Oversite Depth - tile adhesive and screed thickness?

[Ziggy1979]

Hi Everyone

It's been a while since I've been on the forums!

We are having an extension built and are really unsure about subfloor prep and tile adhesive thickness.

We want the new finish floor level to be the same as the existing finish floor level. We are trying to work backwards from there to determine where the top of the oversite/concrete slab should be (ie how much lower than existing floor level). The prep work is starting now with the concrete being poured next week and your advice would really help us get it right!

On top of the slab we will have:
1. insulation (75mm Celotex/kingspan or 100mm Jablite - probably one of the 75mm ones),
2. UFH pipes with a 75mm sand and cement screed (is 75 mm too much/little?)
3. Tile adhesive (thickness unknown) but assume will need a thick one to make up any levels if necessary(?)
4. Large tile probably 800x800mm (a quick look online says that these tiles will be 10-12mm thick)

The room itself is a large open plan sitting, kitchen and dining with the same floor and floor level going into a new utility room and new downstairs shower room. There is also a new downstairs bedroom but that will take wooden floor to match existing downstairs (again all at the same finished height).

I guess the main question is how much should we allow for the tile adhesive and is the screed too thick or thin?

We are also thinking of running the same tiles to an outside terrace (back and side alleyway) through bifold (level access) doors, again at the same finish floor level. We think we need a 100mm reinforced concrete base for this. Is this enough or would we need more eg 150mm? Also, how much should we allow for tile adhesive on the outside?

Sorry for the long post I terrible at making them short!

Thanks for all your help!

Zigs

 

[Chalker]

Hi, welcome back.
15 mm tile and adhesive
55mm screed.
100mm insulation. Always put the best you can get. This will pay back in energy costs throughout the lifetime of the property.

don't forget about drying times, before you tile.

 

[Ajax123]

Hi, welcome back.
15 mm tile and adhesive
55mm screed.
100mm insulation. Always put the best you can get. This will pay back in energy costs throughout the lifetime of the property.

don't forget about drying times, before you tile.

55mm sand cement is too thin especially with ufh. Should be minimum 65mm so with 15mm pipes that means ideally 80mm.

you could use anhydrite at 55mm

 

[Chalker]

55mm sand cement is too thin especially with ufh. Should be minimum 65mm so with 15mm pipes that means ideally 80mm.

you could use anhydrite at 55mm

thanks for for correcting me on that one .
Is that still relevant when using fibre reinforced screed?
So used to liquid screed these days.
In fact fitting underfloor to a new bungalow tomorrow.

 

[Stef]

Take Alan's (Ajax) advice.
I would always spec a calcium screed on a job with ufh because of the benefits.
Specd a new build floor with ufh start of the week, 300m2 of floors..

 

[Ajax123]

thanks for for correcting me on that one .
Is that still relevant when using fibre reinforced screed?
So used to liquid screed these days.
In fact fitting underfloor to a new bungalow tomorrow.

yes it is still relevant. In fact the British standard says that floating screeds and unbonded screeds (where there is UFH it is most likely floating) must be reinforced with either poly fibres or d49 steel mesh. This is to try to control cracking. Sadly the same standard then goes on to say that sand cement screeds crack randomly....

 

[Ziggy1979]

Hi Thanks Everyone for your help.

Other than extra time to heat up for the thicker sand and cement screed are there any benefits of the anhydrite screed? We're looking at about 60m2 with approx 45m2 of UFH. I read on other sites that some people prefer sand cement as easier to patch repair especially if needing to fix a leak in pipes, etc. (will likely have other plumbing and electrics in screed).

On the terrace and side of the house the tiles will be tiled straight onto a concrete slab (which I am told won't be as level as a screed and will have a runoff towards the back garden). Should we also allow 15mm for tile and adhesive here or possibly more for a thicker bed?

I also thought to have Ditra over the screed internally which if you guys recommend needs another 3mm? Would/should we fit this outside too?

thanks again!

 

[Dave]

The anhydrite is a much stronger screed and as for drying , it can be force dried . But you deffo ain't getting anything tiled before Xmas .

 

[trustiling]

For on top of the concrete slab outdoors you could allow for 50mm or so, rapid drying screed from Kerakoll for a more suitable suface to tile onto then you will still have your 15mm tile/adhesive.

 

[Ajax123]

For on top of the concrete slab outdoors you could allow for 50mm or so, rapid drying screed from Kerakoll for a more suitable suface to tile onto then you will still have your 15mm tile/adhesive.

Whats the point in putting rapid drying screed outside...it's gonna get rained on...

 

[trustiling]

Have you used Kerakoll? You mix it with sand, and can be tiled on the next day

 

[Ziggy1979]

Hi Guys thanks again.

Can the Keracoll be mixed on site and applied like a traditional screed? Would we use it inside and out?

We've been rcommended a screeder(?) who does sand cement screed and were told he's really reasonable and good. So ideally we'd like to use him but it means no liquid/anhydride etc screeds I guess. We were also told it would be a lot cheaper and as always £matters. . Is there any reason to avoid traditional screed? Can drying time of normal screed also be reduced with additives?

Outside, can you tile straight onto the outside concrete slab or do u need some form of screed over it? We need to have a fall from the back if the house to the back end of the terrace (4500mm). If we did go with a liquid screed would it not 'self level' and lose the fall?

Thanks again for all your help.

 

[widler]

Yes you can tile straight onto outside concrete, as long as its nice and flat [emoji15]
Nothing up with traditional screeds ,they have been tiled onto for decades with no problems, anyldrite is relatively new , well it is around my parts,ive only seen one in nigh on 30 years [emoji33]

 

[Ajax123]

Hi Guys thanks again.

Can the Keracoll be mixed on site and applied like a traditional screed? Would we use it inside and out?

We've been rcommended a screeder(?) who does sand cement screed and were told he's really reasonable and good. So ideally we'd like to use him but it means no liquid/anhydride etc screeds I guess. We were also told it would be a lot cheaper and as always £matters. . Is there any reason to avoid traditional screed? Can drying time of normal screed also be reduced with additives?

Outside, can you tile straight onto the outside concrete slab or do u need some form of screed over it? We need to have a fall from the back if the house to the back end of the terrace (4500mm). If we did go with a liquid screed would it not 'self level' and lose the fall?

Thanks again for all your help.

Anhydrite is not suitable for this project anyway. You might as well just use a traditional sand cement screed with SBR added to give flexibility and water resistance rather than faffing about with special fast drying screeds.

 

[Ajax123]

Yes you can tile straight onto outside concrete, as long as its nice and flat [emoji15]
Nothing up with traditional screeds ,they have been tiled onto for decades with no problems, anyldrite is relatively new , well it is around my parts,ive only seen one in nigh on 30 years [emoji33]

how long does it take for something to be no longer new. Anhydrite has been used in burrnley and the surrounding areas for thirty years plus.