Subfloor Specs

[WillB1]

Hi all,

I'm currently renovating my kitchen and will be laying porcelain tiles to the floor but am after a bit of advise from you guys first.

The room is roughly 3.6m x 4x6m, part of which is a concrete floor, part timber joist construction at 16" centres. (Timber joist area approx. 3.6m x 3.2m).
I have ripped up all the old floorboards which were in extremely bad condition, fitted 100mm polystyrene insulation btween joists and layed 18mm wbp plywood onto the joists (haven't screwed it down yet).
I have added noggins to the floor where the ply joins meet.

My plan was to lay ditramat and then tile onto that as I'm trying to prevent any large steps between rooms.

My concern is, after having read many posts on this and other sites is that 18mm ply might not be strong enough. I don't really want to take out the 18mm ply as it cost me a fortune.
Is there anything I can do or do you think 18mm ply will be ok???

Hope this all makes sense.

Thanks in advance,

Will.

 

[david campbell]

you could overlay the ply with 6mm cement boards,you wil also need an expansion joint where the 2 floor types meet due to expansion and shrinkage differences between wood and concrete

 

[Alan.P]

As David says, also I would add extra noggins so that your able to fix down at 300mm centres, and prime the undersides and edges of the wpb, it is wpb you've used ?

OH, and welcome to TilersForums.

 

[WillB1]

Thanks David/Alan,

I think I will add a few more noggins for peace of mind.
I've read that the cement boards don't actually add any strength to the floor - is this true??? That was why I was going to use the ditra, so it didn't add too much height.
Also, could you clarify what an expansion joint is? (please excuse my ignorance).

Cheers,

Will.

 

[david campbell]

an expansion joint is a gap of about 10mm that allows for lateral movement(ditra helps to prevent this) but with 2 different surfaces i would still use an expansion,they can be in similar style to your threshold strips between carpet and tiles(the brass bar or wooden strip)
i will post you a link to some other types

http://www.schluter.co.uk/2095.aspx

check above

 

[Alan.P]

The more noggins the merrier You want the floor as solid as can be.

Are / will your wooden floor level and concrete levels be the same height ?

An expansion joint is just that, it's takes up any expansion / contraction between two surfaces, in your case, where the wooden floor meets the concrete floor, the expansion joint will have to run the length of the joint and all the way through, in other words, don't board directly over the join, and when you fix your tiles, fix the expansion joint over the top if that makes sense.

 

[WillB1]

Thanks for the link and feedback regarding the expansion joint.
Is there any way round using this? Possibly using the ditramat??
It's just that if i put that in it will run through my kitchen and an island unit so won't look the prettiest. (or have i missed something here?)

And yes, with 18mm ply down the concrete floor is the same height.

Cheers,

Will.

 

[deno3]

screw the ply down, ditra mat over the whole floor then fill the top of the mat with rapid flexible adhesive or latex and let it set, the ditra will take out any movement underneath and give you ridged floor to work on it will be fine.

 

[WillB1]

Thanks deno3,

That was my original plan but have become confused over the last few days with all the different techniques and products out there and want to get it right and avoid cracked tiles in the future.

My (current) plan is to now go home, take up floor, add millions of noggins, screw floor down tight, ditramat then tile.

Really appreciate all the extremely speedy feedback guys.

Will.

 

[WillB1]

I forgot to ask earlier what I should prime the back of the ply with?????

Will.

 

[deno3]

I forgot to ask earlier what I should prime the back of the ply with?????

Will.

no need just use 1 part flex adhesive or unibond it if you like

 

[grumpygrouter]

If you read through the specification of ditra matting it does specify that you should run your movement joints through the matting as well. It will isolate the tiled surface from lateral expansion/contraction but NOT vertical movement which is possible when you have 2 different substrates meeting. Sealing of the back and edges (with an acrylic or SBR based primer, NOT PVA) helps to prevent moisture ingress to the ply from the back and help reduce expansion caused by the moisture.

 

[The Legend; Phil Hobson RIP]

If you do go for the ply+ ditra, I would recomend you prime (SBR) all area of ply ie top back, sides. To prevent future water ingress. Re expansion joint a must, you can get any colour match now. Good luck

 

[The Legend; Phil Hobson RIP]

Sorry Grumpy, Post overlap. My bloody slow typing

 

[Terry Cottar]

Thanks deno3,

That was my original plan but have become confused over the last few days with all the different techniques and products out there and want to get it right and avoid cracked tiles in the future.

My (current) plan is to now go home, take up floor, add millions of noggins, screw floor down tight, ditramat then tile.

Really appreciate all the extremely speedy feedback guys.

Will.

You still need a movement joint where the two substrates meet as well as the Ditra.

tel