I am planning to tile over an existing concrete slab in my backyard.
The slab is level from one side to another, but there is a gentle fall
from end to end. Although the slab is fine overall, there are a few
ups and downs and I want to make sure that the tiles don't have highs
and lows as I tile down the slope. I presume a string line would do
the trick, but perhaps someone has better ideas for the amateur.
Thanks Philip Lock

if the slope in the slab is for drainaige away from the house, it's a
good thing, and I would tile with the slope. The only thing you'll
have to worry about are the dips and crowns on the floor. But if it's
flat, and only sloping, go with the flow.

tim1198