How Do I Know Whether or Not I Need To Use A 2x12 or 2x14 For My Stair Stringer?
You would need to contact a structural
engineer for the exact answer you're looking for. However, I can provide
you with a few suggestions.
Most of the stairways I built before the 1990s were built out of 2 x 12 and the ones after were built with 2 x 14. I really don't know the reason why, but I think it has more to do with protecting the structural engineer from frivolous lawsuits than anything else.
In the construction world of structural engineers, bigger is usually always going to be better and will provide them with a little extra security, just in case there was any type of building material failure.
With that said, if I was going to build a stairway today, I would spend the extra money on the 2 x 14. The additional cost won't usually be much, but could provide you with the same additional security the structural engineer is looking for.