple of mm from where the stairwell was cut so I've had to replaster the stairwell having disturbed the walls there :(