Stetten Home Services advocates for “an approved roof-decking-assembly to be directly below all roofing materials”.
In reality, the coverage area of the roof (ignoring the gutters) is substantially larger than the decking as it is installed. Why? How? No! 
Work by others: (Shingles bridge the gap between framing and trim)(floppy tails)…

The roofing contractor prices their work “by the square”, without concern for who the framer is and what the specs are, then stands in line to get whooped. Then the framer builds are entire home and is told to install decking last, unless they LEAVE HALF OF THE GABLE-END RAFTERS UN-DECKED, their decking will not reach the last rafter on the other side.  



You see, over the years layers of trim are added, and the roof decking falls way short. There is no way to fix this “NEW ROOF” other than ripping it off and re-decking the entire perimeter so that it overhangs the rafter edges and tails generously.

The most common installation in NC is the “half-tab offset” even for nicer “architectural shingles”, because it is the easiest and fastest method for installation. Often the installation of half-tab-offset and the “short decking epidemic” means that half of the roofing material within 6″ of the gable-end is nothing more than half-tabs with one nail in them each, and that nails is over 3″ from the edge of the shingle. With “short-decking” there is always an increased likelihood of leaks around the perimeter of the roof allowing for wind-driven rain to get under the shingle, long before the shingles themselves begin to fail. Flopping and flapping related to short-decking has been known to reduce the lifespan of shingles too. Short-decking also causes the gable end shingles to be inclined on the edge and depressed 3″ in from the edge, essentially causing the water to run down a slight valley where there is no decking or tar-paper beneath it.