Subfloor strength

[68breakbreak]

Hi,

Please could somebody let me know whether they think my subfloor is strong enough to support ceramic tiles.

It has been constructed as shown in the attached image with 47x75mm timbers, 400mm supports, and with 18mm plywood screwed on top. Plan was rio install 6mm cement board, and tile straight on this.

I can't use one of the online deflection calculators because of the unusual way it has been constructed.

Any advice somebody could give me as to whether my floor is strong enough for tiles would be much appreciated.

View media item 68719 View media item 68718

 

[Skimmers]

you could use 18mm22mm ply then the tile board stuck with thin set then screwed down then ditra matting over that id feel better doing this

 

[68breakbreak]

Thanks, do you think the joist structure is ok with regards to deflection? I'm concerned there is deflection when somebody jumps up and down at the weakest part of the structure

 

[Skimmers]

as long as you have packed any voids and screwed it down.
the dirtra matting is a decoupling membrane for movement.
that's why i said maybe 22mm ply and the tile board glued down with thin set then screwed down then the ditra matting over the top.
as long as te floor is level and voids filled and all screwed tight allow for say 5mm gap all round the edge for movement.you can use a foam gun to fill the void or you can buy the foam strips

 

[AdamCH]

That floor looks pretty strong, can't imagine it will have much deflection.

You'll need noggins to support the edges of the ply and you could put more of them in.

Ditra won't help with deflection, it's for lateral movement not vertical.

You could use thicker ply 22/25 say & put 6mm Hardie on top.

 

[Skimmers]

ditra will help as its a decoupler for any movement and bounce
If there is un/down movement there has to be side to side movement

 

[AdamCH]

Decoupling matts will help with lateral expansion & contraction in the sub floor but won't help with deflection.

Get it rigid and then put a surface on that's good for tiling onto. Ditra or Hardie would be okay for tiling to but needs to be deflection free at the start.

 

[68breakbreak]

Thanks. I already have 18mm ply on top, but no noggins. The ply therefore delfects slightly when I put weight at an unsupported area. I suppose I'll have to remove the ply and fix that. Not sure how though - just screw across and through the top joists into noggins with very long screws? It won't be strong...

If I crawl below the floor and have somebody jump up and down then I can see a small amount of deflection at the weakest points. I'm not sure if it's good enough. Once the noggins are in place and there is another layer screwed over the top, perhaps the weight will be more evenly distributed and deflection will go? I could sister the joists at key areas if you think it would be worthwhile... I'm most concerned with the deflection at the moment. Advice appreciated - thanks

 

[tpt]

adamch is right about the ditra. no need in a lot of cases except to deal with lateral movement.

From what i can see, overboarding the ply with cement board and using a single part flexi adhesive would be ample. floor in general looks very well supported compared to most

 

[Skimmers]

ditra is less in cost as per sheet or board and has better imperturbability then backer boards

 

[jctilingservices]

there is absolutely no need for Ditra, as you would be gaining absolutely nothing by using it apart from extra expense, fix a cement backerboard to your existing ply floor then tile away.

 

[68breakbreak]

thanks. do I fix it down with flexible tile adhesive, and then screw it down? can anyone recommend a suitable tile adhesive?

 

[jctilingservices]

thanks. do I fix it down with flexible tile adhesive, and then screw it down? can anyone recommend a suitable tile adhesive?

yes mate, trowel a bed of flexible addy ( bal, mapei, sovereign etc ) then screw down, the boards are marked for the screws, make sure you use the correct screws for the boards.

 

[AdamCH]

Get that ply supported first though. Put noggins in and make sure the ply edges fall on a noggin. 6mm Hardie backer will give you a good surface to tile to but it won't stop a bouncy floor bouncing.

You've got lots of brick piers and I'd have thought the best place for noggins would be between the piers. You'll need to take the ply up first though.

Don't skimp on the preparation. It'll cost you a new floor later if you don't get this bit right.

 

[68breakbreak]

Right ok. Noggins it is then. The only way I can think to do it is drill coach screws across the top timbers into noggins.. The piers are lower down at joist level so I can't see how I could use them.

It doesn't seem like the best way to do it as they won't be supported. Do let me know if there is a better way.