Ray K wrote:
The books stress having the slab flat, grinding the high spots and
filling the low ones. But just how flat does it really have to be? Can't
I just use a little thicker layer of mortar to fill the known low spots
as I lay the tiles, especially if I "beat" the tiles into the mortar to
assure good contact as some books recommend?

Some books and websites also stress the importance of a level slab.
What's so bad about it not being level to within 1/4" per 10 feet?

My slab is at grade.

Thanks,

Ray

We had two large rooms in our condo (on a slab) tiled several years ago.
It never occurred to me to see whether the slab was LEVEL - good
grief! If it wasn't level, it was long past the point of doing anything
about it. Being an older building (build around 1975), the major
concern was cracks in the slab. We tore up our old carpet and padding
to make sure of what we had before we ordered tile. We found the tile
we liked and then had a contractor install it. Not one crack in the
slab. If there was any unevenness, it wasn't obvious - seems like we
slid a level around a few places to check for dips, but nothing noteworthy.

One challenge of our job was bringing tile to the level of the terazzo
floors in adjoining rooms....solution was to apply the mortar a little
thicker and slant the tiles ever-so-slightly. Can't tell by eyeballing
it, but the difference would have been about 1/4". The tile guys did a
perfect job, even with several challenges.