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floor prep for porcelain tile over hydronic heat

My head is spinning w/sites for prepping hydronically heated floor for porcelain tile finished floor. I laid the pex tubing over my subfloor( 12"spacing), will cover pex w/aluminum plates and use 3/4 plywood btwn pex runs, then put backer board over the plywood, thin set and tile. What needs to be done to ensure no future cracking. Even though my house is 45 yrs old, I think it probably still settles. I'd love to have this finished nxt wkend. Thanks in advance for any help~:confused:

Re: floor prep for porcelain tile over hydronic heat

Aside from the proper base to apply the tiles the floor structure is just as much or more important.
Many existing floor structures are rarely built for tiles. In many cases they were designed for the minimum standards with considerations only for sheet goods or carpeting or maybe hardwood for the finish surface.
The current thing with new homes is if there are to be tiled floors the floor structure is modified at the time of construction.

The main consideration for tiled floors is the flex or bounce the floor structure has. If there is too much then there is an increased likelyhood of the tiles or grout cracking ----- in which case stiffening the structure maybe in order. This could range from doubling the existing floor joists , adding extra joists between existing , increased blocking.

Another good method , especially with in floor radiant heat , is to use an uncoupling membrane laid on the subfloor first then the tile is adheared to the memebrane. This membrane is designed to provide a movement barrier ( uncoupling ) between the subfloor and the tile.
Ditra by Schulter works very well.

Re: floor prep for porcelain tile over hydronic heat

Thanks for the info. I pulled up 8x8 tile from the area I will be adding the heat to and tiling over. Never had cracking in tile or grout. The subflooring is exposed and there is a wee bit of flex in one spot, that's it. The subfloor has a bit of a dip in 2 corners so I was worried about putting down ditra and its ability to adhere to the lower spots. W/tiling, I was just going to back butter those 2 areas to keep it level. I live in SE VA where hydronic floor heat is rare, so I want to have all info before I start, or hire someone to tile over it. It was much easier where I had hardwood put over the hydronic heat. I just laid 1x3s and ran the pex btwn, nailed down the flooring and voile! Thoughts?

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