Designing Kitchens and Bathrooms - uneven bathroom subfloor

I planning to install bathroom tile on the floor of a small bathroom ( < 30 sq. ft.). My problem is that the subfloor is not quite even, due to minor variations in the boards between the floor joists. I am thinking of laying wonderboard or a hardybacker down to back the floor, probably over a piece of plywood, but I don't want the floor to have any flex in the future. One option I have thought of is to pour an actual concrete pad in there, but that is a much larger undertaking than rigid backer boards.
Any thoughts?

-Chris, in Palo Alto

You're headed down the right path in your thinking, a cement underlayment is the way to go to correct the problems you have discribed, also it the best. Floating is the term used by tile contractors to discribe the placement of the cement, they use a portland and sand mix, and combined with just enough water to hold the mixture together, this is refered to as a dry pack. The process of floating a cement underlayment with a dry pack is not that complicated, once you've done it, or have seen it done,it would take some time to discribe it here, instead there's a DVD at www.tradesecretsofthepros.com it's called Master Bath Floor, it shows all the steps and process's it takes to float a floor. Hope this helps you and good luck on your project.

myke

christyler,

What Mike is suggesting is the best way to go but if there is any flex in your current floor system, I would suggest reinforcing the existing joists - double them up if you need to, then procede with the dry pack application, it is considered the best substrate for tile or use the cement board which in most cases will prevent issues like having to cut down existing doors, trim locations, etc. If you do use cement board, install as per manufacturers directions.

Good Luck!

Thanks for the advice. It looks like I'll float a mortar bed and go from there. It will be a bit harder, but hopefully will increase the longevity.

Chris