Replacing timber floor joists into a stone wall

[bernardseafield]

Hello everyone.

I need to replace most or all of the timber floor joists on the first floor. My problem is that the old joists were sitting in recesses in both sides of the wall, so I don't know how to get the new timbers in.

Do I have to put wall plates up to support the joist ends? Can I chip away stone above the recesses on one side then lower the timbers in from above? Could I sister two shorter joists together?

The walls are 600mm thick sandstone and the joist ends currently sit about 150mm recessed into the walls at both ends.

Thanks for any help!

Bernard.

 

[tony1851]

You could bolt timber plates to the walls but it might be difficult in stone.
If the current joists sit 6" into the walls each side, you could cut the joists just 6" wider than the gap, and let them bear 3" into each wall. This may seem a small amount, but it is perfectly adequate. The bearing area will be OK for the loads involved, and the floor will not move once the boarding is fixed.

 

[bernardseafield]

Thanks Tony, that's good advice. I'm still not sure if I could get a joist that is 6" longer than the gap into position and into a hole on one side, but maybe. I'll have to try it to see what angle I can get it into and if the hole is slightly bigger than it needs to be I might be able to jiggle it in.

Is 3" the minimum length I can get away with seated in the wall? The room is 3.5 metres wide if that makes any difference.

Thanks, Bernard.

 

[tony1851]

Is 3" the minimum length I can get away with seated in the wall? .

In theory, you can get away with a lot less than 3" bearing. The Building Regulations used to recommend a minimum length of bearing of 38mm The crtitcal factor is the compressive strength of the timber perpendicular to the grain.
For your span of 3.5m, assuming you have approx 400mm centres, and you are using normal C16 grade timber 47mm wide .and with normal domestic loading, your compressive stress works out at about 0.4 N/mm2.
You are allowed to go up to 1.7 N/mm2, so theoretically your bearing could be a little less than 20mm, but I wouldn't recommend that!
You should be able to juggle the timber to get approx. 3" bearing; you can cut a short angle off the tops of the joist-ends if it helps.

 

[bernardseafield]

Thanks, that's really, really helpful!