You gotta read the instructions.I've never seen spec with an exception when a heat mat is used.
I think we just witnessed a drive by posting! :laughing: Where have you been? I had to verify that the thread was not 2 years old!I've used both mat systems and loose lay cable systems and I prefer the latter.
That said, it is more of a PITA from a labor perspective. The manufacturer should have provided a spacing guide based on the square footage you were covering, and the length of cable--but I know it still takes a little thought and sometimes t&e to get it just right.
In answer to your last question--yes, you'll get a much better result by leveling the entire floor to the depth of the cable. Once the cable is installed, we run screws throughout the room around 2' o.c.--leaving them up at the height we want the top of the leveler pour.
With an uneven floor, a cable system kills two birds with one stone. Even coverage throughout the space, and a perfectly level floor.
Hope that helps.