2 x 12 Outside Stair Stingers
In the picture above they used 2 x 12
stair stringers as the main support for the staircase. This is fine as
long as you can support the stringers with a wall or a post.
The stringer with the black arrow pointing to it is attached to the building giving it the support it requires and the 2 x 12 stringer with the white arrow pointing to it is supported by a post. A free standing staircase like this would require at least a 4 x 12 stringer on the outside (white arrow) with no supports.