Hello, I am a first-time do-it-yourself tiler in the US, though long ago I lived for half a year outside of Bath, in beautiful Somerset. There's lots of good info and many knowledgeable folk on this site, and I need your advice on a particular problem. I'm using wedi, and I think wedi is more widely used in the UK and Europe than it is here in the States.

I am using a wedi presloped Fundo shower base. I bought 50mm mosaics (2" x 2") and quite a bit of matching quarter-cove trim pieces, which are non-returnable. It turns out the mosaics are thinner than advertised, 4mm rather than 6mm. 6mm would have been perfect for the wedi base; but 4mm creates a problem. The 2mm difference looks small but there's more to it than meets the eye.

The plastic strainer frame on the particular wedi drain that shipped with my unit rises about 9-10mm above the cementitious surface; in order to have the mosaic tile be flush with the top of the drain, a ~6mm-thick adhesive bed would have to be placed under the mosaic tile.

The manufacturer of the thinset I bought says their thinset would lose much of its compressive strength if laid so thickly, especially beneath small tiles. The 100% solids epoxy mortar I bought from another manufacturer can't go thicker than 3mm beneath the tile either.

Why am I concerned about compressive strength? A former wedi distributor in the US told me that they didn't recommend tiles smaller than 4 inches when wedi is used underfoot, and I recalled a wedi recommendation to use epoxy with mosaics. So I'm taking the point-load issue seriously, even though on another tiling forum people told me not to worry, unless I happened to weigh 20 stone.

To build the surface up, I've put down a layer of thinset down on the wedi Fundo base, combing it with a 3mm U-notch trowel. It has cured for several days. Now I want to fill in the valleys to create a smooth surface for the final mortar bed to go on top of. Can I use epoxy mortar to fill in the valleys, let it cure, and then bed the mosaics in another layer of epoxy mortar? Or should I use thinset again to fill the valleys, and only use the epoxy as the final bedding layer? Is it 6 of one, half a dozen of the other?

Thanks
Tim

Hi tim...

Maybe using ditra as well might have helped with the compressive strength..but your project is underway so not an option now..

Back to the mosaics.. I think filling the valleys with epoxy will give a better hold....with what you have done already..

Originally Posted by Dave

Hi tim...

Maybe using ditra as well might have helped with the compressive strength..but your project is underway so not an option now..

Back to the mosaics.. I think filling the valleys with epoxy will give a better hold....with what you have done already..

Thanks, Dave. I'll go with the epoxy then.

hi wedi fundo base is that a fibreglass base!!!!!!!!!!

Originally Posted by jay

hi wedi fundo base is that a fibreglass base!!!!!!!!!!

No, not fiberglass, but wedi Fundo is made of extruded polystyrene (waterproof) foam core with a cementitious outer layer.

thanks

Tim I think you'll find the advice from Wedi here is no smaller than 50 x 50 mm - the size you have. In any case that is referring to standard boards. The fundo base is like Plattro board - for exteriors - which both have a heavier gauge mesh / cement facing to take care of the heavier workload under foot.
The use of epoxy seems a bit onerous, when I have had this issue I refloat the base level with thinset - one that can be built up a little - as much as is needed to get the tile level to finish at the drain grid level you set. Leave to set. Then lay the 3mm bed and tile. As long as you check the falls and new surface carefully and dont form pooling spots, its fine.