Better to level first. You want to make your slab as smooth as possible prior to tiling or your tile job will be much more difficult. There are several leveling products available. Just use one that is cement-based, not gypsum-based.

You might also want to contact the makers of the radiant heat system and see what leveling compounds *they* recommend, if they have any to stay away from, etc. Sometimes warranty requirements can be very specific wrt product, sometimes not--it's always good to check.

Diane-- before anything else-- you say you're using flagstone-- how thick is it?

Thick. We would never do this if it was not for the fact that flagstone was already laid in the entry of our home in view of the new addition. I've tried to convience DH to bust it up and we'd do it all in the slate, since we've slain that dragon before. I've looked at the stoneyard (we haven't bought it yet) and I think I've spotted ehough that is gauaged to a half inch, obviously the thiner the better. Of course flagstone is flagstone and thick and rustic.

Alice and Flyleft thanks for your words of advice too. Appreciated. We've got tons of photos and I've got to get my act together and learn to post them.

That's what I thought. I've yet to see flagstone that wasn't about 2" thick, but I wanted to make sure. There's no need to "prelevel" the floor. That stone should be set in mud, so the floor would be leveled as the stone gets set.

Any time the stone you want to use is more than about 1/2" thick, it really should be set in thick bed mortar, using thinset only as a bond coat, if at all.

Thanks, I was sort of thinking that. A good thick bed of mud, with lots of squishing and leveling. Always good to get experienced advice. Dian