Floor joist support

I have had to remove all my living room floor and joists due to woodworm damage. The variably sized beams were variously cemented in and resting on lime portated stone interior walls. I The question I have is … to repair the jointing of the stones in the beam sockets is concrete mortar stronger than lime mortar (the wall is interior and not generally vosible so damp and cosmetics won’t be an issue).

I would be tempted to stick with lime mortar rather than cement mortar always. Cement mortar is so much harder but unforgiving to minor movement.

I am sure however there are others far better qualified than me to comment.

I would use Baticem or similar, it’s a mixture of OPC anc Lime.


I would also install ledger plates to hang the new joists on to, so much tidier and simpler than trying to concrete them in to the wall, which I don’t think is a great idea for the timber either in stone walls. If you’re interested in how to do that, I can explain further. :slight_smile:

Do not use Mortar/Cement in any application in an old house. In fact don’t use it in a new house.

Chaux will be fine.

Yes please. Just removed all traces of slightly rotted beam from the sockets so advice would be well received

Some damp too - render with chaux/cement? Cellar is well ventilated but one corner has efflorescence and boxes get damp. I also need to block mice out (had a plague last year) so rendering could solve both?

Here you go


Rendering won’t stop the damp. It should stop the mice though!

The damp may be coming from back fill, if it’s a cellar though that’s unavoidable. What’s on the other side of the wall?

Its below ground, or the problem wall is.

Is there anything extra I could coat that wall with?

Unfortunately I have nothing substantial to hang the joist on, they must rest on a supporting wall (with varying degrees of repair to the wall required).

Not sure if the attached illustrates, but the afaics my best option is to build up carefully and place the joist ends on pieces of dpc.

There is stone there, you just need to select one every 50cm or so that will provide a good fixing, as you drill 40cm in, you probably hit something else good and solid too. Each one in itself won’t be carrying a major load, and that load will be shear as well, as the joist will prevent them coming away from the wall. As you will have so many fixings it’s not really an issue if some of them are not brilliant.

You could try adding a waterproofing mix to your render but it may lead to further problems if the stone can’t breath, they may eventually disintegrate.



Or build a new wall in front of it with vents in, this will just give you a nice clean wall, the old one can still breath but you don’t have to look at it!


If you are going to build a wall in front of it (which is a good idea) you would use beton cellulaire. No damp will penetrate beton cellulaire. That is the only material I would use in an old stony house. Well, probably a new house as well.

This stuff looks ideal no? Apart from the price!



Bloc de plâtre, de couleur bleue, hydrofugé dans la masse, destiné à la réalisation d’ouvrages intérieurs verticaux dans certains locaux humides, principalement de cloisons, doublages et gaines.

Réaction au feu : M0
Poids du produit au m² : 54,61 kg
Nombre d'éléments au mètre carré hors joint : 3.00000000000000000000
Résistance thermique : 0.14
Usage du produit : Cloison intérieure, Contre-cloison, Doublage des murs extérieurs
Si habillage, type d'habillage : Pièce humide
Destination : Locaux humides privatifs
Norme : CE
Composants essentiels du produit : Plâtre
Norme européenne : EN 12859
Documents techniques (DTU, DTA...) : 25.31
Réaction au feu -Euroclasses : A1
Domaine d'emploi : , , Locaux humides privatifs
Classement sanitaire : A+

The only advantage of using carreaux de platre is if you want to run gains through it. Most have space for that. Otherwise, it is very heavy (especially when you get to ladder height and expensive. I don’t think it glues as well to be fair. I have created a bathroom with it. I don’t think I would use it again.

If you want to hide a stone wall (especially in a cellar/cave type environment) that has damp problems, beton cellulaire is the easiest, cleanest, product on the market. Very cheap as well. I build everything out of it. More importantly, once built and plastered it looks and sounds like a proper wall. You can even cover it in chaux with a bit of effort to make it look nice. In fact a think beton cellulaire is essentially blocks of ‘chaux’. Or near enough.

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Can you go in with a concrete floor?

Is that a question for me?

No it was for @Patrick_Bell

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