1/2 story bathroom remodel

[bjh125]

I am currently in the process of remodeling my upstairs bathroom in my 1 1/2 story cape cod home. This is the type that the upstairs ceiling is angled at a 45 degree angle starting at about 4 ft up the wall. Anyways, the existing plumbing and such is that the tub sits parallel with the short wall , in turn making you lean a little if you were to take a shower. This doesnt bother me, as it is the kids bathroom.
My problem lies in the ceiling rafters. They are 2x4s 24 inches on center. I dont think they are strong enough to hold the weight of backerboard and tile. I have beefed up all of the surrounding walls to be backerboarded and tiled , but am at a loss as to what to do with the angled part of the ceiling. It had sheetrock on it, but I figured it would get wet when showering so I elected to tile it. Now that its all tore apart, I dont think 24 inches on center will suffice for the weight . Is there another material I should use on the ceilings or some other method used for these type of ceilings?
My origional plan was to use 12x12 floor type tiles in the shower instead of the little 4x4 ceramic tiles. the smaller is lighter, but I think still too heavy for the 2x4s. Should I just sheetrock the ceiling and paint it with waterproof paint? I really wanted to tile it , but I think Im not going to be able to.
Any help is appreciated, and I can take pics if needed. Thanks

[jimbo]

First, here is a very good reference for tile work:
www.johnbridge.com

I do think that you are correct in your intuition that 24" OC rafters would not properly support backerboard + tile.

One idea might be to use regular drywall, and cap it with FRP. This a plastic panel, comes in 4X8 sheets and they have accessory joint caps, corners, etc. Find it at HD or any building supply. It is lightweight, glues onto the drywall. This is the stuff you frequently see on the walls in commercial restrooms. It is available in white, and some other colors and textures.

DO NOT use the 4" square tile pattern board you will also see in HD, that is acrylinc coating over masonite. That is NOT for showers. One tiny scratch, or drip through a caulked seam, and the masonite explodes!