Whole house subfloor refit. Floorboards vs Sheets?

[ToastyToes]

Hi all,

I am installing new flooring in my house refurb and wasn't sure whether I should install standard pine floorboards or some sort of sheet material like T&G chipboard or OSB.

I'm not a fan of sheet material as I find it harder to install if walls aren't square but I am finding it hard to find 18x150 floorboards.

Also if I go ahead with floorboards, do they need to be treated?

I plan to install underfloor heating panels on the flooring before the finished flooring.

Thanks!

 

[Deleted member 174758]

OSB isn't really a suitable material for sub-flooring AFAIK, which leaves you with plywood or chipboard in the sheet materials.

As to 18 x 150 planking, why so thin? Softwood in my experience is installed at a minimum 25mm thickness (actually 22mm finished) and TBH 18mm seems a bit on the thin side. Because the sizes quoted for T&G softwood are always "ex" sizes (i.e the size of timber before machining) if you ask for ex-18 x 150 you'll actually receive something like 15 x 144 finished which is way too thin. Ask for ex-25 x 150 and what arrives will be around 22 x 144mm. Softwood T&G flooring isn't normally treated simply because you don't expect it to come in contact with water (timbers installed in roofs to support Veluxes and the like are somewhat different - there you really do need treated timbers).

My biggest concern with using softwood sub-flooring for UFH is that it will probably shrink to an extent when the heating comes on and with so many joints I'd have to be concerned about the floor creaking in due course. P5 T&G chipboard just doesn't do that (providing you've installed the joints tightly and glued and screwed everything adequately) and handling 2400 x 600 pieces is a lot less onerous than manhandling 2440 x 1220 plywood (which will also need to be cut to fit the joists to ensure that any joints fall on and are supported by the joists)

In so far as cutting to size goes, what's the issue? No planking or sheet material is ever going to fit exactly, so you'll need to cut it somewhere, even if it's only to allow installation of the sheet material in a hit and miss (brick) pattern across the joists. The cuts by the walls should be covered by the skirtings when they are installed, so you don't have to be super accurate cutting it or tremendously neat.

 

[ToastyToes]

OSB isn't really a suitable material for sub-flooring AFAIK, which leaves you with plywood or chipboard in the sheet materials.

As to 18 x 150 planking, why so thin? Softwood in my experience is installed at a minimum 25mm thickness (actually 22mm finished) and TBH 18mm seems a bit on the thin side. Because the sizes quoted for T&G softwood are always "ex" sizes (i.e the size of timber before machining) if you ask for ex-18 x 150 you'll actually receive something like 15 x 144 finished which is way too thin. Ask for ex-25 x 150 and what arrives will be around 22 x 144mm. Softwood T&G flooring isn't normally treated simply because you don't expect it to come in contact with water (timbers installed in roofs to support Veluxes and the like are somewhat different - there you really do need treated timbers).

My biggest concern with using softwood sub-flooring for UFH is that it will probably shrink to an extent when the heating comes on and with so many joints I'd have to be concerned about the floor creaking in due course. P5 T&G chipboard just doesn't do that (providing you've installed the joints tightly and glued and screwed everything adequately) and handling 2400 x 600 pieces is a lot less onerous than manhandling 2440 x 1220 plywood (which will also need to be cut to fit the joists to ensure that any joints fall on and are supported by the joists)

In so far as cutting to size goes, what's the issue? No planking or sheet material is ever going to fit exactly, so you'll need to cut it somewhere, even if it's only to allow installation of the sheet material in a hit and miss (brick) pattern across the joists. The cuts by the walls should be covered by the skirtings when they are installed, so you don't have to be super accurate cutting it or tremendously neat.

Thank you very much for your reply.

18mm finished level was the idea. I'd buy wood to suit this.

How well of a competitor is Plywood to chipboard? I'm more concerned about quality and it lasting many years then cost.

If plywood is suitable, what grade or type is best?

