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I am renovating a bathroom and installing radiant heat under a tile floor. The subfloor and cement backer board were installed without considering the finished height of the floor. What is a good way to ensure that the self-leveling cement is thick enough, but not too thick?

The floor is a long, narrow room, 5' × 12'. The heating mat covers 22.5 sq ft in the middle with unheated areas along all 4 walls. Per the mfg, Thermosoft, I have stapled the mat to the backer board over a plywood sub floor. Next I plan to embed the mat in a layer of self-leveling cement prior to laying 1-inch square tiles using thin-set. The height difference from the backer board to the adjacent hardwood floor is 9/16". Mat is 1/8". Tile 1/4". How do I make sure that my self-leveling cement is a consistent depth for the entire length of the room?

I am thinking using nuts that are the correct height and securing them in place every 18 inches with a recessed screw. When I pour the cement, I can let it come up level with the top of the nut. It won't matter if it's visible since the tile will cover this surface. Will this work?

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  • Why are you using self leveling cement, is the floor uneven? – Alaska Man Mar 4 '19 at 18:36
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    The floor has a slight low spot in the middle that this would fix, but mostly I need to cover the radiant heating wiring and I need to raise the floor about 1/2 inch to be level with the adjacent hardwood floor. – sjones171 Mar 4 '19 at 18:50
  • Sometimes you can not get two adjacent floor surfaces to be at the same height. They make transition strips for just that situation. – Alaska Man Mar 4 '19 at 19:01
  • Right, and I may have to go there with the transition piece. The wiring mat does not cover the entire floor, just the open areas. No need to heat under the claw foot tub, vanity, etc.What I'm hearing is pour the self-leveling cement and hope for the best, or use a transition piece. – sjones171 Mar 4 '19 at 19:08
  • "What I'm hearing is pour the self-leveling cement and hope for the best" you did not hear that from me. Using self leveling cement is an acquired skill and hopping for the best can result in big problems and drive you NUTS. – Alaska Man Mar 4 '19 at 19:11
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Self leveling cement is not used to raise the whole floor up to where you want it, it is for filling low spots or slopes to make the floor FLAT.

Take up the cement board, If it was not put down with some Thinset or adhesive, and put down the appropriate thickness of plywood so that when you put the cement board back down you have you floor at the proper height. ( OR ad a new layer of cement board to get your height.)

Fill the low spot on the sub-floor before you put your cement board back down.

Save your nuts !

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  • What about covering the radiant floor wiring? The instructions with that say to cover with self-leveling cement. – sjones171 Mar 4 '19 at 18:53
  • can you post a link to the instructions for the radiant floor product. ? – Alaska Man Mar 4 '19 at 18:56
  • thermosoft.com/en-US/radiant-under-floor-heating/… – sjones171 Mar 4 '19 at 18:59
  • I cannot use the thin-set mortar approach because my tiles are 1-inch square and will not stay level over the mat. – sjones171 Mar 4 '19 at 19:01
  • OK you need to fix your question by adding all of the pertinent information so we do no keep playing this game. Put a detailed description of what your plan is, what product you are using, the manufacturers instructions and what you have done. Using nuts is NOT the proper way to do it. – Alaska Man Mar 4 '19 at 19:06
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Simply use the screws without the nuts, this will let you adjust them up or down for the high or low spots.

If I see your thicknesses properly, you will need to keep the pour just enough to cover the heating wires, 1/4" at the thinnest to account for a little thinset for the tile. Did you prime the floor first? Most self-levelers require a primer.

For what it is worth,if the floor is a little higher than the adjacent floor, you may be able to set a stone threshold as was the custom back in the day. This will break up the differences in the floors, if any

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  • Thanks for your input on the screws Jack. I am putting the primer down this morning and waiting the required 2-3 hours before pouring. I knew the heights were going to be tight which is why I wanted a method for estimating the height of the self-leveling cement. – sjones171 Mar 5 '19 at 14:36

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