We just had an 18ft. X 18ft. three-season room put in over our old patio. The contractor built a floor of 2X10, 16in. on center and covered it with 3/4 inch pressure treated plywood. It seems a little squishy in a couple of areas. I'm thinking I should maybe put down a layer of half-inch plywood and then quarter-inch Hardibacker to support 12 in. ceramic tiles. Or could I get by with just using half-inch Hardibacker over the existing plywood? I'm also thinking of putting down an electric heating mat under the tile, such as Suntouch. Do those things work o.k. and hold up over time? It's a big investment, but it would sure be nice to warm up those tiles a little bit in the winter to try to extend it to more of a four-season room.

Hi Tigrena
I would install another layer of 5/8" exterior grade plywood to increase the strength of your floor and then a layer of 1/4" cement board for your tile.

Make sure that there is a vapor barrier under your floor if there is exposure to the elements especially in a sun room where humidity changes are extreme.

I'm not familiar with Suntouch heating elements but maybe someone else knows their product.

After doing some research, it looks like I will be using a tile-warming cable under the tile. The manufacturers recommend that the cable be covered with a scratch coat and allowed to dry, then install the tile over this. The scratch coat with embedded cable would be about 3/16 inch thick. Would this take the place of the 1/4 inch backer board? In other words, I would put 5/8 exterior plywood on top of the existing 3/4 inch plywood, then the cable, then the scratch coat, then the thinset and tile. Or should I still put the 1/4 inch backer board on top of the 5/8 plywood before I lay down the cable? If so, would I still have to use thinset under the backer board?

I'll usually install the heating element (wires) onto to existing subfloor and then apply a layer of self levelling compound (SLC) which flows evenly over the cables and clips. Because you're installing an extra layer of plywood you can install your tiles directly over the self levelling compound. When I apply the primer for the SLC I roll it generously right over the cables and subfloor.

Bri and Ron might have some advise in this area because I know they do a lot of heated floors.

Hi
I agree with Harry...but I'd like to know what cable you are using...is it attached to a mat, or is it separate wire that is run around the room?

The product is called "Warm Tiles" by a company named EasyHeat. It is not a mat. You put down plastic straps with notches in them and then snap the heating cables into the notches, routing it back and forth with alternating 3" and 1.5" spacing (recommended spacing because the floor is not heated underneath). Theoretically they say you could thinset and tile right over the cable, but it takes some skill to make sure all the tiles have proper contact with the thinset, so they recommend the scratch coat method.

From other postings on your great web site I picked up the hint that I could use Ditra around the perimeter of the room (a one foot space between the cable and the wall) to help keep the scratch coat level. Do they sell this stuff in small quantities?

Hi
That's the heating system I use all the time....and I'll tell you how I do it. I don't use the plastic tabs they give you...I put nails in the floor at the spacing you want to use, at both ends of the room...starting at the spot where the thermostate goes into the wall. I then will string the wire back and forth around the nails. This will let you pull the wire pretty tight and take out the slack in the wire. So instead of the plastic tabs holding the wire, the nails are doing all the work. After you have it strung around the room, I use a hot melt glue gun, and spot glue the wire to the floor. After the glue sets, you can take out the nails..and the wire will be nice and flat against the floor. I found using the plastic tabs, that there were always spots that were too loose, and gave you trouble when doing the tile(they would stick up too high).
Then I put a self leveling cement over the wires...using the primer that is needed first and then the cement. When it drys you have a nice flat floor for laying ceramic tile. I always put Ditra over everything first, and then the tile...but other installers go right over the self leveling cement.

Thanks Brian. Sounds like a good way to do it. But it brings up a question: when you pull the cable tight, wouldn't it cause the cable to bend at a right angle around the nail? The info on their web site says not to bend the cable at a right angle (minimum bending radius is 1/2 inch). Or is the amount of tension used enough to pull the long runs straight without actually kinking it around the nails?

Bri .... that's a great way to do it. Those clips are too damn annoying to use.
Maybe using 3 nails at each loop end to create an arch would take care of the sharp angle.
What do you think Bri?

Hi
Good point...but I do string it around 2 nails at the ends of the loop...this is how I get the 2.5 inch spacing that is required. After you glue the wire, and then pull out the nails..the loops at the end spring back to a nice smooth round radius..so the wire in not damaged. But now that you mention it Harry, the third nail would work great too.