installing 12x12 unguaged slate over heat mat [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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12-19-2016, 07:02 AM
3 season porch with windows that has cement board,leveling compound and Schluter heat mat and cables installed, Want to install 12"x12" ungauged multicolor slate from China on this in diamond pattern. Should I isolate the mat with a layer of thinset and allow to harden before slate install? What are some concerns I should be aware of before the start? Looking from the side these tiles are extremely variable. How can I install level and without lippage or can I? What is acceptable? How thick can the setting bed be and still allow for the heat to transfer to the tile face? Any advice would be appreciated.


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12-19-2016, 11:42 AM
Hard to believe no one has any feedback on this question. I am feeling shunned.



12-19-2016, 02:10 PM
Sometimes you gotta wait more than just a few hours for responses, Bookie. Most of our unpaid helpers have day jobs and some of them even think they should have lives of their own outside of this website, eh? :)

12-19-2016, 02:49 PM
Ungauged slate can be a major bear to install while minimizing lippage! It's more like trying to assemble a jigsaw puzzle, choosing tiles that both match the coloration you like and then finding ones that will end up with edges close to even. You'll probably end up using a much wider grout line than you might otherwise want to help even out those least 3x the maximum delta in sizing and offsets. Thinset is a fairly decent conductor of heat, so as long as you choose a mortar that can handle the thickness required, it will be okay. FWIW, radiant heat works best if it is turned on and left doesn't have a really fast response time.

Regarding the slate, sometimes, you can chip off high areas, but that takes some skill, and can make the tile unusable if it flakes where you didn't expect. That can also cause color variations. Also note, some of the stuff from China they call slate is more like hard sandstone or mud...not a true slate. You might want to take a few sample tiles, place them in a pan covered in water for a day, then see if you can wipe them without color transfers or the surface becoming soft. Often, they literally start to dissolve! A true slate won't. No amount of sealing will help. If the area is well covered, it might work out, but when you wet mop things and you end up with a muddy pail of's hard to detect whether you've cleaned anything or just washed off the top of the tile's surface! A true slate is quite hard, lots of the pretty colors of Chinese, so-called slate, is fairly soft, and probably only suitable for maybe a wall covering in a dry area.

12-19-2016, 07:19 PM
Yep, what Jim said. The tiles will probably vary from 1/8 to 1/2 thick and they won't be flat. If you see one that has a high corner on it when you set it on the floor, set it aside until you get 4 of them and set all the 4 high corners against each other. The sizing will be off too so find a few of the larger tiles and figure your layout off of those using about 1/4" joints. Still, your joints will vary from 1/8 to 3/8.

12-20-2016, 10:20 AM
Very appreciative of the responses. Excellent feedback on testing the slate for its hardness and colorfastness and the importance of patience and culling out for the install. Learning everyday.