Whole house subfloor refit. Floorboards vs Sheets?

[Deleted member 174758]

Maybe the way to think about this is to consider the unsupported length of a joint. On standard 400mm centres with 50mm thick joists the unsupported length of joint between joists is about 350mm. On your 350mm centres it will be about 300mm. 18mm plywood or P5 chipboard under domestic loading won't really deflect a great deal over such a short span especially as the material is rigidly fixed into both joists (whereas in a shop or office building point loadings may be far higher which explains why we go to greater lengths to add strength in the long joints). The unsupported end of a board will tend to move and droop under loading and whilst you might get away with your solid strutting supporting it, you might find that in service it creaks or cracks under variable loadings as two unsupported ends move independently. Personally I'd rather not take the chance because once your UFH and everything else is down it'll be a helluva task to dig things up to sort out a creaking floor

Is this suitable? I find it easier to source materials from big retailers.

https://www.selcobw.com/products/ti...ood/structural-plywood-ce2-2440-x-1220-x-18mm

Should be. Bear in mind that you'll still need to trim the ends off so that they land on a joist because if your non-standard joist spacings (350mm centres)

 

[JohnD]

Looking at the photo attached, imagine the sheer is a full sheet of ply. It is supported at joist ends but it won’t full on every nogging. Is this okay?

I would say not. I've recently had some floors replaced in 18mm WBP. The sheets are big so don't have many joints, and the ends have been strutted where they aren't on a joist. If you hold a piece of 18mm it feels totally rigid.

But in a few places, you can feel the edges of adjoining pieces flex enough that you can feel a lip when you stand on one piece and the one next to it stays higher (they are not T&G, and are screwed, so at least they're easy to take up and add a strut.)

It seems to be worse on smaller pieces than on the full sheets.

In your picture it looks like you will have a lot of unsupported joints, and I think that will be annoying to you.

With the large sheets, I think it is preferable to have the full sheets in the main part/centre of the room. If you have any trimmage round the edges it will not get much foot traffic, and will also be less noticeable to the eye.

Pay extra attention to any joints near doorways, or where there will be extra traffic, say, in front of a sink or at the bottom of stairs. This is where my previous chipboard floors lasted badly.

 

[ToastyToes]

Maybe the way to think about this is to consider the unsupported length of a joint. On standard 400mm centres with 50mm thick joists the unsupported length of joint between joists is about 350mm. On your 350mm centres it will be about 300mm. 18mm plywood or P5 chipboard under domestic loading won't really deflect a great deal over such a short span especially as the material is rigidly fixed into both joists (whereas in a shop or office building point loadings may be far higher which explains why we go to greater lengths to add strength in the long joints). The unsupported end of a board will tend to move and droop under loading and whilst you might get away with your solid strutting supporting it, you might find that in service it creaks or cracks under variable loadings as two unsupported ends move independently. Personally I'd rather not take the chance because once your UFH and everything else is down it'll be a helluva task to dig things up to sort out a creaking floor


Should be. Bear in mind that you'll still need to trim the ends off so that they land on a joist because if your non-standard joist spacings (350mm centres)

I would say not. I've recently had some floors replaced in 18mm WBP. The sheets are big so don't have many joints, and the ends have been strutted where they aren't on a joist. If you hold a piece of 18mm it feels totally rigid.

But in a few places, you can feel the edges of adjoining pieces flex enough that you can feel a lip when you stand on one piece and the one next to it stays higher (they are not T&G, and are screwed, so at least they're easy to take up and add a strut.)

It seems to be worse on smaller pieces than on the full sheets.

In your picture it looks like you will have a lot of unsupported joints, and I think that will be annoying to you.

With the large sheets, I think it is preferable to have the full sheets in the main part/centre of the room. If you have any trimmage round the edges it will not get much foot traffic, and will also be less noticeable to the eye.

Pay extra attention to any joints near doorways, or where there will be extra traffic, say, in front of a sink or at the bottom of stairs. This is where my previous chipboard floors lasted badly.

I really can't lift the insulation and fit supports for every joint so I assume using a T&G product (probably OSB3 as it is reasonably priced and Selco stock a good product) would be ideal in this scenario?

Or am I really over complicating and thinking this... as I tend to do.

 

[Deleted member 174758]

.... am I really over complicating and thinking this... as I tend to do.

