expectation of 5/4 lumber

[meackerman]

planed it down on both sides and it is and has stayed very flat, but this is for garage door frames, not fine furniture.

that's the key.  take off an even amount on both sides when planing.

[MichaelMouse]

What are your expectations of the final thickness when you purchase 5/4 lumber? 

Also would you expect S2S to be flat?

I was hoping for 1" thick seat on my hall bench. I got some 5/4 lumber unfortunately S2S was all I could get. Then I noticed it was not flat. I started working it carefully since it was only about 1 1/16" thick. I stopped bothering with it once I realized it was going to be a little less than 15/16". This is red oak. I have done better with some 4/4 cherry. Back to the store and get 6/4 this time. To make it even worse the 5/4 at the expensive store was more than the 6/4 I can get at the less expensive store. I should check prices meter before I leave the house.

As indicated, 5/4 is to finish to 1 1/16 to meet standard.  Band mills probably cut 1 1/8, while old circular mills use a real 5/4, because the blades vibrate a bit.  

https://www.engineeringtoolbox.com/hardw..._1462.html

I would expect S2S to be surfaced both faces.  Up north, where growth is slow and the rings in a flatsawn board don't show much curvature, or boards from tropical woods will normally show little cup.