How can I replace a rotten section of a bathroom joist, attached to the ceiling below?

[Gary111]

Hi, I am aiming to cut out a rotten section of upstairs joist and sister to the existing part of the joist that's sound.

The whole joist is around 6 feet long its around 5 x 3 inches.

The new piece I want to add want to add is around 20inches from the pocket.

The part that's still ok is still attached to the ceiling below with lath and slats so I want to cut it in situ, by adding a few more screws through the floorboards to hold it once it's cut.

What I can't work out is how to insert the new joist into the pocket as the hole is the exact size for the joist, but it will have to go in at an angle.

The only thing I can think is to leave a bit of a gap between the new piece and old joist in order to get it straight into the pocket, does that seem ok?

Alternatively maybe I can cut the end of the joist that goes in the pocket at a slight angle, but it seems like that may affect the strength?

Thanks

 

[blup]

The original joist went in before the lath and plaster, and it will have to do so again, so you will have to chop out the relevant part of the ceiling, you will find it gives you more room to work and get the fitting right.

Blup

 

[Gary111]

Thanks mate, sorry, I thought I had posted the pic

 

[Deleted member 174758]

What I can't work out is how to insert the new joist into the pocket as the hole is the exact size for the joist, but it will have to go in at an angle.

What I've done on the past is to break out the masonry to the side of the existing pocket so that the sister joist can go in. The masonry is made good after the new joist end has been fitted. Bear in mind that your sister joist should be as near as possible to the dimensions of the original with at least a 1 metre overlap and something like 3 no M10 coach bolts to join the two joists - so NO end to end joins, fishplates or going in at an angle, which are not acceptable

The part that's still ok is still attached to the ceiling below with lath and slats so I want to cut it in situ, by adding a few more screws through the floorboards to hold it once it's cut.

One problem you've got is that the lime plaster "snots" project through the laths for 10 to 30mm (or more) on the upper side of them, so to retain these your sister joist would need to be shallower - not much of an issue if you are dealing with 12 x 4in originals and can go to something like 20-1/2in C24 timbers, but with 5 x 3s you don't have that leeway, so as @blup says you'll need to cut out a section of lath and plaster before you start and make good by boarding and skimming afterwards. In any case attempting to screw lath and plaster is a hit and miss affair and you likely as not will break off some of the snots - which can cause the plaster to droop, crack or fall off

 

[Gary111]

Thanks for your help, a clot of the paths have rotted away and the ceiling is drooping anyway, which I will also need to sort out.

The existing joist is only 1800 in total, and I want to cut out just 500, plus what's in the wall.
(I was originally going to use bower beams as it seems easier but they say for a maximum cut back of 350, they do a longer one but it's 150 deep so too deep for my joist.)

I was planning to have the new sister around 1100 plus what's in the wall, as I would need to lift more floorboards to get further which I don't want to do.


Just one 10mm bolt in the short piece near the wall and 2 more on the other side?

Does that sound ok?

Thanks again for your help

 

[Deleted member 174758]

The existing joist is only 1800 in total, and I want to cut out just 500, plus what's in the wall...

I am a working carpenter and joiner, and I've installed a few sistered joists (over 6,000 alone on one big project) - and every one has had 1000mm or longer overlap and been secured with three bolts after the overlap, because two bolts won't get past a structural engineer or a building inspector. We always aim for 70 to 100mm bearing (on the masonry) in the pocket. Just forget about cutting little bits out here and there and connecting it all together as it gains you no strength. Cut away the section from the wall to the end of the rotted area plus at least 300mm (to ensure that any rot inside the damaged joist is got rid of), treat the cut end of the original joist with Cuprinol or the like and attach a new sister joist to the shortened original joist ensuring that you have an overlay of 1000mm or more using three coach bolts, and with 70 to 100mm bearing in the widened masonry pocket.

If you need to remove lath and plaster from beneath to do the job and then board and skim to make good, then you need to.

Doing the job from below side steps the need to remove floor boards and is actually much less disruptive and less physically taxing I find. The adjacent joists will hold it all together

Just one 10mm bolt in the short piece near the wall and 2 more on the other side?

Does that sound ok?

Not really, please see my previous comments

 

[Gary111]

Great thanks so much for your time and effort in your detailed reply.

 

[Gary111]

Would two c24 2x 4s (which is actually slightly smaller due to regularised sizing) bolted together be strong enough to sister the 3x 5 joist?

With one 2 x 4 on the other side.