Unprecedented idea for floating floor

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I removed some old 27mm T&G flooring. To remove bounce, I then double-sistered and leveled the 75mm floor joists with 2x 45mm sisters. Now I'm thinking about how to install the new 20mm engineered floorboards.

I have 7mm spare. The final height of the floor cannot be raised.

Ideally, I'd make a floating floor, but could screw them directly to joists if not possible.

How crazy is my idea in the attached image?
 

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You vould just lay 6mm ply across the top of the lot, glued and screwed to the joists. If you need to decouple just lay 7mm thick soundproofing on top of the joists/sisters instead. No need for the OSB.

What is your finished flooring made from? Will it actually carry load on the unsupported bits between the joists?

To fully remove the bounce you could have just inserted 75mm deep solid strutting rather than sistering the joists - this is the conventional method and is well proven. It would have been faster, used far less material and added less load to the floor. Adding 45mm deep sisters may or may not give you the desired additional stiffness. Bounce is caused by the joists twisting under load, hence solid strutting being the solution
 
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Whats the purpose does the osb strips?
It holds the T&G flooring surface together tightly.

You vould just lay 6mm ply across the top of the lot, glued and screwed to the joists. If you need to decouple just lay 7mm thick soundproofing on top of the joists/sisters instead. No need for the OSB.

What is your finished flooring made from? Will it actually carry load on the unsupported bits between the joists?

To fully remove the bounce you could have just inserted 75mm deep solid strutting rather than sistering the joists - this is the conventional method and is well proven. It would have been faster, used far less material and added less load to the floor. Adding 45mm deep sisters may or may not give you the desired additional stiffness. Bounce is caused by the joists twisting under load, hence solid strutting being the solution
6mm ply is great, but it leaves only 1mm for soundproofing on top.

I could lay 7mm soundproofing on top of the joists, but I still need to hold the T&G flooring together somehow, and the flimsy membrane wouldn't cope with this. I also couldn't screw through the membrane into the joists, as this would void the decoupling.

I wasn't aware that joist twisting was to blame for bounce, I talked to a structural engineer who agreed that sistering was a good plan. I also replaced the noggins with more beefy wood.
 
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I wasn't aware that joist twisting was to blame for bounce
Well, it is, and the standard approach to deal with bouncy floors is the one I stated above. Your S/E should have known this, but there are certainly some S/Es out there who don't realise this (normally the ones straight out of college in my experience)

I talked to a structural engineer who agreed that sistering was a good plan
Then he should have told you that sistering timbers should be the same depth as the original timbers, i.e 75mm. So "beefy" is less of an issue than depth

I also replaced the noggins with more beefy wood.
In a floor they are normally called strutting (and there is a technical definition of constitutes solid strutting) - noggins are generally in walls. Unless your solid strutting is the same or greater thickness than the original joists and at least 80% of the depth (i.e. minimum 60mm deep) they won't be effective as strutting

I'm not saying that what you have done won't work, but I am saying you could have achieved the same result with less work and less materials

What I am still concerned about is the structure of the new flooring material. Does the manufacturer specifically state that it is capable of carrying structural loads? In my experience most laminate and engineered flooring isn't rated to carry loads, it is instead designed to be laid on a sub-floor which is structural, but which you have done away with by lifting the planked sub-floor. What does your finished flooring manufacturer say?
 
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Well, it is, and the standard approach to deal with bouncy floors is the one I stated above. Your S/E should have known this, but there are certainly some S/Es out there who don't realise this (normally the ones straight out of college in my experience)

Then he should have told you that sistering timbers should becthe same depth as the original timbers, i.e 75mm.

In a floor they are normally called strutting - noggins are in walls. Unless your solid strutting is the same or greater thickness than the original joists and at least 80% of the depth (i.e. 60mm deep) they won't be effective as strutting

I'm not saying that what you have done won't work, but I am saying you could have achieved the same result with less work and less materials

I misread your previous previous message. To clarify, the original joists are 220mm deep and 75mm wide. The sisters are 220mm deep and 45mm wide. My S/E friend is actually quite old, and so is maybe only familiar with the tried and tested overkill timber methods for floor stabilisation.

The new strutting (what I was calling noggins) is made of the sister offcuts, so also 220mm deep and 45mm wide.

You're right in that I feel I could have saved a lot of time and money by perhaps opting to add only extra strutting or something. Regardless, it is extremely solid feeling now.

Anyway, this all feels a little off topic as I've finished fortifying the floor now. I'm only really interested in hearing feedback of my proposed sound proofing idea.
 
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I misread your previous previous message. To clarify, the original joists are 220mm deep and 75mm wide. The sisters are 220mm deep and 45mm wide. My S/E friend is actually quite old, and so is maybe only familiar with the tried and tested overkill timber methods for floor stabilisation.
Well I am quite old, too, and the tried and tested method of stiffening floors is the one I quoted in most cases - unless your unsupported span for a 9 x 3in joist is considerably more than about 26 feet on 16in centres. Strutting to stiffen floors even appears in Victorian texts, like Tredgold's, and I come across it from regularly "in the flesh" in Victorian buildings that I do refurbs on, although TBH herringbone strutting is more common than solid strutting

Anyway, this all feels a little off topic as I've finished fortifying the floor now. I'm only really interested in hearing feedback of my proposed sound proofing idea.
OT for you, maybe, but not for someone doing a search in the future.

