Best Subfloor for UFH with Suspended Timber Floor?

[hammerandnails]

Hi everyone,

I'm having the majority of the ground floor of my house tiled (35m²) and I'm not sure which way to go from a subfloor / prep perspective. Here is the current set-up:

• Suspended timber floor at 450mm centres
• Wet underfloor heating system between joists in a pug screed
• Tiny bit of deflection (<3mm) in one corner of the room which will be covered by kitchen units anyway
• Large (~600x900mm) porcelain tiles to be the finished floor

Based on cost, floor build-up, ease of installation etc. I've narrowed it down to a few options, and would really appreciate anyone's advice on which one to go for (or something entirely separate!):

Option 1: 18mm t+g plywood on joists > 6mm backerboard > tile

Option 2: 22mm t+g plywood on joists > 6mm backerboard > tile
Option 3: 22mm t+g plywood on joists > ditra mat or dura base > tile
(plywood will be sealed on both sides before installation)
Option 4: STS/NoMorePly 22mm t+g floorboard > tile 

I'd prefer the option with the lowest floor build-up, highest heat output, and the best structural stability. I'd also prefer to be doing as little screwing as possible into a floor with underfloor heating pipes in it! However, I've heard mixed opinions on using ditra/dura matting on plywood so would appreciate your input. 

 

Is plywood not a poor conductor of heat?

What does everyone think??

Edited May 29, 2022 by hammerandnails

[Russell griffiths]

What is stopping the heat going downwards out of the bottom of the screed. 

[hammerandnails]

1 hour ago, Russell griffiths said:

What is stopping the heat going downwards out of the bottom of the screed. 

100mm of PIR underneath the screed. Sorry should have mentioned that! The void beneath is a good 800mm with plenty of air flow too.

[JohnMo]

Options 1, 2 and 3 all put an insulator (plywood) above the UFH pipes.  Option is a cement board and much less of an insulator, so I would go for that.  Could the boards be glued down?

[Russell griffiths]

How about more insulation, loose the screed and fit a board that takes the pipes on top in grooves and tile directly onto it. 

[hammerandnails]

9 hours ago, JohnMo said:

Options 1, 2 and 3 all put an insulator (plywood) above the UFH pipes.  Option is a cement board and much less of an insulator, so I would go for that.  Could the boards be glued down?

Yes, all options involve gluing and screwing to joists, as well as glueing the tongue/grooves. 

[hammerandnails]

7 hours ago, Russell griffiths said:

How about more insulation, loose the screed and fit a board that takes the pipes on top in grooves and tile directly onto it. 

We did consider this, however with our current door and ceiling heights we wanted to minimise floor build-up as much as possible. The pipe and screed on top of the insulation have already been laid so I’m reluctant to rip all that out and pay for a new system!

[JohnMo]

18 minutes ago, hammerandnails said:

Yes, all options involve gluing and screwing to joists, as well as glueing the tongue/grooves. 

Not sure why you asked the question then about minimising screwing, if you are going to answer yourself.  Some glue systems only require screws at the perimeter, where there should be no pipes anyway.

[hammerandnails]

On 30/05/2022 at 17:40, JohnMo said:

Not sure why you asked the question then about minimising screwing, if you are going to answer yourself.  Some glue systems only require screws at the perimeter, where there should be no pipes anyway.

What I meant is that with some options it involves screwing additional backerboard down once the subfloor is screwed down, therefore the joists will not be visible and the risk of hitting an underfloor heating pipe will increase.

[Adsibob]

On 30/05/2022 at 19:22, hammerandnails said:

We did consider this, however with our current door and ceiling heights we wanted to minimise floor build-up as much as possible. The pipe and screed on top of the insulation have already been laid so I’m reluctant to rip all that out and pay for a new system!

I’m confused why you would need T+G boards. Is it because that is meant to act as a structural floor?

i would consider speaking to an SE to find out if a mesh of steel/rebar grid covered in a further 30mm of screed could act as a structural floor, and if so pour that and lay tiles directly onto it 30 days later.

[Temp]

On 29/05/2022 at 18:00, hammerandnails said:

Here is the current set-up:

• Suspended timber floor at 450mm centres
• Wet underfloor heating system between joists in a pug screed
• Tiny bit of deflection (<3mm) in one corner of the room which will be covered by kitchen units anyway
• Large (~600x900mm) porcelain tiles to be the finished floor

 

So currently nothing over the pug screed?

