Consider using Ditra under the tile.

I'm not positive about the outdoor application, where are you located?
But check the Schluter (manufacturer) website, I'm sure you'll find
some decent info there.

M
smcjensen wrote:
I have a screened in porch that currently has increasingly gross
outdoor carpeting on it. A dog, two kids, a sandbox, spilled
bubble-juice, and one end that gets soggy each time it rains has made
it pretty gross out there. It's now beyond the help of a steam
cleaner.

I was thinking of putting tile down out there. The subfloor is
pressure treated 3/4 plywood. It feels structurally stiffer than the
floors in the house, seems well built. I figured I'd put down cement
board over the ply, mortar those joints, and put down some one foot
square tiles with some texture to them to get traction when it's wet
out there. Any recommendations along the lines of do it or don't do
it, adhesive type, grout type, tile type? My biggest fear would be
expansion and contraction with the seasons causing cracks since the
subfloor's not in a climate controlled area.

Thanks, all.