Sistering joists

24 Nov 2020
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United Kingdom
Hi ,
We have recently moved into a large property (3000 square foot ) and it has 2 X double extensions on it .
Both of the extended parts of the house have large steel beams supporting the 1st floor joists .
The rear extension has an open plan kitchen with a approx 7m steal RSJ supporting the joists above .
The problem I'm having is that above this part of the house is an ensuite and the extended part of my son's bedroom.
The ensuite has a really bad bouncy floor and all the floor tiles have cracked all over the floor .
In the middle of the floor it has dropped about 30mm
I've had my son's bedroom floor up and I herinbone strutted between the joist and this stopped the bounce .
The ensuite is badly done so I'm ripping it out , I need to sort the floor first before even looking at doing the plumbing work etc .
The joists in my opinion are under spec , 3300 mm span with 7x2 joists ..
The architects drawing asks for 8x3 timber but this hasn't been adhered to .
The extension was signed off by building control
I plan to sister the joist up with lengths of 2400x18mm structural plywood .
Glued and screwed .
I'm going to sister each side of the joist with the ply but I need to create a new level as the floor has dropped in the middle .
I'm unsure what to fill the gap with ontop of the joists as in the middle of the floor where it is dipped the original joist will be lower than the ply either side .
Has anybody any ideas on what I could use to fill this void as I'm out of ideas ?
Sorry for the long text .

Thanks col
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Plywood just won't give you the extra strength or stiffness you require - structural plywood doesn't work like that. Similarly strutting (solid or herringbone) confers stiffness (reduces bounciness) but doesn't add much strength, as you have discovered.

The right way to approach this is to install timber joists of the correct size and structural rating. That will give you a level floor capable of carrying to load with acceptable deflection but will raise the floor level.

Sistering the joists with 7 x 3in C24 timbers (bolted to the originals with M12 coach bolts on centres no greater than 600mm - C24 because it is stiffer than the C16 grade normally specified for domestic builds) would get you quite some way towards where you need to be, especially if you add solid strutting on multiple centres, and may be enough to cure most of the ills of the floor.
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Should you not be talking to your solicitor before rectifying the problem? You may have a claim against the building control inspector for negligence.
Hi , thanks for the replies .
I have thought about going down the legal route but I'm pretty sure this will just end up in a long drawn out battle .
Unfortunately replacing all the joists with 2 inch bigger joist is no go .
The costs involved ,dropping ceilings etc would very high .
I've been told off a structural engineer that structural plywood glued a bolted acting like a fletching plate would cure my problem .
I just wanted to know if anybody had done this before .
Do you think the information I'm getting is flawed ?
Thanks for any help
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Its a recognised fix and depending on the deflection will probably fix it.
Only ever used steel flitch plates. The structural engineers we work with (Arups on the last 4 structural projects I've been on) won't countenance plywood flitch plates because of the problems in getting performance data for plywood in this sort of application, not to mention the impossibility of calculating the strength of an on site produced glue joint. They tend to Insist on proper sistering of joists

Screwing or nailing the sub floor into the edges of the plywood, causing delamination, won't do much for its' strength or integrity, either
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Cheers ,
I won't be screwing into the edges of the ply ,I'll be screwing into the existing joist that the ply sandwiches .
I intended to screw and glue Barton's ontop off the existing joists the plane the batons down to meet the ply at the corrected level .
I'm just digging out the drawings now and building certs to see who I need to contact about it being signed off ..
From a legal perspective, it's not my area, so I did a bit of reading up. Sadly, there are several decided cases which will limit a claim against the inspector unless you can prove some level of deceit/fraud (a very high bar to pass). I was surprised, but they appear to owe no duty to ensure compliance. You might have a claim against the builder but you've no idea if your previous owner didn't agree to the reduced spec, because of fit/fabrication challenges.
Yeah , I thought that would be the case .
I might phone my local authority and ask for advice ,maybe worth spending a bit of money .
There's no way I could rip them out ,I'm looking at a repair really .
Even running steel plates on the sides of each joist sound viable .
Thanks for your input
The steel plates really need structural calcs to be done if only because of the weight issue - badly done the extra weight of the steel could exacerbate the dip in the middle rather than stop it. And you'll still need to add packers to the tops of the joists to carry the floor. Note that it is also possible in some instances to install an extra bearer steel beam below and at right-angles to the joists. These can be sectional (for ease of handling) and will often be hidden inside tight boxing or even inside walls

The reason we often sister is to level the floor whilst keeping the cost down and being able to work from either above or below. Flitch plates, for reasons of weight and safety, are normally installed from below using a Genie lift or the like

As an aside I did wonder if the joists were crowned and some muppet installed them crown down by mistake (it should always be crown up)
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That's exactly what they've done , crown down .
There is a bit if spring hence why I suggested ply to stop it .
I'd put strutting across the middle to stiffen it all up

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