 

I'm not sure if cement board is rated for use as a structural floor but if it is that's what I'd use as it conducts better than plywood.

 

Whatever you do you will need to discuss it with the tiler. He won't warranty his work unless he's happy with what he's laying onto.

[Adsibob]

4 hours ago, Temp said:

I'm not sure if cement board is rated for use as a structural floor but if it is that's what I'd use as it conducts better than plywood.

I’m fairly sure that it’s shear strength is not strong enough, so you can’t use it as a structural floor. 

[Nickfromwales]

2 hours ago, Adsibob said:

I’m fairly sure that it’s shear strength is not strong enough, so you can’t use it as a structural floor. 

Yup.

The deflection is the biggest concern for me. Is it caused by the end of a joist dropping? If so, the whole run would be affected not just the corner. Tiled floors do not like movement, especially if it is to get worse over time due to the degradation of a wooden joist etc.

On 30/05/2022 at 17:22, hammerandnails said:

We did consider this, however with our current door and ceiling heights we wanted to minimise floor build-up as much as possible. The pipe and screed on top of the insulation have already been laid so I’m reluctant to rip all that out and pay for a new system!

Ah. I would have said more insulation under, prob another 50-75mm of rockwool maybe, but that's not happening now. Is the pug mix up dead level with the top of the joists?

12 hours ago, hammerandnails said:

What I meant is that with some options it involves screwing additional backerboard down once the subfloor is screwed down, therefore the joists will not be visible and the risk of hitting an underfloor heating pipe will increase.

Marking out is simple. Chalk line will sort that out in minutes.

Mark the left and right hand side of each end of the joists onto the walls, and then use the chalk line to ping 2 lines on each substrate for accurate fixing ( anywhere between the two lines is safe then ). Use a tape measure and take photos for any intersections / tricky areas, drawing each onto the substrate before even picking up the screw gun . Don't do this when tired / fatigued, as most of my major F_ups have been late on a Friday afternoon after a long week.

 

Lay P5 chipboard deck, not plywood, and go for 22mm thickness if tiling, with that bonded with PU glue at the joist top and a minimum of 5 screws per joist. Then fit a layer of 4 or 6mm plywood atop the P5, glued ( PVA combed on with a mosaic tile trowel ) and screwed, with screws ( 4.0 x 25mm ) going in at 100-120mm OC. With the ply, you 100% won't need the decoupling membrane, as long as the deflection is as you say and no more.  

[hammerandnails]

16 hours ago, Adsibob said:

I’m confused why you would need T+G boards. Is it because that is meant to act as a structural floor?

i would consider speaking to an SE to find out if a mesh of steel/rebar grid covered in a further 30mm of screed could act as a structural floor, and if so pour that and lay tiles directly onto it 30 days later.

The t+g floor is there to act as the structural deck that the UFH company specifies. They don’t say what it needs to be exactly, only that it needs to be a “structural deck” of some sort (they give plywood as an example). The NoMorePly floor is structural. 

[hammerandnails]

16 hours ago, Temp said:

 

So currently nothing over the pug screed?

 

I'm not sure if cement board is rated for use as a structural floor but if it is that's what I'd use as it conducts better than plywood.

 

Whatever you do you will need to discuss it with the tiler. He won't warranty his work unless he's happy with what he's laying onto.

Correct, currently nothing over it as we decide what deck to go with. I’m also leaning more towards the NoMorePly floor as it is structural. I will speak to the tiler to confirm but he says he has filed onto it before. 

[hammerandnails]

11 hours ago, Adsibob said:

I’m fairly sure that it’s shear strength is not strong enough, so you can’t use it as a structural floor. 

https://nomoreply.co.uk/tg4-floorboard/
 

This is the product I’m talking about - it is a structural 22mm floor that can span up to 600mm. Board ends apparently don’t even need to end on a joist. 

[hammerandnails]

8 hours ago, Nickfromwales said:

Yup.

The deflection is the biggest concern for me. Is it caused by the end of a joist dropping? If so, the whole run would be affected not just the corner. Tiled floors do not like movement, especially if it is to get worse over time due to the degradation of a wooden joist etc.

Ah. I would have said more insulation under, prob another 50-75mm of rockwool maybe, but that's not happening now. Is the pug mix up dead level with the top of the joists?

Marking out is simple. Chalk line will sort that out in minutes.