Thank you so much

 

[Deleted member 174758]

Chipboard is cheaper, and if you go to P5 grade it's about as waterproof as WBP (exterior grade) plywood. The problems you have with plywood are that it only comes in 8 x 4ft (Imperial-size) sheets which are large, heavy and awkward to handle (especially if working alone in a room with no floor). In order to drop the sheets onto the joists, which in a new build should be on metric centres (i.e. 400mm not 16in or about 407mm) it will be necessary to trim the ends of each and every plywood sheet you lay because they will all be 40mm too long. But you say that you can't cut stuff. Well, you are going to have to be able do that to install any sort of sub-floor. Aghainst that P5 T&G chipboard comes in 2400x 600 pieces which are not only far lighter, but also drop straight onto 400mm centre joists. Chipboard has been around in housing since the 1950s and whilst it isn't as durable as plywood it should see you out. The only time you get problems with it is when it gets soaked repeatedly - and if that were to happen your UF heating would have been wrecked in any case. Like any other material it needs to be properly installed, i.e glued and screwed on the correct centres with the joints nice and tight (for which I tend to use a 6ft length of 4 x 2 softwood and a 14 or 28lb sledge hammer.....)

The other thing I'm concerned about is that you seem dead set on having an 18mm sub floor. Why? Were I doing this job for myself I'd want to replicate the standard thickness of a timber floor, about 22mm, or possibly go thicker to 25mm (depending on the thickness of the screed and/or tile), if only because the extra weight loading you'll be placing on the floor by adding UF heating, screed and presumably tile really requires as much rigidity and strength as you can get in the sub-floor. I think that 18mm is too thin and it will potentially move or creak in use, if not immediately then in the future. Getting the structural part of this right should be your first concern, even if it means having a transition in the floor or dropping the joists slightly. Otherwise you'll be spoiling the ship for a happorth of tar, as they say

 

[ToastyToes]

Chipboard is cheaper, and if you go to P5 grade it's about as waterproof as WBP (exterior grade) plywood. The problems you have with plywood are that it only comes in 8 x 4ft (Imperial-size) sheets which are large, heavy and awkward to handle (especially if working alone). In order to drop the sheets onto the joists, which should be on metric centres (i.e. 400mm not 16in or about 407mm) it will be necessary to trim the ends of each and every plywood sheet you lay. But you say that you can't cut stuff. Well, you are going to have to do that to install any sort of sub-floor. The P5 T&G chipboard comes in 2400x 600 pieces which are not only far lighter, but also drop straight onto 400mm centre joists. Chipboard has been around in housing since the 1950s. The only time you get problems with it is when it gets soaked repeatedly - and if that were to happen your UF heating would have been wrecked in any case

The other thing I'm concerned about is that you seem dead set on having an 18mm sub floor. Why? Were I doing this job for myself I'd want to replicate the standard thickness of a timber floor, about 22mm or possibly 25mm (depending on the thickness of the screed and/or tile), if only because the extra weight loading you'll be placing on the floor by adding UF heating, screed and presumably tile really requires as much rigidity and strength as you can get in the sub-floor. I think that 18mm is too thin and will potentially move or creak in use. Getting the structural part of this right should be your biggest concern, even if it means having a transition or dropping the joists slightly. Otherwise you'd be spoiling the ship for a happorth of tar, as they say

In hindsight, I would have made many changes to how I laid my new ground floor joists, but I installed 75x100 joists with 1.5m clear spans at 350 centres. Noggings every 2ft or so. I'm using cement overlay UFH boards which are 18mm think screwed to the subfloor. Another 6mm ply on top screwed into the overlay boards, with either engineered or vinyl as the finished floor. I could use 22mm subfloor but I'm concerned I'm losing too much room height with an already very low ceiling. 2.2m in the kitchen with an even lower arch into the conservatory.

I regret not remove 1 course of brick on my dwarf walls and using large joists with a pipe in screed system (much cheaper to buy and install UFH). But I can't do much about that now.

Is 18mm ply at 350 centres adequate? Is the WBP plywood what I should be buying?

Thanks for your help.