Decoupling with sound proofing works, but your new flooring will need to have structural strength - does it? In fact, fixing the ceiling below to suspended resilient bars and using soundblock plasterboard probably works better, especially if you also fill the voids between the joists with mineral wool batting cut tightly and packed in. Whilst the ceiling below might not be feasible, the mineral wool is
 
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Well I am quite old, too, and the tried and tested method of stiffening floors is the one I quoted in most cases - unless your unsupported span for a 9 x 3in joist is considerably more than about 26 feet on 16in centres. Strutting to stiffen floors even appears in Victorian texts, like Tredgold's, and I come across it from regularly "in the flesh" in Victorian buildings that I do refurbs on, although TBH herringbone strutting is more common than solid strutting

The unsupported span is 16.4 feet. I have other rooms to go, so I'll certainly look into strutting as supposed to sistering. Although more likely I'll just use shims to level these floors as the spans are quite short and it's not worth taking out the small amount of bounce.

OT for you, maybe, but not for someone doing a search in the future.

Fair enough.

Decoupling with sound proofing works, but your new flooring will need to have structural strength - does it?

The new flooring is 20mm and is approved for spanning joists. It should be very strong when installed. The fact that it doesn't require chipboard panels underneath gave me the idea for floating it in my attached picture.

In fact, fixing the ceiling below to suspended resilient bars and using soundblock plasterboard probably works better, especially if you also fill the voids between the joists with mineral wool batting cut tightly and packed in. Whilst the ceiling below might not be feasible, the mineral wool is

Yes, ideally I would simply decouple the ceiling. I should have mentioned that my property is a tenement flat, with a different property owner below, so I can't touch the ceiling.
 
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The unsupported span is 16.4 feet.
if your joist centres are 16in and you are getting bounce I'd normally take a look at the masonry pockets that the joists terminate in to see if they were a snug fit, or if there is any rot in the joist ends. 16.4ft really isn't a huge distance for a 9 x 3in joist, unless the timber is almost balsa (and I have had that, too). Traditionally you start installing a centre line of herringbone or solid strutting above around 14 or 15ft span, so I'd have expected to see a single centre row of strutting at that span in your floor (assuming something like 16in centres)
I have other rooms to go, so I'll certainly look into strutting as supposed to sistering. Although more likely I'll just use shims to level these floors as the spans are quite short and it's not worth taking out the small amount of bounce
You'd probably find that 3 x 2in or 4 x 2in CLS fixed with loads of 6 x 100mm screws would be more than adequate if you run a row or two of solid strutting. Full sistering the joists both sides would be a b***s-ache of a job to do on 3in thick material with 16in centres. It is hard enough (i.e. awkward) when doubling-up to centre drill the bolt holes and get the bolts through (we used M12 x 180 bolts on the last job, often with only 250 to 260mm clearance to get the angle drill head in)
The new flooring is 20mm and is approved for spanning joists. It should be very strong when installed. The fact that it doesn't require chipboard panels underneath gave me the idea for floating it in my attached picture.
As it is structurally rated you should be OK, then.I assume the joints will be glued
Yes, ideally I would simply decouple the ceiling. I should have mentioned that my property is a tenement flat, with a different property owner below, so I can't touch the ceiling
In that case adding mineral wool batting between the joists will benefit you
 
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if your joist centres are 16in and you are getting bounce I'd normally look at the masonry pockets that the joists terminate in to see if they were a snug fit, or if there is any rot in the joist ends. 16.4ft really isn't a huge distance for a 9 x 3in joist, although traditionally you start installing a centre line of herringbone or solid strutting above around 14 or 15ft span, so I'd have expected to see a single centre row of strutting at that span (on 16in centres)

It was a bit tricky to tell, but there were no signs of rot or rotational play. The room has a bay window, and a double perpendicular joist just before the bay. The 16.4 ft joist spans from an internal wall, to this perpendicular joist, so there isn't really a possibility of rot due to both ends being internal. The 4 ft joists between the perpendicular joist and the bay window did have a little bit of play.

You're right, there were two lines of strutting (either side of the fireplace) along the 16.4 ft span.

You'd probably find that 3 x 2in or 4 x 2in CLS fixed with 6 x 100mm screws would be more than adequate

I might consider that too.

As it is structurally rated you should be OK, then

Good good.

In that case adding mineral wool batti g between the joists will benefit you

Way ahead of you. For this room, the Victorians put 1.2 tons of fly ash between the joists. I removed it all to add sisters. I have put back the shelf (this time made of fireproof plasterboard) and will retrofit the same weight/m2 of Quietex deadening. (regs ask for >80 kg/m2).
 

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