Oh what have I let myself in for....

[Ruth]

Ambitious or mad, not sure which, I've laid floor tiles before (terracotta etc) but not limestone tiles onto a suspended floor. Having read horror stories of cracked stone floors, I've scoured your pages for advice. I'd be hugely grateful for comments on what follows:

Subfloor To my horror the joists are only 47x97mm (tho consulting a table it tells me that should span 1.92m, and although the room is 3.6m wide it has a supporting wall at the half way point). Have access below the floor, will brace joists with noggins (same size as joists?), how far apart? Then screw floor boards to both noggins and joists? Lay 18mm WBP ply (back and edges primed with SBR), two boards spanning the room so meeting in the middle, at the under floor wall support, leaving gap of ??mm between each board and wall, screwed down every 200mm. Any need to glue boards? Prime surface with SBR or PVA or nothing at all?
Tiling I've heard of a decoupling mat, don't really know what it is, how or when it is used?? Ditramat? Next, apply adhesive as solid bed, how thick? I don't understand the reason why there must be no air under the tiles? I have seen it done using a notched trowel which would leave plenty of air beneath the tiles.
I hope I can then manage the rest of the job! (may be back here...)

Any top tips or advice would be hugely welcome since the stone has arrive, there's no backing out now!

 

[Dan]

I can help with a bit of the advice there but the pro's will need to correct me and have their say, I don't actually tile these days.

A decoupling mat such as ditra or dural (another make that's often cheaper but just as good) allows the subfloor /substrate to expand and retract at a different rate to the tiles. Often used on large floors, floors with UFH under tiles, dodgy old substrates that are just about good enough to tile on, and like in your case on wooden substrates.

You basically tile (if I can use that word, it's not a tile) it to the floor, then tile over it with your tiles.

As for using a trowel, it will need to be a notched trowel (a thick-bed solid-bed trowel, with sort of half moon shapes cut into it) but there's a way you apply the tile that forces the air out of the voids, then you apply pressure and twist the tile a little which gives it its solid bed. There HAS to be 100% coverage under the tile which prevents it cracking. If you had an air void/pocket and tapped it with something solid it would crack as easy as a tile would in your hand holding it in mid air. The solidness of the tile is only achieved when it's fixed to the floor with 100% coverage, it then becomes as solid as the substrate.

You need to pull up a tile every now and again and ensure it has actually been covered fully with adhesive. Tiles sinking a little often gives it away, but not all tiles will sink, especially if the adhesive is a bit thick.

I'm lost on the noggins thing though. You generally need them to strengthen the floor and prevent deflection, I know that, but what span they have to be etc I'd need to ask about myself to be honest. As for fixing to them, it's fine to fix to them AND the original joists. Always use good screws too and not nails.

As for over-boarding, you need to prime both the back and front and the ENDS TOO! (the important bits that often get left) And I think you leave a 2mm gap but let the professionals correct me there.

Good luck with it all. Not all DIYers would take on such a big task with so much money involved! I think you're very ambitious and perhaps a little mad. But hey-ho

 

[grumpygrouter]

Dan has pretty much answered your question. Only thing I will add is that priming of the ply tiling face isn't always necessary. It depends on the adhesive manufacturers specifications. BAL for instance say not to prime, Ardex say prime with P51 primer and tile with Ardexflex 7001........

 

[david campbell]

not much correction needed with that post danspot on:smilewinkgrin:
apart from grumps post which is as he says is down to the manufacturers spec!
as for the extra noggins-the more movement the more you will need as this is where you will be screwing the overboarding into,i think building spec is every 450 or 600mm depending on where you are and what is being built

 

[DHTiling]

If those joists are only 47mm x 97mm , then they are not strong enough to support the weight of a stone tiled floor..

You can get over this by adding new joists to run along the length of the old ones and tied into the adjacent walls etc.. but this is only possible if you have the room to lift the old floor boards etc.

 

[Dan]

If those joists are only 47mm x 97mm , then they are not strong enough to support the weight of a stone tiled floor..

You can get over this by adding new joists to run along the length of the old ones and tied into the adjacent walls etc.. but this is only possible if you have the room to lift the old floor boards etc.

That's more the area I was worried about. Don't really know how far you go with this noggin thing. :lol:

I'd end up with a solid subfloor 1' deep just to be sure lol due to the amount of failures I hear about due to this!

 

[mikethetile]

4x2 joists supported mid way make two spans of 1.8 which is too close to capacity for comfort, this span will just about support the floor and items placed on it adding 18mm ply , addy and stone will probably overload the floor

is this a loft conversion as the minimum is usually 2x8

 

[faithhealer]

welcome to the forum Ruth, smallest joists I've heard of

 

[The Legend; Phil Hobson RIP]

:welcome:Ruth hope you got your answers

 

[mikethetile]

Ruth

if you can lift the floorboards you can add extra joists in between the existing joists to increase the carrying capacity of the floor

 

[Ruth]

Thanks for all your comments, it's not looking good. I must say I was pretty alarmed when I saw the size of the joists. They are around 340-380mm apart. It is a ground floor, the floor is built up to around 1.5m at it's highest end, then gradually slopes to around 1m at the other end of the room, with a concrete floor beneath. I'm thinking tieing in additional joists is not going to be an easy task, let along the implications of such a rise in floor height.... Oh dear, I think I might be in trouble.

 

[Ruth]

Sorry Mike, I was taking too long to write the last message so missed your suggestion. If I removed the floorboards, would it be feasible to cut a portion out of the blocks between joists in order to insert further joists, or are they vital structurally?

 

[mikethetile]

you can use 2x4 joists in between the existing joists , theres no need to increase the depth of joist or remove the existing joist

 

[DHTiling]

Yes you can remove a block/brick to accommodate the new joist depth..

 

[mikethetile]

yes you can remove them and fill them in again around the new joists