 

[Deleted member 174758]

On 350mm centres I think plywood makes better sense than chipboard - just remember that you still need to support the end joints in the plywood sub-floor so you will need a circular saw, a straight edge (although a 6ft level would do) and a couple of clamps to make the sizing cuts. Standard 18mm WBP plywood should be OK but I'd screw to the joists on something like 200mm screw centres. The plywood need only to be construction grade (i.e. it can have surface defects, knots, repairs, etc as it'll never be seen) - ask your timber merchant for the best deal, which is probably going to be a Chinese sourced plywood, or even look for salvaged boards (generally new boards with a defect such as oil staining or over-orders from construction job. You didn't mention the 18mm cement board earlier; I'd recommend screwing that through into the joists 65/70mm screws) and I'd also screw to the plywood on 200 to 300mm centres (40mm screws) which make for a more rigid sub floor. Be careful to set-out the cement boards so that the joints are all staggered from the plywood joints (if needs be make a few cuts to accommodate this) and lay the boards at right angles to the plywood. Wear a good quality mask when cutting cement board (outside!) as it is terribly dusty stuff to work with (and use a cheap blade as it wrecks TCT saw blades or better still a PCD blade designed for the task).

 

[ToastyToes]

On 350mm centres I think plywood makes better sense than chipboard - just remember that you still need to support the end joints in the plywood sub-floor so you will need a circular saw, a straight edge (although a 6ft level would do) and a couple of clamps to make the sizing cuts. Standard 18mm WBP plywood should be OK but I'd screw to the joists on something like 200mm screw centres. The plywood need only to be construction grade (i.e. it can have surface defects, knots, repairs, etc as it'll never be seen) - ask your timber merchant for the best deal, which is probably going to be a Chinese sourced plywood, or even look for salvaged boards (generally new boards with a defect such as oil staining or over-orders from construction job. You didn't mention the 18mm cement board earlier; I'd recommend screwing that through into the joists 65/70mm screws) and I'd also screw to the plywood on 200 to 300mm centres (40mm screws) which make for a more rigid sub floor. Be careful to set-out the cement boards so that the joints are all staggered from the plywood joints (if needs be make a few cuts to accommodate this) and lay the boards at right angles to the plywood. Wear a good quality mask when cutting cement board (outside!) as it is terribly dusty stuff to work with (and use a cheap blade as it wrecks TCT saw blades or better still a PCD blade designed for the task).

You're a star, thanks for the advice!

 

[ToastyToes]

On 350mm centres I think plywood makes better sense than chipboard - just remember that you still need to support the end joints in the plywood sub-floor so you will need a circular saw, a straight edge (although a 6ft level would do) and a couple of clamps to make the sizing cuts. Standard 18mm WBP plywood should be OK but I'd screw to the joists on something like 200mm screw centres. The plywood need only to be construction grade (i.e. it can have surface defects, knots, repairs, etc as it'll never be seen) - ask your timber merchant for the best deal, which is probably going to be a Chinese sourced plywood, or even look for salvaged boards (generally new boards with a defect such as oil staining or over-orders from construction job. You didn't mention the 18mm cement board earlier; I'd recommend screwing that through into the joists 65/70mm screws) and I'd also screw to the plywood on 200 to 300mm centres (40mm screws) which make for a more rigid sub floor. Be careful to set-out the cement boards so that the joints are all staggered from the plywood joints (if needs be make a few cuts to accommodate this) and lay the boards at right angles to the plywood. Wear a good quality mask when cutting cement board (outside!) as it is terribly dusty stuff to work with (and use a cheap blade as it wrecks TCT saw blades or better still a PCD blade designed for the task).

Just a thought, why do sheet materials such as plywood need the end joints supported if they aren't tongue and groove, yet traditional floorboards don't?

I've noticed a problem. My noggings are staggered with insulation filled to the top of the joists so I can't installed supports for the joints... any solution to this?

 

[Deleted member 174758]

Just a thought, why do sheet materials such as plywood need the end joints supported if they aren't tongue and groove, yet traditional floorboards don't?

