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Is it possible to lay wood planks on tile mortar and grout in between with 1/8" gaps, like you would tile. I have a concrete slab sub floor that is all above ground level. I live in North Central Arizona. We have a good rainy season during the summer, but are high and dry for most the year.

The only thing that I think could go wrong would be the boards warping and separating from the mortar beneath, or perhaps shrinking and leaving gaps in the mortar. I guess I wouldn't mind cracks or gaps; those are easily filled, but warping would be a problem. I want to use 3/4" or 1" heartwood pine boards that I can get locally. I would be able to let them sit in my garage for a few months or even a year if needed.

I want to try mortar for the look, but also for maintenance. The gaps should make replacing a board easier.

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    Why not use a "floating" floor installation? Even with manufacture lumber, there is some size change with humidity; I don't think grouting would work well. – keshlam Jun 23 '16 at 7:15
  • I cant say I have ever seen wood embedded like this. I would think it would shrink causing it to become loose in the summer and in the winter swell and bow. maybe shorter lengths ? Pine is a soft wood and may cup more than a hard wood. I would not try it out doors because the rain standing in the troughs would rot it out. I would be curious to know how it works through several seasons. – Ed Beal Jun 23 '16 at 13:20
  • @Ed - yeah, indoors is where it would go. – Ian Jun 23 '16 at 14:58
  • @keshlam - The planks I'm thinking about getting are not tongue and groove. There just have straight edges. You need the interlocking edges for a floating install, right? – Ian Jun 23 '16 at 14:59
  • Yes, floating needs the interlock, a s far as I know. – keshlam Jun 23 '16 at 16:56
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No you cannot use a tile mortar or thinset. If you want to adhere wood to a concrete floor you need to use a urethane based adhesive. using a water-based mastic or a cement-based thinset will lead to swelling of the wood fibers and premature failure. For grout you would also need a urethane-based or Epoxe based substance that would remain flexible for expansion and contraction. You could mix in some colored sawdust to make it interesting. Your concrete must absolutely be cured and moisture free.

Replacing a board easier will never happen, you might be able to replace a board but it will not be easy.

You could also do all of this on top of a tongue and groove plywood subfloor on top of the concrete, I would glue the tongues in the grooves together so the whole thing would be like a floating floor.

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No you can't do that. Even in Arizona. Wood has a high expansion and contraction ratio, they would work themselves out of the mortar in under three years in the driest of climates.

Your best option would be to membrane underneath the boards, secure them with concrete nails, and epoxy, or caulk the joints. That might last a year.

In terms of code, in the north it's not allowed. Untreated wood cannot bare directly on concrete.

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