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Discussion Starter · #1 · (Edited)
Hello all =]

I am planning a 200sqft heated marble floor installation. The floor layering plan was: Original 1/2" planking>3/8" plywood>1/4" thinset>1/2" wonderboard>warming wire embedded in 3/8" self leveler> 1/2" thinset>Marble.

Would it be okay NOT to use wonder board on top of my 3/8" plywood (this would basically leave 1/2" tongue and groove planking, and 3/8" plywood beneath the warming wire+ 3/8" leveler layer). Once the self leveler hardens I would use a 1/2" trowel to thin-set in the marble.

I suppose the self leveler could replace the wonder board in this application since it is creating flat, level surface for the thin-set and marble. The only negative I can think of is that there is still some flex in the 3/8" plywood 1/2" planking which could lead to cracking in the leveler layer.

What does everyone think?
 

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Paul
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It's only 7 sheets of plywood. Why not strip it down to joists and start with 3/4" ply? Then build off that.
No question about it. 200sf I would demo everything to the joists and start over. Pour the SLC deep enough to give you a smooth, flat surface to lay directly on. Less **** in the way of your heating mat to boot.
 

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Discussion Starter · #5 · (Edited)
Actually, I double checked and the planks are 3/4" plus the 1/2" plywood I put down, so I now have the 1 1/4" necessarily for natural stone. After priming the plywood I will secure the heating mat then embed it in ~3/8" SLC. After the SLC has set I will install Ditra using 1/4" unmodified thinset. I will then install the marble on top of the Ditra using 1/2" unmodified thinset.

Good to go?
 

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What brand of heatmat and slc?

Another important thing is the size of the floor joists( or truss )and their span and spacing between the joists. What do you have? Remember natural stone requires a stiffer floor.
 

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Discussion Starter · #7 · (Edited)
What brand of heatmat and slc?

Another important thing is the size of the floor joists( or truss )and their span and spacing between the joists. What do you have? Remember natural stone requires a stiffer floor.
Calculated weight deflection for this floor installation is well above what is necessary for natural stone. I am only slightly concerned with the flex in between the spacing of the joists because I do not have two true layers of plywood (original 3/4" planks <re-screwed where needed> topped by 1/2" plywood). However, to answer your question the joists are Fir 2x10s spaced 16OC with a maximum span in each room of about 9.5 feet.

The heat mat brand is LATICRETE, the SLC is TEC Ultimate Self Leveling Underlayment. TEC says thier SLC does not need lathe over plywood (just primer) if you have two layers of PLYWOOD totaling 1-1/8" - I am hoping my 1-1/4" plank/plywood combination is good enough. Since I will be installing ditra between the heat-wire embedded SLC and the marble that should help mitigate effects on the marble due to horizontal sublayer movement.

What do you guys think?
 

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Paul
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Should work but I'd get rid of the plank and double layer 3/4 if it was mine.
 

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I've done it like that tons of times without problem. You gain space for out of level older floors. Make sure to use a good bonding agent and secure the heating Mat down well. Currently I prefer the fabric style elements that you can thinset down first too avoid them floating up in the self-leveler. And like everyone else said, a tight subfloor is key. Good luck
 

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Actually, I double checked and the planks are 3/4" plus the 1/2" plywood I put down, so I now have the 1 1/4" necessarily for natural stone. After priming the plywood I will secure the heating mat then embed it in ~3/8" SLC. After the SLC has set I will install Ditra using 1/4" unmodified thinset. I will then install the marble on top of the Ditra using 1/2" unmodified thinset.

Good to go?
Everything you stack on top of the floor is irrelevant without knowing the deflection of the joists that support it.

Modified under ditra.
 

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Talking Head
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If you are getting flex with the 1/2" ply then I'd at least swap that out for 3/4" ply. The cbu doesn't really do you any good so I'd take it out to save the height.
 
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