I'm building a cabin and plan to use that T11 siding. If you dont know
what that is, it's plywood with vertical grooves to make it look like
individual boards. In some ways, it's similar to paneling except for
exterior use.

Anyhow, the sheets are 4x8 foot, just like regular plywood. The walls
will be 8 ft. so on the side walls these sheets will cover right to the
roof overhang. The problem is that on the end walls, where there is a
gable, I will have a horizontal joint where the wall sheets meet the
sheets on the gable. I really dont like the idea of just butting the
sheets together, because water will get in, which will not only cause
decay of the plywood ovfer time, but also get inside the walls.

I've considered applying a bead of silicone caulk to the joint before
placing the sheets in place, but there is no guarantee that I'll get
100% of them sealed, not to mention that I intend to apply a paint or
wood stain when the walls are complete, and I know well that paints do
not adhere to silicone caulk.

However, I'm wondering if there is some sort of material made
specifically for this use? I'm aware that when butting together some
pole barn steel, they sell a special metal trim (divider) piece that
sits under the upper sheet and has a lip over the lower sheet. I've
already considered using that same metal piece, and just getting a color
similar to that color I intend to paint the siding. This will at least
keep water out of the walls, but that upper sheet's lower edge will
still get moisture. I suppose I could paint that lower edge before
applying it to seal the wood. I think this is preferred to just using
caulk. However, I'm wondering if there is a specific material made for
this purpose? Anyone know?

Note: I asked the clerk at the "big box" store about this, and the "kid"
had no clue what I was talking about. I figured it was a waste of time
to even ask, but I tried.....

Thanks