Hi,

Something I've been confused about. I think this should be a simple
answer, but I've not found it yet.

This is the way I understand construction. Not my field, so I'm
sure I'm wrong.

In stick built construction for the walls you have floor joists,
subfloor, bottom plate (one thickness), wall studs (92 5/8"?), and
double top plates. The three layer of plates is 4.5". The subfloor
could be 3/4" thick. That's 100 7/8". A sheet of T1-11 is 96" and
shorter than the 100". Seems like it would be good for the external
sheet to extend over the subfloor and over the outside of the floor
joists.

What am I not getting here?

TIA

Mike