Floor height wrong after screeding !

[sh]

Hi
It seems that the plasterer didn't do as good a job as I'd thought.

After discussion about heights before the work starting it has ended up 15 mm too high and I need to try and claw back at least 10mm.This will take it to 2-3mm below the height of the wooden threshold.

To that end a couple of questions :
1. Can I screw the 10mm Marmox UFH insulation straight to the screed rather than lay it on a bed of adhesive ?
2. I had allowed 5mm of height for the encapsulation of the UFH in SLC, Is that about right ?
3. If I lay the 10mm thick porcelain tiles on a 10mm bed of adhesive how thick will the adhesive finish once bedded ?
4. I know it's not ideal but would it be better to just leave out the Marmox ?

So many questions I know !
Many Thanks

 

[Richard C]

1. Can I screw the 10mm Marmox UFH insulation straight to the screed rather than lay it on a bed of adhesive ?

It would be extremely ill advised; the boards must be fully supported on an adhesive bed, even the slightest movement could cause tile failure. What thickness are you using, can you reduce it?

2. I had allowed 5mm of height for the encapsulation of the UFH in SLC, Is that about right ?

Sounds about right but depends on the size of the heater mat cable. What is this room & is the UFH to be your primary or secondary heat source? IMO electric UFH is impracticable & vastly overrated as a secondary heat source, unnecessary in many cases & expensive t run; a further alternative might be to ditch it altogether!

3. If I lay the 10mm thick porcelain tiles on a 10mm bed of adhesive how thick will the adhesive finish once bedded ?

You only need an adhesive bed with 100% coverage to support the tiles. The final thickness should be between 3-6mm depending on the size of the notched trowel you use which is dictated by the size of your tiles; solid thick bed trowel up to 300mm large format trowel for tiles over that.

4. I know it's not ideal but would it be better to just leave out the Marmox ?

If your intent on having the UFH that might be the best option but it will affect the warm up time & efficiency of the system which is the main reason I think it’s impractical as a secondary heat source.

What type of screed has been laid? Do you have any insulation in/under it?

 

[sh]

1. I was going to use 10 mm Marmox so at best I would gain 4mm

2. It is a shower room being tiled and the UFH is as asecondary heat source. We have it in the kitchen and even though it may be expensive to run we really do find it very good.

3. The tiles are a 10mm thick porcelain which are 820mm x 410mm so I was going to bed them on a 10mm bed of adhesive. Once laid I guess that would reduce to 6-7 mm ?

4. Not sure what type of screed has been laid (technical name) but I do know it is sharp sand & cement which is 70mm deep with 50mm of Insulation under that.

5. As there is some uncertainty about the finished height with this stack of product perhaps it would be better to just leave out the Marmox and immedtiately gain 15-16mm ?

6. As I mentioned we have it in the kitchen and as that was put in very last minute we didn't have the height for marmox in there either and that seems ok. I am sure if it had it it may warm up a bit quicker but we will never know !

Many Thanks[/quote]

 

[Richard C]

3. The tiles are a 10mm thick porcelain which are 820mm x 410mm so I was going to bed them on a 10mm bed of adhesive. Once laid I guess that would reduce to 6-7 mm ?

You need to use the correct trowel, large format trowel for tiles that size - 20mm round notches, 13mm deep, at 28mm centres.

4. Not sure what type of screed has been laid (technical name) but I do know it is sharp sand & cement which is 70mm deep with 50mm of Insulation under that.

If it’s conventional sand/cement render then OK. Anhydrite screeds (gypsum based) must be acrylic primed to avoid a reaction between the cement in the adhesive & gypsum in the screed.

How long has it been down? It must be fully cured (28 days) but more importantly dry before you tile; allow 1 day per mm thickness drying time.