Tiling floor on sistered joists - Advice needed

22 Sep 2021
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Hi All,
I am doing up our new but 50 year old dormer bungalow and need some advice on the floors. The upstairs floor is sagging and appears to have undersized joists (16" centres, ~5.2m span, 2x7" joists). To complicate matters, the area is divided by two partition walls (bathroom and wardrobe/storage) and in the bathroom has a section of 2x4 joist with extra timber on top to level the floor. The attached image show the layout. So I am planning on taking out the two walls and sistering the full length of the span (and the extra 2x4 bit in the bathroom) and try level and strengthen the floors, before reinstating the partition walls. Then floor with marine ply for the bathroom and regular ply for the bedroom. I was wondering if the deflection after all this will be reduced sufficiently to install tile backer board and tiles on the floor of the bathroom or will this just crack and fail in short order. I don't really have the head height to install deeper joists so this is all I can think of doing to remedy the floors, but if anyone has suggestions it would be a great help.


  • floor layout.jpg
    floor layout.jpg
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Undersized is an understatement. Even when you double up the calculated deflection is just above the allowable maximum of 14mm and that is using c24 class wood.
169 x 40 isn't 7 x 2in (178 x 50). That would be 6-3/4 x 1-1/2in, which is quite undersized for 16ft 5in span (it should be about 9 x 2in C16). If you go to a higher stiffness spec (C24 instead of standard C16 grade) it would be some help, as would going to a 3in (75mm) wide joist as recommended above. I'd suggest doing both and put in a couple of rows of solid strutting to reduce bounciness. Plywood flooring should be glued to the tops of the joists (D4 flooring glue not PVA) as well as screwed
Thanks Dereekoo and JobAndKnock. That does put me in a pickle. Had a hard enough time finding lengths of 7x2 local, not sure where I'd look for 6 3/4 x 3 in 5 meter lengths. Will hold back on ordering anything till I figure this out. (or the floor gives way...which every comes first!)
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169x40?? Don't put anything up there, taking standard floor loadings would see the strength requirement exceeded by 40% and deflection WOW
Whether or not they were designed that way, those walls appear to be acting as structural elements and cannot just be removed without something else being installed in their place. If you can't increase the depth of the joists sufficiently, it looks like you may need to consider installing a couple of (sectional) steel beams at right angles to the joists to improve the strength of the floor. This is perfectly doable, but is notifiable and will require structural calcs to be done by an engineer
Well for the time being the place is being stripped. Things only coming out including a cast iron tub that weighted a lot. I remeasured the floor joists. Definitely 169...ish some measurements were 167mm some as high as 172 but for the most about 169-170. Did get the width wrong closer, to 42-43mm. Was writing off the top of my head at work earlier. The reason why I thought 7x2 was because my local providers were saying that the finished size for 7x2 was 170mm (link removed) The wood currently there has been lived in for the last 50 years and although has a bow, it isn't very bouncy to walk on. There are noggins fitted in the bedroom part so maybe they are keeping the bounce in check. With out changing the room height , I guess all I can do is get the best grade wood I can and as thick as possible. I'm guessing that tiling the bathroom floor is probably off the cards.

For simplicity in the drawing I showed two walls holding up the offending span, but in reality the prior owners have removed one of the walls and replaced it with an I beam (no pad stone - dodgy red brick and concrete!) I'd be concerned about running more steel of this rsj. Have to get an engineer to look at the steel that is there as it is. Maybe they will do a two for one, if they ever return my call. Might call a company tomorrow about lvl joists. But I suspect if I not building a great big shopping center they won't be too interested. Can I ask, what sources you are using for span tables and deflection calcs? Are they point loads at center of span or how it equates to the edges (ie in the bathroom)
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Actually just had an idea, might fit a couple of tell-tales to the joists to measure changes over a couple of months post sistering and see what level of deflection is before putting any flooring down. That might give me a bit more info to work with.
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A minor point, but joists are always either sawn to size or regularised, not planed, so a 7 x 2in (178 x 50) will come in within a few millimetres of that size, not well undersize like PAR timber where you lose 5 to 7mm off the sawn (nominal) size. On a recent job our nominal 7 x 3s came in at 175 x 73mm regularised. Either way your 169 x 42mm is still very undersized.

I wasn't talking about running any steels off the RSJ (which wasn't mentioned until now, was it?). I was referring to the need to support the joists if you remove the two walls (which will be acting as structural supports for the joists and reducing deflection and bounce). That RSJ, dodgy or not, is reducing the effective span of the floor, and therefore the potential deflection, as is the one remaining wall. If the brick is dense enough it will probably be OK to support the RSJ, although we almost always cast in a concrete pad or insert a pre cast padstone ,(belt and braces). To retrofit one is simply a matter of propping the existing RSJ (Acrow props and scaffolding boards) cutting out a masonry pocket, screwing a plywood shutter to the wall (sealed with expanding foam), then filling with concrete. Takes 2 to 4 days to cure, then the whole procedure can be repeated at the other end. Gaps beneath a steel are generally packed with slate and mortar or steel shim stock and gaps at the sides (if any) are filled with brick and mortar

BTW "noggins" are normally only in walls - solid strutting (in floors) to work properly needs to be the same thickness as the original joists and be at least 80% of the depth of the joists (so for 180 x 50 that would mean 145 x 50 minimum). Just pointing that out in case your SE specifies solid strutting
The span/deflection/strength calcs come from a spreadsheet I developed based on BS5268 - The Structural Use of Timber which allows me to play all sorts of tunes on it depending what loading, timber size and class, spans, etc. I choose to input.

As an aside the loadings used are the standard TRADA tables of 150 kg/m2 which is equivalent to 2 average size blokes standing over the floor area every square metre which you can imagine is a bit OTT and as such any results will have a fair factor of safety within them.

You could of course put extra 170 x 40 joists in to give 133 centres which would just bring you within the 14mm deflection limit although your existing joist will no doubt have long term bow effects and will not spring back to the same level as any new joists.

Bouy (alias Dereekoo, name depending which computer I use)
Oh boy! Thanks JobAndKnock, I know what you mean... now. I think I am starting to see why they don't put me in charge of anything at work. Naturally you meant putting in the steel perpendicular to the joists, not sure why I took it to mean putting in steel joists. Starting to worry now about my measurements. I've lead an insulated (and metric) life! The idea of measuring something and then converting it to ft and inches, then trying to match it to a nominal dimensional lumber value...planed or unplaned sir? So there is only one thing for it, off to the lumber yard with my tape and measure it. Hopefully that way I can minimise the chance of getting the wrong dimensions.

Thanks Dereekoo, just one quick question on those calculations, were you using C16 for the original joists or were they for 2sistered C24 or one solid joist of C24. As in to meet the deflection limit, would I be ripping out the unknown lumber quality and replacing with a solid C24 or would the old + new C24 joist be within the deflection limits?
Worked on C24 as if that failed so does C16. Doesn't matter if you use 1 solid joist or 10 thinner joists, if the overall block dims are the same the results are the same.

Calculating deflection using different grade of woods a bit complicated but assuming all the wood was c16, deflection = 16.4mm, assuming c24, deflection = 13.4, so actual deflection somewhere in between but more to 13.4 (based on 133 spacing). Again all based on theoretical 150kg/m2, in reality probably a lot less. Assuming no load except for a nominal 25kg/m2 wooden floor loading deflection = 4mm

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