I just had an old house reroofed including redecking the entire house. The original decking was spaced 1x4s. The roofers removed the old shingles, shakes, and nails and laid the new 1/2" plywood over the 1x4s. They did not take care to make sure than the edges met on 1x4s and made no attempt to hit rafters w/ nails.
But my real question is this: when you redeck over existing decking you necessarily raise the height of the deck by the thickness of the new decking, in this case 1/2". That means that the decking is no longer flush w/ the fascia boards or crown moulding as the case may be. The edge of the plywood is visible. Is this standard redecking practice and what do you do about the exposed plywood?