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I went to tile upstairs bathroom floor and when we pulled up old linoleum and replace subfloor that was rotted we discovered 1 1/2" of concrete poured on top of old subfloor. Why would somebody do this?

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It was (and still is) common to float cement on top of the floor for installation of ceramic tile. It provides a level and firm substrate to which tile mortar (thin-set) will adhere. 1 1/2" seems a bit thick though, maybe there were irregularities with the floor which had to be compensated for. I try not to float so thick unless I have to, and now I use cementitious tile backer board when appropriate. Or sometimes I mortar the tile directly to a smooth firm plywood subfloor in new construction applications.

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    Perfectly normal thickness for this type of install - it's not thinset, it's not done the same way as thinset. Tile direct to plywood is not a recommended method. – Ecnerwal Aug 25 '14 at 14:09
  • @Ecnerwal, all due respect, there are polymer fortified thin-set mortars specifically designed for direct application to sound plywood flooring that does not exceed maximum deflection specs. I have had very good results, proper application is key. Check it out: custombuildingproducts.com/TDS/TDS-118.pdf – Jimmy Fix-it Aug 26 '14 at 0:57

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