Fixing joist hangers into stone wall

[pandjfielding]

We have 150 year old stone walls built with lime mortar, I need to replace the floor joists in the loft but dont want to cut out any more stone from the walls. The old joists vary between 100 x 100 to 125 x 125 (no two are the same!) My plan is to clean out the existi ng holes, wet them with unibond and water, and fill them with a concrete mi x, then at the top of each hole set in a joist hanger which I could either just ram in more mix or maybe use a resin or cement grout so in effect maki ng a sort of concrete pad for the hanger to rest on. Any thoughts on this? I understand the use of lime mortar in any stone repa irs etc, but thought that a cement based pad would be ok in this case.

[GB]

I haven't a clue, but can I just ask whether the ceilings have been removed in the rooms below the loft? If so, could you screw a joist-size beam to each wall and then hang the joist hangers from those? I just imagine that would be a lot easier to get completely level than trying to fill holes in the stonework with different amounts of mortar.

But it does put the woodwork at risk from any penetrating damp.

As I said, I've never done anything like this, so I should really have kept my mouth/keyboard shut.

[pandjfielding]

The ceilings are "Placo" which is thin hollow terracotta blocks suspended 8 to 10 inches below these old joist by wires (forget to mention this is an old French house) so I can only replace one at a time...