Bathroom wet UFH floor buildup

[Super_Paulie]

On 15/02/2026 at 23:51, Nickfromwales said:

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.

 

I've bought this from Wickes before I even clicked on I should have got marine ply. 

 

Wickes 6mm ply

 

Will this stuff be ok? I'll be tiling, so priming and flexible adhesive on top of this.

[Nickfromwales]

39 minutes ago, Super_Paulie said:

 

I've bought this from Wickes before I even clicked on I should have got marine ply. 

 

Wickes 6mm ply

 

Will this stuff be ok? I'll be tiling, so priming and flexible adhesive on top of this.

Yup. It'll stick like shit to 2 blankets  I've never bought marine ply as if the water is getting to the plywood, you're already wrapped in both of those blankets!

[Super_Paulie]

12 hours ago, Nickfromwales said:

Yup. It'll stick like shit to 2 blankets  I've never bought marine ply as if the water is getting to the plywood, you're already wrapped in both of those blankets!

 

Great, I'll crack on then. Off work for a few weeks so I'm hoping to get the bathroom somewhere near sorted. Just spent the day setting up a temporary bathroom in the neighbouring bedroom... When you gotta go you gotta go!

[Nickfromwales]

8 hours ago, Super_Paulie said:

 

Great, I'll crack on then. Off work for a few weeks so I'm hoping to get the bathroom somewhere near sorted.

Use LOTS of 4.0 x 25mm screws to fix the plywood down, and if the subfloor is even SLIGHTLY suspect I put the ply down over a 3mm notched bed of neat PVA.

[Super_Paulie]

5 hours ago, Nickfromwales said:

Use LOTS of 4.0 x 25mm screws to fix the plywood down, and if the subfloor is even SLIGHTLY suspect I put the ply down over a 3mm notched bed of neat PVA.

 

That's my plan. 25mm seems scarily close to the pipes though. I've got 20mm and 25mm screws, just to see what I dare put in. Seems 25mm is only an impact driver away from spoiling the whole party with the pipes underneath.

[Nickfromwales]

7 hours ago, Super_Paulie said:

 

That's my plan. 25mm seems scarily close to the pipes though. I've got 20mm and 25mm screws, just to see what I dare put in. Seems 25mm is only an impact driver away from spoiling the whole party with the pipes underneath.

22mm deck or 18mm deck?

 

If 22mm deck, plus 6mm ply, then you’ve got 4mm before you poke out the other side.

 

My impact drill has 4 setting for speed / torque, so is much more manageable, but if you’re not rushing you’ll be fine. 
 

I mark a grid out and go for a minimum of 120 centres, 100mm best practice imho. Never had issues so I don’t deviate from this.

[Super_Paulie]

11 hours ago, Nickfromwales said:

22mm deck or 18mm deck?

 

If 22mm deck, plus 6mm ply, then you’ve got 4mm before you poke out the other side.

 

My impact drill has 4 setting for speed / torque, so is much more manageable, but if you’re not rushing you’ll be fine. 
 

I mark a grid out and go for a minimum of 120 centres, 100mm best practice imho. Never had issues so I don’t deviate from this.

 

22mm.

Impact is DeWalt and just one speed, it often gets a bit speedy. Wonder if I can use one of those drywall type depth restrictor bits. 

 

Probably being overly cautious but the thought of all the work required to sort out a hit pipe under this lot, doesn't bear thinking about quite frankly.

[Nickfromwales]

4 hours ago, Super_Paulie said:

 

22mm.

Impact is DeWalt and just one speed, it often gets a bit speedy. Wonder if I can use one of those drywall type depth restrictor bits. 

 

Probably being overly cautious but the thought of all the work required to sort out a hit pipe under this lot, doesn't bear thinking about quite frankly.

Lol. Squeeze the trigger GENTLY!

[Nickfromwales]

 

👌👍. 
 

As the screws will be PZ2 and these bits are PH2, you’ll need to keep a lot of weight on the impact when you’re sending these home. Should work fine, just test fire a few off to get settled in, and bingo-bango. 👌

[Russell griffiths]

Wera do a pz2 bit that has a collar on the bit holder so you can’t sink them in past flush. 

