Crumbling wooden floor

[BenTheDog]

An elderly lady I know has an old cottage and her kitchen floor has become very soft in places. I have peeled back the lino to expose an 18mm marine ply floor resting on 40mm deep 'joists', themselves resting on a DPM laid on top of a concrete slab. The woodwork is plainly rotten and crumbling. There are no proper air vents, only a series of holes at one end, and no means of opening up any new vents as the kitchen is surrounded by other rooms. I would like to help her (she can't afford much) but don't want to spend a lot of time, so concreting the floor is definitely out.
I could replace the floor with similar construction, but I would really like to use materials not likely to be affected by moisture.
Does anyone know, would it work to use 18mm cement board on top of 40mm Celotex (or similar insulation board), or would the flooring be too flexible underfoot?
If not, has anyone any other ideas?

Read more: //www.diynot.com/diy/forums/floors-stairs-lofts/create-thread#ixzz3iocLmwTA

 

[ree]

The kitchen will have to be stripped of all base units and appliances to lift the floor.
Are the skirtings and wall plaster sound - you cant leave any rot in place.
The membrane to be re-newed, and the floor treated with anti-fungal chemicals.

Treated 50mm x 50mm can be ripped and planed to fit level (chemically treat the worked surfaces).
Ply or T&G chipboard can be screwed to the sleepers.

Are you saying that all surrounding rooms have solid floors?

Thats one way to do it. Another way would be after lifting the old floor to treat and the lay a semi-dry sand & cement screed. Done properly (beaten into compaction) and well dried it will take loose laid sheet vinyl or ceramic tile

 

[BenTheDog]

Thanks for your comments Ree.
There is no actual rot only weakened and life expired marine ply. The issue about using wood for any part of the construction is the inability to satisfactorily vent the underside of the wood surface or joists so any wood put down would eventually rot itself.
The existing joists all run in a direction between the next door neighbours' on both sides and there are solid floors on the other two sides, so venting is not possible.
This is why I was keen to try something different like cement board on Celotex.
Your last idea sounded good but I don't trust myself to do it properly.

 

[geraldthehamster]

I'd put celotex down and float new ply on top of it. Joists can't rot if there are no joists. My conservatory floor is constructed on this principle and is quite stable.

Cheers
Richard