(Sort of) sleeper wall problem: any ideas?

[rrh]

10-12 years ago I packed out the gaps which had developed between the sleeper walls on top of the concrete float and the joists supporting our timber kitchen floor (1920s semi, North London). The floor had become bouncy again and with new kitchen units en route and some electrics needing attention, it was time today to rip it up again. I found some small gaps from extra settlement over the intervening years, but no more than you'd expect. But the real problem appears to be around the boiler alcove in the middle of one wall - presumably once a fireplace. This is an area I didn't even look at when packing the joists before. There seems to be a brick sleeper-type wall built out around the former fireplace area. The middle of it seems to be full of old builders' rubble into which I can poke a stick for a long way but there is a concrete shelf above - thankfully sound as far as I can see - supporting the boiler. As you can see from this pic:

of the joists are supposed to rest on the wall top but the wall has settled and a gap has opened up (there was also originally a length of joist timber along the concrete edge at right angles to the joists which was completely rotten and providing no support). To judge from the amount of curve in the floorboards, the joists have dropped some 20mm and are now not properly supported at their ends - although they are at the sleeper wall in front (packing shown is undisturbed from last time). It may not be clear from the photo but the top of the problem wall has a pronounced lean towards the camera - enough to make me unwilling to just put more packing under and seal up again, for fear of it just collapsing under the extra weight.

The local BCO won't visit to advise and it's next to impossible to get a builder round here these days for this sort of thing. So I'm planning to build a new sort of mega-sleeper wall between the one in the foreground and the problem wall, filling in the whole intervening area to try to both provide support for the floor and prevent any further sliding out forwards of the fireplace support wall. I don't forsee any problem with ventilation - there is plenty down there and I will leave gaps.

Does anyone have any tips or suggestions please? TIA.

[Scott Mills]

I'm a bit confussed, but can't you just pack the 20mm with broken slate tiles? I raise part of our floor that way. Jack up the joist with a car jack, put in the slate shimm and relax the jack.

Scott

[rrh]

Sure I could, but the point is that the wall around the fireplace area hasn't just settled downwards a bit, it has developed a noticeable lean out of vertical. If I just pack onto the top of it, I'm worried that it will collapse (at some future point when the floor is well and truly covered with expensive tiles...).

[Scott Mills]

Ahh, now I have re-read your post and looked at the picture it has all become clear. I would build a wall next to it as you suggest ie on the side away from the camera. Use a few large breeze blocks so that the job is quick and cheap - remember to put in a DPC. You don't have to be neat on with the cement either, and it might be worth treating the wood too while you're there.

Scott

[Peter Taylor]

Scott Mills wrote

OK, but remember the settlement is likely to be due to shrinking clay (North London) so if you build it straight off the oversite it will probably settle (relative to the main house walls) again in time. Ideally it needs a decent foundation, to the same depth as the main walls. Also, make sure you leave some holes in the wall to allow air to circulate.

Peter

[rrh]

I thought this method of house construction put everything (from outer walls inwards) onto one large concrete slab, rather than have the main walls on separate foundations? That's certainly what it looks like as far as I can see. Either way some further settlement seems inevitable but at the current rate of less than 5mm per decade I think I can live with it.