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Cut the cement board to size. Place the cement board on the subfloor.
Use a screw gun to drive screws into the cement board every 8".
Set an ohmmeter. Connect the wires from the floor mat to sensors. The reading should be within 5 percent to 10 percent of the factory reading (28).
Move to critical areas where heat is desired. Mark the outside of the mat with a pencil line to use as a guide so the thinset doesn't extend beyond the mat.
Mix the thinset, and add the thinset mix to water. Mix until it's of soupy consistency.
Scoop the thinset out of the bucket with a trowel. Spread the thinset with a notched trowel in pre-marked areas.
Lay the mat over the thinset. Smooth it out with rubber float. Make sure to apply enough pressure to secure the mat to the floor (Image 1).
Apply the probe 1'-2' off the end of the mat, near the electrical connection, between two hot wires on the mat.
Tape down the probe with electrical tape (Image 2).