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SLC & Radiant Heat Flooring

Self-Leveling Cement & Radiant Heat Flooring

When installing electric radiant heated flooring, you will find that every manufacturer has slightly differing recommendations in their installation guidelines. Following the manufacturer’s recommendations for installing the wires or mats is important. Following their guidelines for installing tile directly over these mats generally leads to frustration and in many cases an installation that has an inferior finish due to trying to set over a surface that is not flat, and sometimes one that has sensors that are thicker than the thinset bed you are trying to set tile into.

As a professional installer, I have found that installing the electric heat and following this with a Pour of Self Leveling Cement to a thickness of ⅜ – ½” provides an excellent flat surface for tile installation and additional mass to retain heat. Once poured it also protects the Heating System from damage while setting tile.

The following method is what I… what my company, Tilewerks, uses:

  1. Apply one coat of Self-Leveling Cement primer.
  2. Install radiant heat system as per manufacturer’s instructions.
  3. Test the heat system with an ohm meter and note the numbers.
  4. With bare wire system install plastic lath carefully with staples (I recommend Mapei Mapelath*).
  5. Install self-sticking foam strip insulation for expansion around perimeter of room (I recommend Frost King ¼ x ¾” high density foam strip available at home centers). See notes below concerning Expansion Space.
  6. Cut 1 x 2″ wood trim into dam blocks for use at doorways or anywhere tile will end and meet with another flooring type. Duct tape leading edge and bottom to ensure easy removal.
  7. Protect floor air ducts by stapling heavy cardboard strips into them, caulk around edges and duct tape inside for added protection.
  8. Plug escape routes by sealing any potential leak areas with caulk, or larger areas with expanding foam. See the notes below concerning Sealing Escape Routes.
  9. Test again – this time, the wire leads with ohm meter and note the numbers.
  10. After all material used to seal potential leak areas has dried, coat area with SLC Primer using a pump type garden sprayer for insecticides, available at garden supply centers. See the notes below concerning Priming The Substrate.
  11. Wait for primer to dry, approximately 1 hour. While this is drying prepare your bags, buckets mixing equipment etcetera.
  12. Test again all leads with ohm meter again making certain no damage has occurred. This is your last chance before wires are covered.
  13. Pour Self-Leveling Cement using the following instructions.
    • A 200 square foot area, ¼” thick is really not that big of a deal. Two men can handle it. Once it is primed and ready to pour, I am going to figure 8 bags of SLC, if the yield per bag is 25 sq. ft. per ¼”. To do this you will need:
      • 3 clean 5-gallon buckets (1 to hold and rinse mixing drill, 2 for mixing in rotation)
      • 1 mixing drill with egg beater type-mixing paddle
      • 1 margin trowel
      • 1 extension cord
      • 1 water-measuring pail (mark with sharpie to exact required amount of water per bag)
      • 1 garden hose
      • 1 20-30 gallon clean plastic garbage can pre-filled with water (for dipping measuring bucket into)
      • 1 plastic tarp (about 8×10 minimum for mixing area)
    • Open 7 of the 8 bags of material with a utility knife, completely removing the top (you will open the 8th bag only when you need it).
    • Start mixing keeping in mind the instructions on the bag. Most companies require you mix each bag a full 2 minutes and with a recommended wet edge time of no more than 5 minutes we need to keep moving throughout the process until the entire area is complete.
    • Allowing 3½ minutes per bag we will complete the mixing and pouring of this project in approximately 25-30 minutes.
  14. Allow the Self Leveling Cement to dry overnight.
  15. Remove all dam blocks from doors, air ducts, etc. Carefully scrape down any high spots. DO NOT remove extra perimeter insulation material. Leaving it in place will ensure proper expansion is achieved for next steps.
  16. Using a straightedge that is at least 8′ long, check all areas for flatness. If you do find any areas that are not flat you will want to scrape or fill before proceeding to the next step.
  17. Install a membrane that is designed for use over a radiant floor heat system. See notes below concerning Membranes for Radiant Heat Floors.
  18. You are now ready to layout and install your tile following recommended guidelines and being certain to include field expansion joints as per TCA recommendations in detail EJ171-05.

Good luck and enjoy the warmth and luxury of your new radiant heated tile floor.

noteNotes about Expansion Space: The perimeters of the floor needs expansion and to achieve this attach some self-sticking insulating foam in strips ¼ x ¾” high. It is available in 17’ rolls. You can trim off the excess with a sheetrock knife after the SLC hardens. This permits expansion of the SLC and will prevent tenting of your tile. I generally leave the excess above as a backer for our tile to insure expansion for the tile as well.

noteNotes about Sealing Escape Routes: Any cracks, holes or potential leak areas should be filled with caulk, minimal expanding foam sealant or a rapid setting thinset. Stuff floor heat registers with rags & cover before pouring. Cardboard and duct tape will seal these off. Mark where these are, writing the dimension on the wall above the pour line but behind the baseboard and leave a piece of folded over duct tape on the cardboard raised so it will stick above the floor as a reminder.

noteNotes about Priming The Substrate: Read the directions on your primer product for preparing the substrate prior to priming. This is the most important and not doing so is the cause of many failures. Oil, grease, sealers, adhesives, etc must be removed preferably mechanically rather than chemically. Spray the primer into the substrate with a garden insecticide sprayer or roll on with a paint roller. If you do not prime the floor the job is going to fail.

noteNotes about Membranes for Radiant Heat Floors: I recommend quality membranes from manufacturers like Durock, Schluter, Noble and Custom Building Products. This step is not included in the 2005 TCA handbook detail RH140-05, but is recommended by most radiant heat manufacturers and better tile contractors in North America and around the world. For more about membranes, see the article Ceramic & Stone Tile Underlayment, by Bud Cline.

*Mapelath Data Sheet: pdfEnglish pdfSpanish


This article was originally published October 15, 2006. If you would like to discuss this article, or any of the other articles you have found here at TheFloorPro.com, please join us in the Articles Discussion Forum. If you are not yet a member of The Floor Pro Community, we invite you to register now, it’s free and gives you the opportunity to enjoy all the features of the site.

Todd Groettum
Todd Groettum, also known here at TFP as tileguytodd, has been in the tile trade for over 25 years. He has been located in Warba, MN for most of that time under the business name Tilewerks. Todd does mostly commercial and high end residential work, and with too little time left over, Todd still manages to participate on flooring forums, helping other professionals and the DIYer too. Tile Guy Todd joined TFP in July 2006 and has been a valuable asset to our members and visitors.

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