For the third time in this thread - you put in your joists on the WRONG centres without reference to the size of the boards you were intending to install. In fact you hadn't even decided which boards to install before you went ahead and put in the joists (not even an apprentice's error, that one). TBH to me it is immaterial which board you decide to install, but you are now faced with crosscutting each and every board so that the short edges butt up to each other over joists. To make the cut will require a circular saw, a straight edge and either screws or a clamp. It isn't rocket science, but if you butt the short edges of the board anywhere other than over a joist (where they are siupported fully AND can be fixed to the joists) the floor stands a good chance of moving at those joints and creaking at some time in the future. I also hope that you've thought to put in joists near the edges of the room (within about 100mm, ideally), even if it means one of them being "wrongly" spaced - this needs to be done to ensure that the overhang isn't so excessive that the ends of the board at those points flex and bend under load.

 

[ToastyToes]

For the third time in this thread - you put in your joists on the WRONG centres without reference to the size of the boards you were intending to install. In fact you hadn't even decided which boards to install before you went ahead and put in the joists (not even an apprentice's error, that one). TBH to me it is immaterial which board you decide to install, but you are now faced with crosscutting each and every board so that the short edges butt up to each other over joists. To make the cut will require a circular saw, a straight edge and either screws or a clamp. It isn't rocket science, but if you butt the short edges of the board anywhere other than over a joist (where they are siupported fully AND can be fixed to the joists) the floor stands a good chance of moving at those joints and creaking at some time in the future. I also hope that you've thought to put in joists near the edges of the room (within about 100mm, ideally), even if it means one of them being "wrongly" spaced - this needs to be done to ensure that the overhang isn't so excessive that the ends of the board at those points flex and bend under load.

I completely understand boards must end on a joist, my only concern has been the long part of the boards not landing on a nogging.

Joists either end are 50mm from the wall.

 

[Deleted member 174758]

Not an issue, as I already stated. Personally I wouldn't have put in solid strutting because in most domestic floors it simply isn't needed

 

[endecotp]

Because it's not normally used for those purposes. Having talked to a few people today, none of us has ever come across it being used for structural work such as flooring - roofing and wall cladding, yes, sub-flooring on top of a solid concrete sub-base, yes, self-supporting sub-flooring, no. Despite what Norboard say I'd be wary of using it in a flooring project, as would the four colleagues I chatted with today (one of whom happens to be a structural surveyor). One of the things I'd be concerned about is that OSB has a very coarse face making glue bonding, etc tenuous. Also with P5 chipboard the boards have a tongue/groove all round to support them, T&G flooring plywood (generally spruce - specifically manufactured for flooring) there are tongue/grooves on the long edges to permit glueing (the short ends don't need them as they will always be supported on a joist) and if you are installing plain plywood I've always been advised to groove the edges and insert a glued loose tongue if at all possible. The reason for all this is to help prevent squeaking/creaking floors. What about OSB? The nature of OSB will mean that a large amount of a gap-filling adhesive would be required to glue any joints and cutting a groove with a router that would be a thankless and frustrating task.
I'm told that there apparently are also issues when screeding onto it prior to flooring such as vinyl or carpet being fitted which is probably another reason w edon't see it in commercial jobs where almost every floor is screeded these days. BTW I'm not saying it can't be used - just that in the experience of 5 of us (with a combined length of time in the industry of over 150 years) it isn't. But then we aren't trying to market a product

Thanks for taking the time to reply, and sorry ToastyToes for “hijacking” your thread.

FWIW, I’ve used some 2400x600x18mm T&G osb3 (which is T&G on all 4 sides, and has relatively smooth sanded surfaces) for a floating floor, over kingspan, over the original screed. Most of it will probably have carpet eventually. It has been easy to use and the result seems good and solid so far. My feeling is that it would also work OK over joists, but that is a theoretical question as it’s not something I’m planning on doing!

 

[ToastyToes]

Thanks for taking the time to reply, and sorry ToastyToes for “hijacking” your thread.

FWIW, I’ve used some 2400x600x18mm T&G osb3 (which is T&G on all 4 sides, and has relatively smooth sanded surfaces) for a floating floor, over kingspan, over the original screed. Most of it will probably have carpet eventually. It has been easy to use and the result seems good and solid so far. My feeling is that it would also work OK over joists, but that is a theoretical question as it’s not something I’m planning on doing!

@endecotp Regarding the floating floor, what was the process to do this? I have a concrete floor in my conservatory after ripping up the old tiles and screed. I want to get 25mm kingspan on it before I finish it to the same level as the rest of the house

 

[endecotp]

@endecotp Regarding the floating floor, what was the process to do this? I have a concrete floor in my conservatory after ripping up the old tiles and screed. I want to get 25mm kingspan on it before I finish it to the same level as the rest of the house

I don’t know much about UFH so I can’t really comment, but - you can lay insulation boards on the concrete, possibly with a bit of sand to to even up any variations in the level if necessary.