Help!: What trowel / spacer size do i need?

[mrgdclark]

I'm laying my 1st polished marble floor for my sister and need some advise. She has a new screed floor (been down about 2 months) and has wet underfloor heating installed which has been on up until a week ago and is perfectly level with no joins / cracks in it. the tiles are 600 x 600 x 12mm polished marble and the room is approx 7.1m x 4.6m.
1. what trowel size do i need. adh is powder flexible for underfloor heating
2. what spacer size should i use.
3. do you just "drop" the tile into place and tap it down?
4. do i need to treat the screed floor with anything?
5. do i give the tiles a 1st "seal" after i have put them down and before i grout and then reseal them after i have grouted?

i will be leaving a gap around the edge of approx 8 - 10 mm as i'm going to use the same tile as skirting.
many thanks for any advise

[nybor62]

min trowel size 10mm round notch, white flexible adhesive, spacers 2-3 mm, also back butter the tiles with flat side of trowel for 100%solid bed , lay the tile in place and twist slightly, the screed will need priming with suitable primer same as adhesive manafactuer , i would also use ditra matting when fixing stone products on to under floor heating,
seal before grouting and after, work as clean as poss .
gap around edge will be fine also

what type of screed is it.

mapei products are very good , from tile giant if you have one near you

[mrgdclark]

Hi, many thanks for your advise. With regard to the ditra matting is it possible to tile direct to the screed or will it cause problems? It's just budget is tight at the moment and it would cost near to £300 in materials for this matting.
The screed had a very watery mix of pva put all over it when it was first put down to stop any dust kicking up as my sister need to use the floor before xmas / tiling.
The screed i think is just a 4.5:1 mix of sharp sand and cement and ranges from 60mm to 80mm deep sitting on kingspan with the pipes fixed with plastic clips.

[nybor62]

i would use ditra , the cost of ditra far out weighs , the cost of failure .
im sure there will be more advice from fellow tilers

[doddy41]

The screed should not just be normal sharp sand if wet ufh is being installed could have problems with cracking would def use ditra mat

[Dave]

The screed i think is just a 4.5:1 mix of sharp sand and cement and ranges from 60mm to 80mm deep sitting on kingspan with the pipes fixed with plastic clips.

Was there no form of reinforcement used ?

[mrgdclark]

Nothing else aprt from sand and cement.
thanks for all your replies

[johnryan]

your screed may not be thick enough. as far as i'm aware, it should've been 100mm on top of kingspan.