Replacing Part of a 1×6 Roof Deck with OSB – Roofing Guide

Twofold theory:

Perhaps OSB is cheaper at the moment for this builder than ply and wants to use OSB as much as they can. OSB and Ply are apparently seen as the same in terms of performance and code see: http://bct.eco.umass.edu/publications/by-title/choosing-between-oriented-strandboard-and-plywood/

However, OSB, when cut, is more susceptible to water damage on the cut (the cut exposed raw wood not covered in the bonding agent/glue). Once OSB edges get wet, it expands like a sponge...which would ruin the roof.

So, it looks like they might be using as much OSB as they can, but have to switch to ply whenever cutting.

Skip sheathing for shake roofs used to be really common around my area of the country and I've never removed it (or seen anybody else remove it for that matter). OSB or plywood over skip sheathing makes for a much stiffer roof deck, resists problems with expansion better, and obviously doesn't require all the hassle of removing it.

After you strip the roof, do spot repairs to the existing sheathing where needed, then go right over it. You'll want to use both longer sheathing nails and longer roofing nails. The sheathing nails should be long enough to give a good hold in the studs, and the roofing nails should be long enough to penetrate both layers of sheathing.