And now it's time for the Covid "I guess" lumber shortage

Yeh, but....
Your talking about strength of the stringer. The treads should be supported every 16" min for 2x's.
I used 2x12x36" SYP PT treads, I wanted more depth on the tread.

Got me 2nd guessing myself, I'm within limits 2x12x36" tread attached to solid 2x12 stringers w/2x4 ledgers(now I can sleep)
Edit: Found the charts


upload_2020-7-29_21-53-42.pngupload_2020-7-29_21-54-50.png