There's just too much info here. The more I read, the more confused I get, so please be gentle

I'm ripping out our existing bathroom that's 3mx3m. The current chipboard floor isn't all in great shape, and most of it is water damaged or covered in old tiles so would just require extra work to remove the old cement etc.

My current plan is to remove the lot, pack the joists where necessary to get it as level as possible and put an 18mm wbp subfloor down in its place. I was planning on covering this with 6mm ekoboard or similar, then ufh, then slc. Depending on how flat it ends up I'll see whether I feel confident of putting the 300x600 tiles down or whether I get a tiler in.

Now I'm starting to think that instead, maybe I should go for 22mm wbp, sacrifice the insulation board and go with durabase-wp or similar. I also wonder whether an extra 4mill of ply really makes any difference in a 9m2 room as long as the ply is securely screwed at 300mm centres.

As the chipboard+tiles are only around 20mm currently, I'd rather not add any more height than necessary, so any help would be appreciated.

Thanks,

Pat.

I would say 18mm wbp as a replacement for your 18mm chipboard and overboard with 6mm hardibacker to give the ideal surface to tile to

it really is down to how much deflection you have in the floor but if you brace the floor and as it is a relatively small room you should get away with 18mm

for a larger room with potentialy more deflection I would bump up to 25mm wbp as a belt and braces approach

i'm with you Mike. 18mm ply but with "No More Ply" for me.

I think you answered your own query pat, 18mm plus 6mm backer boards is fine.

Thanks, I felt I was going round in circles. Its nice to get a second opinion

personally I'd go for 22mm wbp, ekoboard, ufh, slc and then uncoupling membrane, oh and then tile

it depends on the size of the room,18mm is the min i would think about but i would add extra noggins and make sure the floor has no movement,if the floor was larger i would be looking at 22mm or 28mm then the 6mm cement boards

the trouble is, everything adds to the height, that route takes me to 22mm + 6mm board + 3mm membrane + 3mm ufh + say 5mm adhesive + 10mm tiles = 49mm vs around 20mm currently (ok my current plan is already only 6mm less than this but...), the step is growing.
In the fullness of time, I'll have to overboard the rest of the upstairs to match the height of the bathroom but I'd rather not lose any more height than I can get away with.

if you lose the ufh, that'll bring the floor depth down quite a lot

Why use themal boards on a timber ply floor, it dos'nt suck the heat out like a screed floor

But then, I'd still need a decoupling membrane wouldn't I? So I'd only be swapping the 6mm insulation for a 3mm membrane, so only a marginal saving in height. Or have I missed something?

Originally Posted by doug boardley

if you lose the ufh, that'll bring the floor depth down quite a lot

Sadly, the UFH is non-negotiable, SWMBO really *must* be obeyed on this one...

What tiles are you fixing to the floor

ok patl, somewhere along the line your ffl is gonna be higher than 20mm from the joists. The least height I can imagine is 22mm wbp, 4mm addy (using a 10mm solid bed trowel well bedded down) and 10mm tiles, so that brings the ffl to 36mm from floor joists.

Originally Posted by patl

But then, I'd still need a decoupling membrane wouldn't I? So I'd only be swapping the 6mm insulation for a 3mm membrane, so only a marginal saving in height. Or have I missed something?

depends on the tile, uncoupling membrane for natural stone definitely, porcelain you could probably get away without uncoupling

I've never done this so just putting out here........(probably to be shot down in flames), but...

If your joists are level, could you not fix batons 18mm lower than the top of the joists, probably also fix noggins between so that you can then screw 18mm wbp "between" the joists at min 300mm centres? That should give you a solid base to build your layers on. That will save 18mm provided the "guys that know" agree that it is feasible.

Originally Posted by Captain Slow

I've never done this so just putting out here........(probably to be shot down in flames), but...

If your joists are level, could you not fix batons 18mm lower than the top of the joists, probably also fix noggins between so that you can then screw 18mm wbp "between" the joists at min 300mm centres? That should give you a solid base to build your layers on. That will save 18mm provided the "guys that know" agree that it is feasible.

I think there would be too much movement that way Captain. It needs the 'laminate' of the flooring structure above it to hold it all together.

If the floor joists are 450 apart then 18mm wbp will be fine, in fact at 600 centres it will be ok providing the floor has enough noggins to stop the deflection.

Just another point, you shouldnt need to level out the joists as once the floor has ply down you can get over any undulations with the self levelling compound (slc) when you put your under floor heating (ufh) down

If you are going to use slc then i wouldnt use the cement boards either.

I had a similar situation to deal with not so long ago so I took advice form this forum and ripped up the old floorboards, put in noggins every 300mm between joists then boarded with 18mm wbp ply. this was sealed on the underside and the edges, also left an expansion joint between sheets and walls, screwed every 150mm with exterior grade wood screws.

Then stuck between worrying about the 18mmply being to thin and the option of thermal board under the ufh.

I opted to put down 6mm hardibacker with spf and screws. When I laid this over the ply I made sure the joins in the hardibacker were not over the joins in the ply,(hope that makes sense)

This gave my solid floor an even more solid feel.

Ok I sacrificed the insulation boards but knowing the ply was under the backers boards justified this for me.

Then I put latex compound over my heat mat before fixing my tiles.

Job done and floor looks and feels great.

Lots of advice taken from this site and the main thing is just ask if not sure(as you have done).

Good luck.

Gary

Originally Posted by whitebeam

What tiles are you fixing to the floor

300x600 porcelain. If I could get away without decoupling, that would make life a lot simpler. I could then easily go up to 22mm ply and still come within my (a bit arbitrary) aim that the floor shouldn't increase by any more than than 25mm.