undersized joists in old house

[Koolpc]

Personally, i would sister the beams and put nogging in. Nice tight fit.

 

[mordzy]

Personally, i would sister the beams and out nogging in. Nice tight fit.

not bother with hangers on the big beam at all?

 

[Koolpc]

not bother with hangers on the big beam at all?

Why put hangers on them?

 

[Mr Chibs]

Are the current joists pocketed into the exterior wall... or on a ledger board?

You won't be able to sister with hangers, unless you get wide ones... that will take two joists.

Are you plaster boarding these over or insetting so you can see the joists?

 

[catlad]

If that was my house I would screw some 2400x1200= 18mm ply to the underside of those skinny floor joist before fitting the plasterboard with a lot of 70mm screws.

 

[mordzy]

Why put hangers on them?

Because the existing are notched over the beam. So half the thickness is missing. As such may also be probe to snapping off rendering the sister useless. Unless you can suggest something else?

 

[mordzy]

If that was my house I would screw some 2400x1200= 18mm ply to the underside of those skinny floor joist before fitting the plasterboard with a lot of 70mm screws.

Why’s that? It’s not going to stop it flexing. And will add a lot of weight. It’s will stop lateral movement but that’s not the issue.


The joists are pocketed into the walls. But not notched that side. I was thinking more on the beam side

 

[Mr Chibs]

It's either leave it, or perhaps add more of the same size joists, in-between the current joists and use a hanger to connect to the beam.
In my mind though, it will be a faff.

 

[tony1851]

Assuming your floor joists are - say 1800 long (?) @ 500 centres (?) and, being old timber, equivalent in strength to something like modern-day C24 timber, which is better quality than the more usual C16 timber.

The bending stress and deflection of the joists themselves would be within acceptable limits. This suggests that the source of the 'bounce' might be the long beams supporting the joists.

As regards the notching of the joists onto the main beam; the only aspect that matters at the end is the shear stress across the reduced section where it is halved. Assuming it is reasonably good-quality timber, the reduced section is still adequate, but near to the limit of allowable shear stress. It might be worth considering screwing a batten to the main beam (s) along the underside of the joists, to give a little extra support.

But before you do anything with the joists, I'd be inclined to check the main beams first; they may be a little undersized for the span and loading.

 

[mordzy]

It's either leave it, or perhaps add more of the same size joists, in-between the current joists and use a hanger to connect to the beam.
In my mind though, it will be a faff.

Yeh I agree with the faff. Thanks for the suggestions any way. I’ll mull it over

 

[mordzy]

Assuming your floor joists are - say 1800 long (?) @ 500 centres (?) and, being old timber, equivalent in strength to something like modern-day C24 timber, which is better quality than the more usual C16 timber.

The bending stress and deflection of the joists themselves would be within acceptable limits. This suggests that the source of the 'bounce' might be the long beams supporting the joists.

As regards the notching of the joists onto the main beam; the only aspect that matters at the end is the shear stress across the reduced section where it is halved. Assuming it is reasonably good-quality timber, the reduced section is still adequate, but near to the limit of allowable shear stress. It might be worth considering screwing a batten to the main beam (s) along the underside of the joists, to give a little extra support.

But before you do anything with the joists, I'd be inclined to check the main beams first; they may be a little undersized for the span and loading.

Thanks for the detailed response. I think the first reaction is to look at the size and assume they are too small by modern standards but as you and others have suggested the old wood was slow grown and much better strength.

I’ve got the wife to gently bounce above and it’s the joist not the beam that moves. I’ve identified it to a few spots where the wood has warn away over time and as such is moving on the beam. I’ll pack up and as you suggest batton underneath for extra support and see how this improves.

There is no bath above, only a shower.

 

[Bosswhite]

I had the same problem regarding the ceiling " Our house is over 200 years old) I picked out alternate joists in middle of room (4) carefuly jacked up the original sagging joists and " sistered " the new joists four bolts spaced out through each one , as the joists were to show on the ceiling I distressed the new ones and stained to match exsisting , They look part of the ceiling .

 

[Dereekoo]

Why’s that? It’s not going to stop it flexing. And will add a lot of weight. It’s will stop lateral movement but that’s not the issue.

Theoretically if you can fix the boards to the beams so it all acts monolithically (lots of screws and glue) then what you have created is an inverted Tee beam which will resist deflection(and be stronger) than a rectangular beam. I did some noddy calcs once and it indicated things were improved three fold but of course it all depends on the particular dimensions used. As for lateral deflection the joists are not particularly slender so I don't think that's an issue anyway

 

[Bouy]

Why’s that? It’s not going to stop it flexing. And will add a lot of weight. It’s will stop lateral movement but that’s not the issue.

Theoretically if you fi

Assuming your floor joists are - say 1800 long (?) @ 500 centres (?) and, being old timber, equivalent in strength to something like modern-day C24 timber, which is better quality than the more usual C16 timber.

The bending stress and deflection of the joists themselves would be within acceptable limits. This suggests that the source of the 'bounce' might be the long beams supporting the joists.

As regards the notching of the joists onto the main beam; the only aspect that matters at the end is the shear stress across the reduced section where it is halved. Assuming it is reasonably good-quality timber, the reduced section is still adequate, but near to the limit of allowable shear stress. It might be worth considering screwing a batten to the main beam (s) along the underside of the joists, to give a little extra support.

But before you do anything with the joists, I'd be inclined to check the main beams first; they may be a little undersized for the span and loading.

Basically dimensions required to give a proper appraisal