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Floor tiles on EPS underfloor boards - some questions

Discuss Floor tiles on EPS underfloor boards - some questions in the America Tile Forum area at TilersForums.com.

Hi All

I'm planning the tiling of our kitchen floor with 600x600 rectified 9mm thick porcelain tiles onto retrofit underfloor boards made of high density EPS/polystyrene with an aluminium foil top surface (Wundatherm Premium+). According to the manufacturer, tiles can be laid directly on these boards using a recommended primer and S2 adhesive but I have several questions on the best way to tackle it.

Firstly would an uncoupling membrane like Ditra25 or BAL Rapid Mat be beneficial despite the UFH manufacturer not specifying it? If so, is there significant functional differences or suitability between Ditra/BAL/something else as we're really limited on buildup height due to a low bifold door threshold so the 0.85mm thickness of the BAL would be preferable to the Ditra at around 3mm.

Second question is on adhesive choice. I've done some DIY tiling in the past but not with tiles of this size (the largest were 30x45's in our bathroom), so given I'll be taking it slow and steady using levelling clips/wedges would I be better off with a slow set S2 adhesive or do I go with the more easily sourced fast set and just mix smaller amounts per batch?

My final question is trowel notch size. I know bigger tiles generally need bigger notches but is there a rule of thumb for a certain tile size with a certain adhesive rather than just guesstimating?

Thanks in advance.
 
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"According to the manufacturer, tiles can be laid directly on these boards using a recommended primer and S2 adhesive"

Absolute nonsense! Instant tile failure!

How would a primer absorb into metal? S2 adhesives can bond to metal, but not recommended to a flimsy sheet of foil, as the bond between the tile and the foil is only as strong as the foil to the subfloor. Which is weak. Very.

I'd want to put down at LEAST a 6mm cement tile backer board screwed down hard onto floor joists, penetrating the existing floor. Then possibly more prep such as levelling compound.

I'd be prepared to get a joiner on board to trim things down.
 
Hmm OK thanks for your thoughts. Mapei and Instamac both certify a process for tiling and screeding directly over these boards that includes the application of a primer onto the aluminium top layer (Eco Prim Grip in Mapei's case) so you'd hope they know what they're talking about. My question was whether there's a case for adding a decoupling membrane to that given it's benefits and because one of the tilers that originally quoted for the job specified it.
 
Yep as I understand it, other than to provide a keyed surface priming is an important part of the process with these boards to prevent the cement based screed/adhesive reacting with the aluminium and causing hydrogen bubbles.

These are indeed 400kpa boards and I've already laid them elsewhere in the house that were primed with Eco Prim (that seemed to stick like **** to a blanket!) then screeded over for rooms that are having carpet/LVT so I'm not unduly concerned about primer not adhering, I just thought probably naively that anything like uncoupling membrane that might reduce potential movement of the tiles was worth considering even if it goes away from the certified process from the manufacturer.
 
:)
Yep as I understand it, other than to provide a keyed surface priming is an important part of the process with these boards to prevent the cement based screed/adhesive reacting with the aluminium and causing hydrogen bubbles.

These are indeed 400kpa boards and I've already laid them elsewhere in the house that were primed with Eco Prim (that seemed to stick like **** to a blanket!) then screeded over for rooms that are having carpet/LVT so I'm not unduly concerned about primer not adhering, I just thought probably naively that anything like uncoupling membrane that might reduce potential movement of the tiles was worth considering even if it goes away from the certified process from the manufacturer.
Hi Locoblade,

What screed did you use to cover the ufh boards for the carpeted rooms? I covered mine with 6mm ply as per Wunda’s instructions in the bedrooms which were carpeted but makes the ufh very inefficient. I’ve now taken up the ply and was going to pour self leveller to put the carpet on top. Looking for a flexible screed which won’t fail.
 
Hi Keith

We used Mapei Ultraplan Renovation with Eco Prim Grip primer in rooms that needed screeding which is a combination that Wunda endorse. We did have some cracking issues in two rooms with that though due to some boards seemingly not bonding properly with the floor and causing a bit of movement, even though they felt secured prior to screeding and only failed a few months later when the UFH was run in anger. I removed any movement by mechanically fixing down with screws and spreader washers in the cracked areas (using a borrowed thermal imaging camera to mark out where the UFH pipes were so they didn't get screwed through!) and then put another 5mm of screed over top but this time using BAL Level Max which hasn't cracked at all. If I was doing it again I'd probably use the BAL product from the off with a suitable primer over the boards as it seems to have a lot more fibre reinforcement in it, it may not be on Wunda's approved list but if you have issues it will likely be blamed on the installer anyway even if you do follow their install guides to the letter. Just make sure there is indeed no movement in the boards at all, I'd also use mechanical fixings on the board joints before screeding next time if there was any doubt with movement just as a belt and braces.
 
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