Bathroom wet UFH floor buildup

[Super_Paulie]

Evenin.

I'm onto planning my bathroom and looking for advice and general opinions, which will be ongoing...

Searching buildhub via Google seems to have gone by the wayside and I can't seem to word what I want in the site search so here I am. 

 

I'm going to extend my UFH to the small bathroom (3x3m) and I'm struggling to find the build-up that would be best. For info the bathroom doesn't really need it, putting it in just to take the edge off a cold floor when barefoot.

 

My initial plan is PIR between joists, pipes clipped to it (or spreaders). 

The original floorboards need to go, so this is where I'm struggling for info. 

Replace subfloor with chipboard/ply? Then if I want to tile, do I need to go over this with say 6mm cement board? Insulated tile backer is adding insulation that I don't need.

I assume I don't tile directly into the subfloor.

 

Cheers friends.

 

 

Edited February 15 by Super_Paulie

[Nickfromwales]

9 minutes ago, Super_Paulie said:

Evenin.

I'm onto planning my bathroom and looking for advice and general opinions, which will be ongoing...

Searching buildhub via Google seems to have gone by the wayside and I can't seem to word what I want in the site search so here I am. 

 

I'm going to extend my UFH to the small bathroom (3x3m) and I'm struggling to find the build-up that would be best. For info the bathroom doesn't really need it, putting it in just to take the edge off a cold floor when barefoot.

 

My initial plan is PIR between joists, pipes clipped to it (or spreaders). 

The original floorboards need to go, so this is where I'm struggling for info. 

Replace subfloor with chipboard/ply? Then if I want to tile, do I need to go over this with say 6mm cement board? Insulated tile backer is adding insulation that I don't need.

I assume I don't tile directly into the subfloor.

 

Cheers friends.

 

 

Go for spreader plates, but allow some rockwool under them, atop the PIR, so the plates are sprung upwards slightly. Then, when you screw the 22mm P5 deck down there will always be great surface contact between the aluminium plates and the underside of the P5. Without this, the heat transfer suffers somewhat.

 

I've never used backer board and only ever 6mm or more of plywood (glued and screwed down) and then tile straight onto the ply. For 6mm ply and 22mm deck you use 25mm 4.0 x 25mm screws to lay the ply so you don't hit a pipe.

[Super_Paulie]

13 minutes ago, Nickfromwales said:

Go for spreader plates, but allow some rockwool under them, atop the PIR, so the plates are sprung upwards slightly. Then, when you screw the 22mm P5 deck down there will always be great surface contact between the aluminium plates and the underside of the P5. Without this, the heat transfer suffers somewhat.

 

I've never used backer board and only ever 6mm or more of plywood (glued and screwed down) and then tile straight onto the ply. For 6mm ply and 22mm deck you use 25mm 4.0 x 25mm screws to lay the ply so you don't hit a pipe.

 

That's what I did in my porch, just forced the chipboard down onto the fibre-bouncy spreaders 👍

 

Thanks Nick, appreciate the info. Any reason for 6mm ply as opposed to a cement based board? I'm thinking for heat transfer.

[Nickfromwales]

39 minutes ago, Super_Paulie said:

 

That's what I did in my porch, just forced the chipboard down onto the fibre-bouncy spreaders 👍

 

Thanks Nick, appreciate the info. Any reason for 6mm ply as opposed to a cement based board? I'm thinking for heat transfer.

I just don't like the brittle nature, and I've done UFH with P5 + plywood so many times with great results, I just don't want to change. If it works, then happy days, plus the plywood conforms to the floor shape / undulations, whereas cement board will just snap or fracture, no Bueno afaic, sorry!

[Super_Paulie]

8 hours ago, Nickfromwales said:

I just don't like the brittle nature, and I've done UFH with P5 + plywood so many times with great results, I just don't want to change. If it works, then happy days, plus the plywood conforms to the floor shape / undulations, whereas cement board will just snap or fracture, no Bueno afaic, sorry!

i wont argue with that experience, thanks Nick.

Just so im on the same page, i always thought the ply screwed over the subfloor was so that if it needs to come up in the future then you dont have to destroy the subfloor. Surely gluing the ply to the subfloor will make that impossible? 
I assume im just overthinking this.

[Nickfromwales]

4 hours ago, Super_Paulie said:

i wont argue with that experience, thanks Nick.

Just so im on the same page, i always thought the ply screwed over the subfloor was so that if it needs to come up in the future then you dont have to destroy the subfloor. Surely gluing the ply to the subfloor will make that impossible? 
I assume im just overthinking this.

Yup. Let’s be honest, if you’re ‘accessing’ the plywood you’ve smashed all the tiles up and binning them so are ‘all in’. 
 

If you want the tiles to stay down, you go ‘all in’ on the installation and enjoy the longevity.

 

You could always fit 27, randomly positioned access panels on the ceiling downstairs.   

[Super_Paulie]

On 16/02/2026 at 13:23, Nickfromwales said:

Yup. Let’s be honest, if you’re ‘accessing’ the plywood you’ve smashed all the tiles up and binning them so are ‘all in’. 
 

If you want the tiles to stay down, you go ‘all in’ on the installation and enjoy the longevity.

 

You could always fit 27, randomly positioned access panels on the ceiling downstairs.   

 

I guess why use the plywood at all? Just to smooth over any wonkyness in the chipboard?

