Waterproof plywood under tiling?

[macadonald]

Hi Guys,

Another question. I have seen a few references to using a water proof plywood as the base for a bathroom floor to be tiled. Exactly what do I buy? I will lift the old floor boards I have in there and the joists are roughly 400mm spacing.

I have seen a couple of different options at timber merchants (including a grey painted option). Is this grey the sealant? Some posts have suggested sealing or painting the underside of the plywood? With what and why?

Any advice would be great?

On a slightly different note, I have pretty old and noisy floorboards in the upstairs bedrooms and had considered lifting these during all this work and replacing it with a plywood or chip board? To soften the noise of people walking upstairs, thought I'd run some silicon along the top of the joists before screwing down the boards if I do replace these?

 

[Richard C]

I have seen a few references to using a water proof plywood as the base for a bathroom floor to be tiled. Exactly what do I buy? I will lift the old floor boards I have in there and the joists are roughly 400mm spacing.

I have seen a couple of different options at timber merchants (including a grey painted option). Is this grey the sealant? Some posts have suggested sealing or painting the underside of the plywood? With what and why?

You don’t what anything painted grey or with anything else for that matter. What you need is WBP (water & boil proof) ply, thickness 18-25mm; in many cases 18mm is sufficient in a light use/load area such as a bath/shower room but it depends on joist size/pitch/span. Boards should be laid as large as possible, screwed into the joists every 150-200mm max. do not use nails. Tape the joints with a few layers of fiberglass mesh (plasterboard) reinforcing tape, working the tile adhesive well in. Support the ply boards down the centre line of a joist, any edges running across the boards must be supported by noggins; this is not usually necessary on the two walls around the edge of the room unless it’s across the door threshold or subject to loading. The underside & edges should be sealed with an SBR or acrylic primer (not PVA); it’s there to prevent moisture affecting the underside & edges of the ply which can cause warping & lifting, leading to tile failure. I prefer to replace the existing floor with WBP ply but another option is to lay tile backer boards over the existing floor & tile onto that.

Do not prime the tile surface unless the tile or adhesive manufacturer recommends it (most don’t). Use only quality trade adhesive/grout products not cheapo DIY stuff, it’s generally crap. You will need a flexible powder addy & grout for the floor tiles, not tub ready mix. If your also doing the walls, it’s important you do the correct preparation & use the correct materials so read the Tiling Forum sticky & Forum archive posts before you buy any materials or start work, it may avoid you making potentially disastrous & expensive mistakes. Post back in the Tiling Forum if you have any further questions.

On a slightly different note, I have pretty old and noisy floorboards in the upstairs bedrooms and had considered lifting these during all this work and replacing it with a plywood or chip board? To soften the noise of people walking upstairs, thought I'd run some silicon along the top of the joists before screwing down the boards if I do replace these?

Proper floor boards are by far the best you can get & there is no reason why some additional screws (not nails) in these should not stop the creaks & groans; check for pipes & cables first. I would do this rather than replace them as you’re unlikely to fair any better with chip board after a few years, it’s crap. I’ve not heard of anyone running silicone down the joists & you certainly DO NOT want to be doing this under any tiled surface, it will cause flex in the floor & lead to tile failure.

 

[^woody^]

You can leave the existing boards down, and just overboard with some 6mm ply

 

[Richard C]

You can leave the existing boards down, and just overboard with some 6mm ply

Sorry Woody but 6mm is not enough & there will be serious risk of tile failure on all but the very smallest of areas. The BS & minimum trade tile adhesive manufacturers recommend is 15mm WBP but it depends on floor construction. It's generally recognised as being some what OTT by most tilers & 12mm is all you need in most cases. A quality waterproof tile backer board is probably a better alternative for over boarding; this can be 6mm if glued & screwed providing the floor material & construction is up to it but you’d be better going thicker if you want avoid failure.

 

[^woody^]

6mm ply, properly fixed to the existing floorboards is more than enough and no worse than anything else

 

[Richard C]

6mm ply, properly fixed to the existing floorboards is more than enough and no worse than anything else

Woody As a daily forum contributor, I respect the quality of your contributions

But I, BS & most decent pro tilers who have to guarantee their work will disagree with you. 6mm ply will flex too much & even distort when you screw it down to the under floor leaving an uneven tile base; it’s not the same as using a decent 6mm cement based tile backer board which is extremely rigid.

 

[^woody^]

But how can it flex if it is properly fitted?

ie not just a few screws around the edge

 

[noseall]

For 6mm ply you are looking at screws every 2" unless the sub floor is perfect.