Mark the left and right hand side of each end of the joists onto the walls, and then use the chalk line to ping 2 lines on each substrate for accurate fixing ( anywhere between the two lines is safe then ). Use a tape measure and take photos for any intersections / tricky areas, drawing each onto the substrate before even picking up the screw gun . Don't do this when tired / fatigued, as most of my major F_ups have been late on a Friday afternoon after a long week.

 

Lay P5 chipboard deck, not plywood, and go for 22mm thickness if tiling, with that bonded with PU glue at the joist top and a minimum of 5 screws per joist. Then fit a layer of 4 or 6mm plywood atop the P5, glued ( PVA combed on with a mosaic tile trowel ) and screwed, with screws ( 4.0 x 25mm ) going in at 100-120mm OC. With the ply, you 100% won't need the decoupling membrane, as long as the deflection is as you say and no more.  

I’m not entirely sure what’s causing the slight deflection to be honest. My builder is confident that once the floor deck is on (likely to be 22mm) it will remove it anyway. 
 

Yes, the pug screed is level with the joists now. 
 

Thanks for your advice with the flooring, I’ll chat to my tiler about it. 

[Nickfromwales]

37 minutes ago, hammerandnails said:

My builder is confident that once the floor deck is on (likely to be 22mm) it will remove it anyway.

It is an odd approach to expect the deck material to correct issues with the substructure tbh. It's normally the substructure which is made robust, and the deck is then installed atop as a non structurally-functional overlay for walking / point loading upon.

 

37 minutes ago, hammerandnails said:

Thanks for your advice with the flooring, I’ll chat to my tiler about it. 

Been doing these for a long time, ask my back and knees, so sound, practical advice there  

 

37 minutes ago, hammerandnails said:

Yes, the pug screed is level with the joists now. 

OK, great. You won't struggle to get heat up through the P5 + Plywood, + tiles, just it will take a lot longer to respond ( but will also stay warm for prob 50% of the initial heat up time after you've turned the heating off again ). Choose a self-learning room thermostat and it will calculate the time required to get the room to temp at the time you want it to be. They usually scrub that info and keep 're-learning' so they make allowances for the changes in season. Poor mans weather compensation if you like.

Edited June 2, 2022 by Nickfromwales

[hammerandnails]

16 hours ago, Temp said:

 

So currently nothing over the pug screed?

 

I'm not sure if cement board is rated for use as a structural floor but if it is that's what I'd use as it conducts better than plywood.

 

Whatever you do you will need to discuss it with the tiler. He won't warranty his work unless he's happy with what he's laying onto.

I've had a look online and plywood's conductivity is around 0.13mK whereas this NoMorePly stuff is 0.172mK. Despite it being cement-based, this doesn't seem like a huge difference to me (or am I wrong here?) - especially when compared to a proper anhydrite screed which is more like 2.2mK.

[hammerandnails]

15 minutes ago, Nickfromwales said:

It is an odd approach to expect the deck material to correct issues with the substructure tbh. It's normally the substructure which is made robust, and the deck is then installed atop as a non structurally-functional overlay for walking / point loading upon.

 

Been doing these for a long time, ask my back and knees, so sound, practical advice there  

 

OK, great. You won't struggle to get heat up through the P5 + Plywood, + tiles, just it will take a lot longer to respond ( but will also stay warm for prob 50% of the initial heat up time after you've turned the heating off again ). Choose a self-learning room thermostat and it will calculate the time required to get the room to temp at the time you want it to be. They usually scrub that info and keep 're-learning' so they make allowances for the changes in season. Poor mans weather compensation if you like.

Cheers, Nick. Having spoken to my tiler, his recommendation is as follows, with justification in brackets:

 

1. Plywood deck 22mm (rather than chipboard because the tile adhesive for the backer board adheres better apparently)

2. 6mm Hardie Backer fixed to plywood with adhesive and screws (better finished tile adhesion to the backerboard rather than plywood which tends to "suck" the adhesion out of the tile apparently).

3. Tile on top of backerboard

 

Alternative and simpler option would of course going with a floor like Karndean!

[Nickfromwales]

12 hours ago, hammerandnails said:

Cheers, Nick. Having spoken to my tiler, his recommendation is as follows, with justification in brackets:

 

1. Plywood deck 22mm (rather than chipboard because the tile adhesive for the backer board adheres better apparently)

2. 6mm Hardie Backer fixed to plywood with adhesive and screws (better finished tile adhesion to the backerboard rather than plywood which tends to "suck" the adhesion out of the tile apparently).

