Hi, I have a 1920's house and demoed the bathroom. As the concrete flr had cracked, I built up the flr joists with sister 2x6", and many cross beams. It is now perfectly flat.

I will screw down 3/4" outdoor CDX on the joists, on top of that will adhere backerboard. On top of that will adhere 18x18" porcelain tile.

Question 1: should I use 1/2 or 1/4" backerboard? Can I get away with 1/4? Now I have a clawfoot tub, but there is extra joist support where the four claw feet will meet the tile.

Question 2: what thickness can I expect the flexbond adhesion to end up being? I have the knotched trowel that is 1/4" deep. With all the knotches, will the troweled flexbond end up being 1/8" thick (betweeen the tile and the backerboard)?

Many thanks!!!

 

Curious, how big is this bathroom? Asking because most 1920's bathrooms allow you to sit on the toilet, wash your hands in the sink and soak your feet in the tub at the same time . Grew up in one. I know it is an exaggeration, but the smaller the bathroom floor, the smaller your tile. Using 18x18 tile in a small area presents cut problems, fit problems, and will look totally out of place. Just an opinion, mind you. Larger "grand" rooms....larger tile is perfect.
Use 1/4" backer. It gives no support, but provides continuity between tiles. Figure on some sort of cushion under the sharp claws as any movement on larger tile will most likely have a bad effect on the tile, either scratching it, or cracking it.
1/4x1/4 trowel will give you 1/4" of thinset. Of course you compress it, if that is what you are getting at.

 

http://www.netdevi.com/bathroom3s.jpgHi Chandler, thanks much for what you sent. The bathroom is 6' x 7'. The guy at the tile store suggested the big tile on the flr to make the room look 'larger', then 9x18" on the wall. Yep, it was in my mind about how it might be hard to deal with cutting tile. OK, the big tile on the flr is out. Might have to rethink the wall tile size as well. Yep, a spacer under the clawfeet to stop scratching.

So you think the 3/4" CDX, plus the 1/4" backerboard is fine. Like I said under the clawfeet will be cross joists for all of the weight of the tub.

 

My opinion but I would not use anything bigger than a 12" x 12" tile in a bathroom that size - I think it will make the room look smaller if you use 18" x 18" tiles