 

[joinerjohn]

Have to agree with the other posters ^woody^. 6mm ply would be next to useless for the OP's purpose. We don't know if the existing floorboards are nice and level, or cupped. He's best to remove the old floor boards and lay 18mm WBP ply.

 

[Richard C]

Cement based (or other) tile backer boards are very ridged & are both screwed & glued to the substrate using a powder cement based tile adhesive so it aint going nowhere & the adhesive makes up any discrepancies in level. You could do that with 6mm ply but the point is it will flex & distort to the original floor profile as you screw it down, not good for tiling.

I normally screw down new ply floors (18-25mm) & over boarding (12mm) every 150mm max; 200mm is the recommended max but, same as plasterboard, screws are relatively cheap. Screwing at 50mm will make the floor more ridged but you will still have a problem with distortion on 6mm ply. Do you feel lucky

Personally for the little extra it costs I would never risk it; trust me, amongst other things, I’m a tiler of quality & never had a failure.

 

[^woody^]

Obviously if the floor deck is crap, then that needs to be sorted out before any overboarding or tiling.

But if the OP's floorboards are satisfactory, then he does not have to rip them up to then lay thick plyboard.

If the floorboarding is OK, then he can lay 6mm ply, he can glue it if he wants, or he can just fix it every 150-200.

It's an alternative not an argument.

It may not be suitable in the OP's situation, but the point is laying new 18mm ply is not the only option. In a bathroom, it is much easier and more practical to overboard than to take up and fit new floor deck.

 

[^woody^]

I have pretty old and noisy floorboards .... and had considered ... replacing it with a plywood or chip board?

No, no, no. You want to prevent squeaks, not increase them with chip or ply board

Refix your existing floorboards with screws and remove the nails. If need be take them up and reset them closer together if they have contracted and joints opened, and buy a new floor board for the end if need be, to close the remaining gap

But often just screwing them down does the trick

 

[Richard C]

Woody; if you read my posts I’ve already advise that with proper floor boards, over boarding it perfectly acceptable with either 12mm WBP or a proprietary tile backer board. A 6mm cement based board glued & screwed would be fine after fixing the original boards down securely with extra screws but I would not or ever agree that 6mm WPB ply overboard, is OK, it’s just not rigid enough for the reasons I’ve already stated & that others have already agreed. I generally like to replace as it doesn’t increase the threshold height (which is a real problem for many customers) & I need to be absolutely sure of what I’m tiling onto; hence why I usually rip up & replace.

Glad to see you at least agree with me regarding proper floor boards

 

[macadonald]

Richard/Woody et all,

Thanks for all the useful advise and it looks like we might all be learning from eachother here..

To clear things up. My bathroom floor boards are not great and a bit of a patch work, so I really want to lift them. There is currently a 2-3mm ply glued to them for some old lino.

I think I'll have to go with the suggestion of 18-25mm WBP board and probably even the thicker end of this. Only reason I say this is because the tiles I bought are 300x600mm porcelain tiles and are of the thin/slim kind. They are only about 2-3mm thick.

Anyone have experience or judgement on these slim tiles?

Happy that you don't suggest I lift the old boards in the bedrooms. Screwing down will be much easiear and I'm sure a good underlay will help with the thumping noise of upstairs walking. The current carpets are so old, they probably do little for soundproofing. Either that or stop the wife walkign on her heals...

 

[Richard C]

I think I'll have to go with the suggestion of 18-25mm WBP board and probably even the thicker end of this. Only reason I say this is because the tiles I bought are 300x600mm porcelain tiles and are of the thin/slim kind. They are only about 2-3mm thick.
Anyone have experience or judgement on these slim tiles?

Can you confirm what size/pitch/span the floor joists are? I would probably go for 22mm min or even 25mm. You will also need a really flat base for that size tile so check the joists for any significant bowing or out of level & pack them out if necessary before laying the ply. The tiles will be fine on a sound base but must be fully supported so use a solid bed large format trowel – 20mm half round notches, 13mm deep, at 28mm centres.

Some porcelain tiles don’t need sealing but polished porcs do & some come ready sealed so check with your supplier which you have, more info here;
//www.diynot.com/forums/viewtopic.php?p=1700591#1700591

Happy that you don't suggest I lift the old boards in the bedrooms. Screwing down will be much easiear and I'm sure a good underlay will help with the thumping noise of upstairs walking.

Remember to check for pipes & cables first, don’t flood the place & electrocute yourself.