3. Tile on top of backerboard

 

Alternative and simpler option would of course going with a floor like Karndean!

1. He's 100% right, tile adhesive sticks to plywood like shit sticks to a fluffy blanket. I would NEVER tile, or recommend anyone tiled onto, chipboard.
2. This is where we differ. Hardie backer board has almost zero flex, buy a sheet and see for yourself. Completely unsuitable for a suspended timber floor with known 'possible' deflection where tiles are to be applied, AFAIC. When tiling over ANY substrate that is porous / dry, it is 1st year apprentice practice to dampen or wet the substrate, and, if necessary, also then utilise a recognised and suitable pre-tile primer to key and 'size' the substrate. Your chap mentions this problem, but goes on to say he would use the same adhesive over the plywood subfloor ( under the Hardie ) to bond that down? Yes, the screws..........If that solution relies on the screws, your in a very precarious position. I've never tiled a floor with this method. I've done "more than one" of these in my days as a tiler.....with some budgets as high as £25k for one room.
3. Tile + adhesive would very happily sit on the 4 or 6mm plywood I mentioned earlier. A significant amount more than the equivalent adhesion to the Hardie, as per your chaps own admission
4. Karndean / other LVT would also need the plywood layer over the subfloor deck board. Plus a feathering compound,  used to fill the screw-holes and fill the board joints, to give a flawless finish. No free lunches here i'm afraid      

[hammerandnails]

10 hours ago, Nickfromwales said:

1. He's 100% right, tile adhesive sticks to plywood like shit sticks to a fluffy blanket. I would NEVER tile, or recommend anyone tiled onto, chipboard.
2. This is where we differ. Hardie backer board has almost zero flex, buy a sheet and see for yourself. Completely unsuitable for a suspended timber floor with known 'possible' deflection where tiles are to be applied, AFAIC. When tiling over ANY substrate that is porous / dry, it is 1st year apprentice practice to dampen or wet the substrate, and, if necessary, also then utilise a recognised and suitable pre-tile primer to key and 'size' the substrate. Your chap mentions this problem, but goes on to say he would use the same adhesive over the plywood subfloor ( under the Hardie ) to bond that down? Yes, the screws..........If that solution relies on the screws, your in a very precarious position. I've never tiled a floor with this method. I've done "more than one" of these in my days as a tiler.....with some budgets as high as £25k for one room.
3. Tile + adhesive would very happily sit on the 4 or 6mm plywood I mentioned earlier. A significant amount more than the equivalent adhesion to the Hardie, as per your chaps own admission
4. Karndean / other LVT would also need the plywood layer over the subfloor deck board. Plus a feathering compound,  used to fill the screw-holes and fill the board joints, to give a flawless finish. No free lunches here i'm afraid      

Hi Nick,

Thanks again for the extensive reply,  I appreciate it. Quick question about the Karndean - if the "subfloor deck board" is the 22mm plywood, surely I wouldn't need another layer over this, and could fix the Karndean straight to it?

 

Cheers

[Nickfromwales]

40 minutes ago, hammerandnails said:

Hi Nick,

Thanks again for the extensive reply,  I appreciate it. Quick question about the Karndean - if the "subfloor deck board" is the 22mm plywood, surely I wouldn't need another layer over this, and could fix the Karndean straight to it?

 

Cheers

At your own peril. Any decent installer should refuse to lay directly over a deck board. You need the plywood as a binder, and to allow you to stagger the joints so movement in the deck doesn’t transfer to the LVT, which it most definitely will. 

[hammerandnails]

On 03/06/2022 at 09:50, Nickfromwales said:

At your own peril. Any decent installer should refuse to lay directly over a deck board. You need the plywood as a binder, and to allow you to stagger the joints so movement in the deck doesn’t transfer to the LVT, which it most definitely will. 

So, having just spent the majority of the bank holiday ensuring the entire area is deflection free I'm now confident that the floor is rock solid. Taking into consideration ease of installation, floor height build-up, thermal conductivity, and structural properties, I'm leaning heavily towards the NoMorePly boards. Is there anything I might be overlooking here? Only downside is the cost (~£300 more expensive than the ply + backerboard).

[embra]

Hey @hammerandnails, what did you go for in the end? I'm also interested in using the NoMorePly TG4 boards on a project