Preference for screws to fix 9mm ply to 22mm chipboard?

[milleniumaire]

My Omnie TorFloor UFH is now in place and has been filled with water at mains pressure with no leaks, so after the pressure test tomorrow I'm ready to overlay the 22mm chipboard with 9mm ply, which will be glued and screwed.

I know fixing plyboard over UFH isn't a great idea but it is required as part of the Omnie specification to ensure the floor has the required structural integrity.

It occurred to me I haven't yet purchased any screws and was thinking of getting the Screw-Tite 2 PZ Double Self-Countersunk Screws 3.5 x 25mm or possibly the 4 x 25mm from Screwfix.

The combined floor thickness will be 31mm, so don't want to risk 30mm as these may push through the chipboard. 25mm seems safe.

Are there better screws or will these do the job?

 

[J K Tiling Cornwall]

I'd use the 4x25 screws. Just out of interest what's the final floor finish? Cheers

 

[milleniumaire]

I'd use the 4x25 screws. Just out of interest what's the final floor finish? Cheers

Once the ply is down, a Tilemaster anti-fracture mat will be fixed, then 600 x 600 x 10mm porcelain floor tiles will be laid.

I'm aware that these days ply isn't the best base for tiling onto when there are so many cement backer boards available, however, the 9mm ply is being laid to provide structural integrity for the 22mm routed Omnie Torfloor boards, according to THEIR specification.

I have also discussed this with a Tilemaster rep as I wasn't sure if the ply should be primed and he was perfectly happy that it would not be an issue and that as the ply was new, primer wasn't necessary as the Tilemaster Ultimate flexible S2 adhesive would bond to the new ply.

I did consider fixing some backer board to the ply, but a) this seemed like overkill and b) it would have made the floor height, which is already going to end up around 26 to 30mm higher than the landing floor, even higher!

 

[Perfect Tiling]

I wouldn't be tiling onto 9mm ply....I'm sure British Standards state it must be 15mm minimum....especially a heated floor with large format tiles. Can you get them to agree to a 6mm cement board on top of the heating. 9mm ply is likely to buckle and cause issues. Also, cement board should be better at heat distribution making it more energy efficient to heat.

 

[milleniumaire]

I'm following the Omnie TorFloor tiling specification, which requires a minimum of 6mm ply screwed and glued to their 22mm routed TorFloor boards. The spec. has been approved/tested by Tilemaster and includes the following layers:

Not sure why they aren't applying the 15mm ply British Standard.

What do you mean by "buckle and cause issues"?

I agree, cement board would be much better over UFH, but Omnie don't approve the use of it as they haven't tested it, which is a shame.

 

[Perfect Tiling]

Hi Milleniumaire......sometimes you see thinner plywood that won't lie flat.....it distorts....and can be made worse with heat. I get that if it's fully glued and then screwed it should stay rigid. But...I'm always dubious and refuse to tile onto plywood...especially if its WBP ply....you make the adhesive up by mixing with water and then stick it to something that is waterproof. I had tiles floors in my last house and tiled onto 154mm ply only for the whole lot to lift off cleanly....found out the ply had a silica coating in it to make it waterproof....hardi or wedi boards now!!!