[Super_Paulie]

On 16/07/2026 at 07:57, Nickfromwales said:

 

I mark a grid out and go for a minimum of 120 centres, 100mm

 

How many screws is this, jesus 😂

[Nickfromwales]

7 hours ago, Super_Paulie said:

 

How many screws is this, jesus 😂

Jesus is my stage name, please use it sparingly........my child. lol.

 

Stop whinging and go buy loads of screws, they're cheaper than shoplifting.

 

https://www.screwfix.com/p/goldscrew-pz-double-countersunk-thread-cutting-multipurpose-screws-4mm-x-25mm-200-pack/17430?tc=ET2&gclsrc=aw.ds&gad_source=1&gad_campaignid=19815174909&gbraid=0AAAAAD8IdPxO0lenZYLD6Z3KcA6zQZjuy&gclid=CjwKCAjwyOzSBhBTEiwAmxvJ-rrQHxbaDIuLj8bFNZ-ErefwWyeaAMa9xO000NkTVbbporAuhnbGrRoCf7QQAvD_BwE

 

Amen.

[Super_Paulie]

9 hours ago, Nickfromwales said:

Jesus is my stage name, please use it sparingly........my child. lol.

 

Stop whinging and go buy loads of screws, they're cheaper than shoplifting.

 

https://www.screwfix.com/p/goldscrew-pz-double-countersunk-thread-cutting-multipurpose-screws-4mm-x-25mm-200-pack/17430?tc=ET2&gclsrc=aw.ds&gad_source=1&gad_campaignid=19815174909&gbraid=0AAAAAD8IdPxO0lenZYLD6Z3KcA6zQZjuy&gclid=CjwKCAjwyOzSBhBTEiwAmxvJ-rrQHxbaDIuLj8bFNZ-ErefwWyeaAMa9xO000NkTVbbporAuhnbGrRoCf7QQAvD_BwE

 

Amen.

 

Aye I've got my screws all ready. Just seems a mental amount to put in but I've already drawn the grid now, looks like a Sunday afternoon job.

 

Ran a few tests, think I'm ok with the depth and 25mm screws, takes a lot to get it to go through the other end of the chipboard to be fair.

[Super_Paulie]

Am I just throwing down a load of PVA onto the P5? Just brush the chipboard down, PVA the lot and go to town with the ply and screws? Seems an easy enough job for today.

[Nickfromwales]

41 minutes ago, Super_Paulie said:

 

Aye I've got my screws all ready. Just seems a mental amount to put in but I've already drawn the grid now, looks like a Sunday afternoon job.

 

Ran a few tests, think I'm ok with the depth and 25mm screws, takes a lot to get it to go through the other end of the chipboard to be fair.

Yup. You’d need to punch them almost completely through the ply to cause any issues, so plenty of time to get off the trigger.
 

Don’t panic if one starts to head in and you didn’t halt at the perfect moment, as that kind of hesitance could mean you're not actually pulling the ply 100% tight to the deck, which is far worse. 
 

Just get the screw to pull in so it’s a little countersunk and job done. 

[Nickfromwales]

3 minutes ago, Super_Paulie said:

Am I just throwing down a load of PVA onto the P5? Just brush the chipboard down, PVA the lot and go to town with the ply and screws?

I use a 3mm notched tile adhesive trowel if it’s a big area, but you can make a ‘comb’ with an 8” bit of the 6mm ply and just cut a load of notches our with a saw. 

 

3 minutes ago, Super_Paulie said:

Seems an easy enough job for today

That’s the spirit, go get after it!  

[Super_Paulie]

Just now, Nickfromwales said:

I use a 3mm notched tile adhesive trowel if it’s a big area, but you can make a ‘comb’ with an 8” bit of the 6mm ply and just cut a load of notches our with a saw. 

 

That's what I'll try my man, thanks for the tip.

[Nickfromwales]

24 minutes ago, Super_Paulie said:

 

That's what I'll try my man, thanks for the tip.

👍👍👍