I'm looking at laying tile (12"x12") directly on a basement slab for my bathroom floor. The expansion joint that is cut diagonally across part of the basement floor is in the corner of this room (probably 3 or 4 tiles will actually be over the joint). Will this cause any problems? Also, is there any other prep that should be done to the slab prior to laying the tile?

You really can't guarantee anything. Design with Flexibility -- Ceramic Tile Installation | Extreme How To, DIY - Do it Yourself, Home Improvement, Home Decorating (http://www.extremehowto.com/xh/article.asp?article_id=60206)

What I ight suggest is to keep the joint in place. Cut the tile and fill with polyurethane caulk through the joint. e.g. a small 1/8" intensional seam. That, in my opinion would be better than an unintentiional crack.

If you could disguse the diagonal to make it part of something else like a zig zag, it might be better. I also believe there may be metal expansion joints that can be used. Building Homes or Extensions: concrete, metal expansion joints, stone balustrade (http://en.allexperts.com/q/Building-Homes-Extensions-2333/2009/7/concrete-15.htm)

TCNA - Technical Services: FAQ (http://www.tileusa.com/antifracture_faq.htm#antifracture2)

Why not tile diagonally and make the grout joint over the expansion joint. You don't use grout there. Use caulk.

Another alternative might be to place a thin subfloor.

The expansion joint that is cut diagonally across...

If it were a true expansion joint, where you have two separate slabs with a filler between them, you would have to honor the joint up through the tile at the exact location of the joint. By describing it as "cut" I will assume this is not an expansion, or "cold joint" but a "control joint" which is a saw cut designed to encourage any future cracking of the curing slab to occur at predetermined places. They still need to be honored through the tile installation, but they can be relocated. You have two options...

To relocate a control joint, the control joint is covered on both sides by an antifracture membrane that extends at least 1.5 tile widths past the joint on all sides. If the tile is on a diagonal to the control joint, make sure to take the diagonal measurement for your 1.5 tile widths. When your tile is done being installed, the grout joint on either side of the tiles that are bridging the control joint are filled with a color and texture matched caulk instead of grout.

In your case, since it's a bathroom, you're probably better off just covering the entire area with an antifracture membrane and letting the clear perimeter gap (or caulk filled perimeter gap) be where any movement is released from the tile assembly, just for the sake of not having caulk joints within the tile field which just like any caulk joints, will need periodic maintenance.

To answer the rest of your post, as long as the slab is free of dirt, paint, sealers, water soluble adhesive residues, and will absorb water, you are good to go.

Thanks for the quick answers; I like the diagonal design idea, since the space is so small I think either that or a thin subfloor may be the answer. One question regarding the subfloor; how is that best attached to the slab?

I use Ditra whenever possible and appropriate. But in this case also consider Noble's CIS. With CIS you can maintain the pattern without make new unwanted joints. Sheet Membranes for Waterproofing, Crack Isolation and Sound Isolation (http://www.noblecompany.com/Products/SheetMembranes/tabid/58/Default.aspx#products)

Jaz

No reason why it can't float especially if the subfloor can be one piece or you can attach at a couple of points (cement anchors) with some play. Just make sure it doesn't attach to both sides of the joint.

Thanks for the quick answers; I like the diagonal design idea, since the space is so small I think either that or a thin subfloor may be the answer. One question regarding the subfloor; how is that best attached to the slab?

There is no thin "subfloor" such as you might think by adding such a product as a plywood or cement board layer over the slab. An unbonded mud job which would be tar paper and lath with a minimum thickness of 3/4" of mud (4-5 parts sand to 1 part portland cement) packed tight is one version of an acceptable "floating" floor system for tile. Antifracture membranes are also a floating floor of sorts, the Noble CIS allows up to 1/8" of horizontal movement below. Schluter refers to it as "uncoupling" as their fleece backing on the Ditra allows the plastic to float above the movement. (Jaz, how it is Ditra "uncouples" while CIS "crack isolates" when they are both fleece backed? :confused: ). Liquid membranes like "Redgard" or "Floor and wall watertight" available from HD or Lowe's, respectively, are liquid membranes that allow elongation of the membrane below where it bonds to the substrate while maintaining the integrity of the tile above. All of these are what you should be thinking of when you think about floating a tile floor. In your case, skip any idea that involves mechanical fasteners, wood products or cement boards.