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SLC & Radiant Heat Flooring

by Todd Groettum - October 2006

SLC & 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 - 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.

About the Author

Tile Guy Todd

Todd Groettum, also known here at The Floor Pro as tileguytodd, has been in the tile trade for over 20 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 moderate on the John Bridge Forum and enjoy helping other professionals and the DIYer too. Tile Guy Todd joined us here at in July 2006 and has been a valuable asset to our members and visitors too.

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

Step 1. Apply one coat of Self-Leveling Cement primer.

Step 2. Install radiant heat system as per manufacturer's instructions.

Step 3. Test the heat system with an ohm meter and note the numbers.

Step 4. With bare wire system install plastic lath carefully with staples (I recommend Mapelath* from Mapei).

Step 5. Install self-sticking foam strip insulation for expansion around perimeter of room (I recommend Frost King x " high density foam strip available at home centers). See notes below concerning Expansion Space.

Step 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.

Step 7. Protect floor air ducts by stapling heavy cardboard strips into them, caulk around edges and duct tape inside for added protection.

Step 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.

Step 9. Test again - this time, the wire leads with ohm meter and note the numbers.

Step 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.

Step 11. Wait approximately 1 hour for primer to dry. While this is drying prepare your bags, buckets mixing equipment etcetera.

Step 12. Test again all leads with ohm meter again making certain no damage has occurred. This is your last chance before wires are covered.

Step 13. Pour Self-Leveling Cement using the following instructions.
A 200 square foot area, " 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 ". 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 8x10 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 3 minutes per bag we will complete the mixing and pouring of this project in approximately 25-30 minutes.
Step 14. Allow the Self Leveling Cement to dry overnight.

Step 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.

Step 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.

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.

Step 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.
Notes about Expansion Space: The perimeters of the floor needs expansion and to achieve this attach some self-sticking insulating foam in strips x " 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.

Notes 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.

Notes 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.

Notes 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: English Spanish

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