Deflection:

Before you install a radiant heat mat over the old bathroom tile and install new tile over the mat, you should check the floor for deflection. This is the maximum amount the floor can move under the anticipated load (you). Ceramic tile is hard and will break or dislodge if the surface bends under the load. Here's a simple test:

Stand in the middle of the bathroom floor and jump up and down. If the floor moves it has a deflection problem and is not a good candidate for tile over tile installation until you reinforce the sub-floor.

Avoid That Sinking Feeling:

Since you're tiling over tile, you must plan in advance to avoid making the vanity, toilet and tub look like they're “sinking” into the new floor.

Fortunately, many of the new radiant heating mats are no thicker than the depth of the mortar you would ordinarily apply for most tile installations. Combine this with a tile thickness of ¼ inch and the maximum elevation above the old floor would be only ½ inch.

One way to keep the bathroom fixtures from looking swallowed up by the new floor is to remove and reinstall the baseboard. Better yet, why not buy new baseboard tile to complement the new floor?

The raised height of the new floor will also require you to adjust the length of the door(s) and possibly install a new threshold.

Preparation:

Remove or re-attach broken tile pieces. If you remove them, wait until the floor has been sanded and thoroughly vacuumed before you fill the spaces with mortar.

If any of the tiles have checks wider than 1/8”, you should consider a crack isolation membrane. This membrane is a roll on product that you apply to the old tile. The membrane allows the new floor to move independently of the old.

Next, sand the old tile so the mortar has a good bonding surface. A belt sander would ensure a consistent bonding surface.

Please make sure you wear a face mask and safety glasses while sanding. The tiles may have been fired with toxic lead glaze.

After sanding, vacuum the tile and wipe down with an all purpose cleaner. Pay special attention to the areas untouched by the sander.

Installation:

Roll out the mats prefabricated to your specifications by the manufacturer. Some radiant systems, like Quickmat, are self-adhesive and require no mortar to secure them to the old tile floor.

If you're re-tiling the only bathroom, keep a board and some scrap 2 x 4's handy to protect the mats when the bathroom is being used.

Throughout the installation process, use a digital ohm meter to check the resistance of the heating mats. This will help you monitor the mats for short circuits.

Have your electrician connect the heating mats to the power source and install the thermostat. Depending on the local electrical inspection procedures, you might have to wait until the job is inspected before you start laying tile.

Trowel out the mortar over the mats. Some manufacturers recommend latex- modified or epoxy-modified mortar and grout instead of water-based multi-purpose adhesives. Mortar beds thicker than 3/8 inch should work fine for most systems; they just take slightly longer to heat up.

Since you won't be covering the entire floor with heating mats, make sure the mortar applied over open areas, (under the toilet) is level with the mortar covered mats.

Don't bang the trowel on the mat or heating wire to remove excess mortar from the trowel. This could sever the heating wire.

At this point, you would lay the tile. If you have no experience tiling, practice on your neighbor's bathroom floor.

Fire It Up!

Your new radiant heated bathroom floor looks beautiful and you can't wait to get warm feet. Go ahead, but only for 10 minutes. Don't put the system into full operation until the mortar is fully cured. This can take up to four weeks. See why thorough planning is so important?