But they do! Planking and sheet materials are both laid in the same direction - at right angles to the floor joists - and if you are laying softwood planking and the end of a plank is left unsupported you will always have to cut it back so that it's supported on a joist. Same goes for sheet materials, you always need to support the joints on a joist. This is why if you know you'll be using Imperial size boards - and all plywood is Imperial - you set your joists at divisions of 8ft (2440mm), so 16in and 12in become natural centres. On the other hand if your joists were carrying plasterboard on the underside (as in a ceiling), because plaster boards are only available in metric sizes (which are slightly smaller than the Imperial "equivalents" - 8 x 4ft translates to 2400 x 1200mm in PB) you'd need to go to 400 or 300mm centres if you want to avoid cutting sheets (and believe me if you are laying a couple of thousand square feet of plywood sub-flooring you'd want to do the minimum amount of cutting humanly possible). Unfortunately you were unaware of this and set your centres at 350mm, so you've effectively "condemned" yourself to cutting every board as a consequence. A small circular saw and a home made straight line guide (or a 6ft level) plus a couple of clamps is the cure

When you get to the cement board/backer board layer you should already have a fully screwed and supported plywood sub-floor so the joint positioning is immaterial - the plywood will always support the end joints wherever they fall

I've noticed a problem. My noggings are staggered with insulation filled to the top of the joists so I can't installed supports for the joints... any solution to this?

Then cut to length so that the plywood boards join on top of a joist (as I've been saying all along)

 

[ToastyToes]

But they do! Planking and sheet materials are both laid in the same direction - at right angles to the floor joists - and if you are laying softwood planking and the end of a plank is left unsupported you will always have to cut it back so that it's supported on a joist. Same goes for sheet materials, you always need to support the joints on a joist. This is why if you know you'll be using Imperial size boards - and all plywood is Imperial - you set your joists at divisions of 8ft (2440mm), so 16in and 12in become natural centres. On the other hand if your joists were carrying plasterboard on the underside (as in a ceiling), because plaster boards are only available in metric sizes (which are slightly smaller than the Imperial "equivalents" - 8 x 4ft translates to 2400 x 1200mm in PB) you'd need to go to 400 or 300mm centres if you want to avoid cutting sheets (and believe me if you are laying a couple of thousand square feet of plywood sub-flooring you'd want to do the minimum amount of cutting humanly possible). Unfortunately you were unaware of this and set your centres at 350mm, so you've effectively "condemned" yourself to cutting every board as a consequence. A small circular saw and a home made straight line guide (or a 6ft level) plus a couple of clamps is the cure

When you get to the cement board/backer board layer you should already have a fully screwed and supported plywood sub-floor so the joint positioning is immaterial - the plywood will always support the end joints wherever they fall


Then cut to length so that the plywood boards join on top of a joist (as I've been saying all along)

Sorry I don’t think my reply was clear. Subfloor ends will be supported at a joist, however there won’t be support for the ends which go against the joists.

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?

 

[endecotp]

OSB isn't really a suitable material for sub-flooring AFAIK

Why do you say that?

Here is a link to Norboard’s T&G OSB web page:
https://norbord.co.uk/our-products/sterlingosbzero/sterlingosb-zero-tongue-groove/

The first FAQ is can OSB be used for flooring, and the answer is “Yes, in fact OSB3 15mm has the same strength properties as 18mm P5 chipboard flooring. In addition, 18mm OSB3 can be a substitute for 22mm P5.”

 

[Deleted member 174758]

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

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?

View attachment 155924

View attachment 155923

Yes. The important thing is to support the ends. If you use a T&G product such as P5 chipboard you'll get additional support along the long edges.

 

[ToastyToes]

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, flooring, no. Despite what Norboard say I'd be wary of using it in a flooring project, as would my four colleagues (one of whom is 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. There apparently are also issues when screeding onto it prior to flooring such as vinyl or carpet being fitted

Hi @JobAndKnock, thanks for your reply regarding OSB3.

Could you check my images in my previous post and let me know what you think?
Thanks for your help

 

[Notch7]

You only need the ends of your boards supported on a joist.

So on an 8ft x 4ft sheet, the 8ft side that runs perpendicular to your joists does not need supporting with noggins.

 

[ToastyToes]

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


Yes. The important thing is to support the ends. If you use a T&G product such as P5 chipboard you'll get additional support along the long edges.

You only need the ends of your boards supported on a joist.

So on an 8ft x 4ft sheet, the 8ft side that runs perpendicular to your joists does not need supporting with noggins.

Thanks both.

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