Hearth extension from block to wooden subfloor.

[NRGarrott]

I'm redoing my hearth, and it is partially built on a block foundation that the chimney sits on, and partially built on a wooden subfloor. Where it transitioned the tile popped off the first time. The original builder nailed wire lathe to the subfloor, then a 2 inch layer of thickset that extended onto the block foundation. The original builder only had a 3 inch extension onto the subfloor, but I am going to make it 8" in order to reach the clearances I need. I am worried about it cracking again. I have Micore 300, and whatever home depot sells to make this work. Any ideas?
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[semipro]

This seems like a tough one to me. It would seem that there's always going to be some vertical movement of the floor with respect to the masonry.
I can see two possible ways to install tile so that cracks don't occur at the interface between masonry and wood.
1. Cut back the flooring and subfloor, reframe the floor joists as necessary, and then build a proper masonry foundation extending out as far as you want to extend the tile.
2. Use steel plate supported by the masonry to create a cantilevered shelf that hangs out over the wood portion of the floor. I'm not sure how you'd tile over that though but I'd bet there's a way.

Neither sounds particularly promising but I thought I'd get some ideas rolling.

 

[Warm_in_NH]

Wood meets concrete, the two will always move independently of each other and will inevitably always crack in that same spot.

Ideal way, build an entirely new pad the right size and go from there. This is messy expensive and time consuming.

Mickey mouse way that may last for several years but will eventually fail. Cut the hardwood flooring up to the edge of your 8" extension, set in 1/2" cement board filling the gap and shimming it up the the level of the concrete that's there, or close to it, a hair lower works. Get fiber webbed tape (like for sheet rock) set the tape over the joint from old slab to new cement board and thinset it in place like you were mudding a sheet rock wall. Then tile it but make sure your tile bridges the new joint and doesn't break on it. Ie 12" tile would go 8" on one piece and 4" onto the other.

Not saying this is the right or only way but it's an easy way out that'll get you to code in a matter of hours and may last years.

 

[bsruther]

I would probably tear out the old hearth, cut out the wood floor or bring the hearth sub floor up to floor level and build a new hearth base.