Sister joists with plywood instead of C16

15 Sep 2019
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United Kingdom
I am about to put a chipboard new floor down in a bathroom and want to level the existing joists as they slope towards the party wall.

It is only a very small span (1m) that needs levelling as the floor that sits underneath the bath has not been removed.

I have looked at sistering the joists with 2x6 c16 to bring everything level, but they seem like they will add a lot of unnecessary weight.

Am I worrying about nothing? Could I use plywood as an alternative sistering material?
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Yes, worrying about nothing really.

you could cut firring strips (long wedges) applied to top of current joists... but it’s a bit of a faff cuttings them.

I used coach screws to connect the last ones I did, 90x10mm.
Thanks for confirming, always nervous about weight for some reason :)

As I am not repairing the joists but merely slightly levelling them, could I just use chipboard 18mm flooring glued either side of the joist? I have lots of pieces already so would be a cheaper option for me.
I wouldn’t, as I don’t like chipboard, and it’s not as strong as wood.

That said others have done it before.

if you are dead set, perhaps glue (5 minute set PU glue) 2 strips together to make a kind of chipboard joist, then fit as previously mentioned.
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Point taken, I can also foresee issues with screwing into the chipboard and it crumbling. Think I am going to go for 47x125 carcassing timber. It is smaller than the original joists but as I am not repairing it then it seems like a good trade off between price and size.
Good choice.

I bought a load of chipboard flooring... trying to save money... still sat in the garage, as didn’t think it was suitable when I came to use it.

Use 63mm cls if the height is not too much, else 89mm. Turbo coach screws from screwfix are good and cheap for this purpose too.
I am having a bit of a strange one with this. I worked out the highest point and started from that joist. I used a spirit level and fitted the length of 47x125 to the joist with poly glue and turbo coach screws.

I then moved on to the next joist and did the same thing. I then went back with the level and checked the first one and to my surprise it was now not level.

I then went back to the latest one I did and that was no longer level either. After a couple of hours I went back and checked again and now they had swapped around and the first one was level!

I am guessing faulty spirit levels are not a thing and what is actually happening is that my joists are moving ever so slightly in the joist hanger? Is this a thing? I can't see any movement but it might be very slight, should I shim the bottom of the joist hanger?
In fact, just turned the spirit level round the other way and the first one is accurate. So looks like faulty spirit levels are a thing after all
I would've laid 18mm plywood and then use bal flexible self levelling compound.
Works a treat and doesn't crack.
You could put a noggin in, joining joist to joist if required to stiffen the floor.

Take a photo of the joist hanger, might Folk be able to offer more advice.

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