A standard 3-tab is 18" by 36" with an 8" exposure, right?
That means that there's a 2" band to put a nail where it
will both penetrate the back edge of the shingle below,
and be covered by the shingle above. Within that
2" strip, I expect you want the nail as low as possible
so that there's less flap to blow around in the wind,
but back far enough that wind-driven water doesn't back up
under the shingle around the nail-head.


8" exposure.....OKaaaaaay.

Ok, so Certainteed's carriage house aren't as standard
as I thought. If you're using a wimpy-ass timberline
shingle that's only 12" deep, then the exposure is 5".

Which still gives you a 2" stripe to hit on the underlying
shingle.