Josh wrote:
I'm getting ready to set some ceramic tiles on my kitchen wall for a
backsplash. A few days ago I screwed a couple of very straight 5'
steel bars end to end horizontally along the wall studs, to serve as a
ledge to mount the first row of tiles on. I was very careful to get
the two bars' ends completely flush with each other and level, using a
good accurate 48" Johnson level. I checked their level later that
night, and again the next day. Both bars were dead-on level.
Yesterday I checked their level one more time prior to beginning the
tiling and guess what: they're both off somewhere between an eight and
a quarter of an inch.

I set the level at the same place on the bars each time, and used the
same side of the level. The bars can't slip....they're still securely
screwed to the studs behind the drywall. And their ends are still
flush with each other, so I have a good 10' straight edge along the
wall. But what's up with the changing level?

This house is a 50 year old single story brick veneer, built on a
concrete slab in West Texas. Is it normal for houses or individual
walls to shift that much from day to day? Maybe due to temperature
variations?

Why steel bars? Perhaps they expanded enough to force one end up or
down a little bit?