[Nickfromwales]

1 hour ago, Super_Paulie said:

 

I guess why use the plywood at all? Just to smooth over any wonkyness in the chipboard?

If you don't ply, you'll be re-tiling 6 months later. Tile adhesive does NOT take to the P5 chipboard at all. Main source of my income for about a decade, doing insurance work repairing such shoddy (and subsequently failed or failing) work.

 

Stop guessing!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Super_Paulie]

18 minutes ago, Nickfromwales said:

If you don't ply, you'll be re-tiling 6 months later. Tile adhesive does NOT take to the P5 chipboard at all. Main source of my income for about a decade, doing insurance work repairing such shoddy (and subsequently failed or failing) work.

 

Stop guessing!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

🤴 Thank you my liege.

[Nickfromwales]

2 minutes ago, Super_Paulie said:

 

🤴 Thank you my liege.

Arise...........you are worthy.

[Super_Paulie]

Just about to order my sub floor for this, @Nickfromwales would you recommend 6, 9 or 12mm ply over the 22mm P5? Got my spreader plates at the ready.

Edited 23 hours ago by Super_Paulie

[Nickfromwales]

3 hours ago, Super_Paulie said:

Just about to order my sub floor for this, @Nickfromwales would you recommend 6, 9 or 12mm ply over the 22mm P5? Got my spreader plates at the ready.

You select this to your advantage, to balance floor levels from room to room, sir.

 

6mm is plenty, just make sure to glue and screw it down to the P5, and you will be triumphant.

[Nickfromwales]

@Super_Paulie

 

I just noticed that you come from a dodgy area.

 

I worked in A&P Tyne for years, both Hebburn and Wallsend, on RFA contracts. All hail the stotty!

 

Many a curry consumed down Whitley Bay.

[Super_Paulie]

6mm it is, cheers 👍

 

Tyneside? How dare you, I'm a Sunderland lad. 6 easy points, we have a beach etc.

Love a stottie like it had to be said, add in a saveloy with pease puddin and you've nailed it.

[Nickfromwales]

4 minutes ago, Super_Paulie said:

6mm it is, cheers 👍

 

Tyneside? How dare you, I'm a Sunderland lad. 6 easy points, we have a beach etc.

Love a stottie like it had to be said, add in a saveloy with pease puddin and you've nailed it.

Geordies and Maccums scrapping it out on the ships all day was entertaining lol. 
 

My chargehand was a Sunderland lad. 

[Nickfromwales]

3 minutes ago, Super_Paulie said:

6mm it is, cheers 👍

I use PVA with a 3mm notched scraper on the P5, and lay the ply down onto that whilst wet. Then screw the ply down with 4.0 x 25mm screws at 100-120mm centres max.

[garrymartin]

Not for this particular topic, but for future reference @Nickfromwales, can I assume that where you are struggling with floor height differences, you could either rip the P5 up and ply directly to the joists, or you could put ply down instead of P5 for tiled areas in the first place?

 

[BadgerBodger]

Out of curiosity, I see these gypsum and cement based t&g floor boards, I was thinking of using these in my bathrooms with spreader plates below. Ok to tile direct? Thoughts on a postcard… 

[Nickfromwales]

1 hour ago, garrymartin said:

Not for this particular topic, but for future reference @Nickfromwales, can I assume that where you are struggling with floor height differences, you could either rip the P5 up and ply directly to the joists, or you could put ply down instead of P5 for tiled areas in the first place?

 

I always use 22mm P5, and then use the plywood as a binder; staggered joints and glued & screwed. 
 

P5 for strength, and ply as bulking material and a binder. Never had a floor crack so that’s my preferred method.

[Nickfromwales]

28 minutes ago, BadgerBodger said:

Out of curiosity, I see these gypsum and cement based t&g floor boards, I was thinking of using these in my bathrooms with spreader plates below. Ok to tile direct? Thoughts on a postcard… 

Gypsum ones may have issues with adhesion, so triple check those before attempting to stick tiles down. 
 

Cement based T&G boards should work well, but I’m always sceptical as these are brittle and are entirely reliant on the joists being straight etc. Wood bends and conforms, so I just prefer that method.

 

Done plenty with alu spreaders under P5 + plywood + tiles. Works well, but needs higher temps than UFH in screed, so some jobs have needed a separate manifold for the FF bathrooms. 

[Super_Paulie]

I'm off planning on running mine from the return on the bathroom rad/one of the bypass rads. Just snake the return under the floor on its way back to the boiler from the UF system. 

[Nickfromwales]

29 minutes ago, Super_Paulie said:

I'm off planning on running mine from the return on the bathroom rad/one of the bypass rads. Just snake the return under the floor on its way back to the boiler from the UF system. 

At what temp?

[Super_Paulie]

3 hours ago, Nickfromwales said:

At what temp?

The boiler runs at 50, the return temp gets to 40 ish before it hits the cycle. I was planning on using a valve on the bathroom bypass to control the bathroom UF. I have another bypass rad in the hall which is always open.

[Nickfromwales]

4 hours ago, Super_Paulie said:

The boiler runs at 50, the return temp gets to 40 ish before it hits the cycle. I was planning on using a valve on the bathroom bypass to control the bathroom UF. I have another bypass rad in the hall which is always open.

That’ll work. If it’s 50 a lot, or high 40’s you’ll maybe find it a bit too warm. 
 

Perhaps reduce the quantity of pipe to just a squiggle where you’ll be standing when towelling off etc.