On Apr 7, 12:09 am, Wayne Whitney wrote:
On 2008-04-07, Harry K wrote:

Well, if the winds and sheer force are great enough to rip off T&G,
a bit of ply over it ain't gonna stop it.

It's not a matter of ripping off the T&G boards, it's a matter of
deforming the framing so that each rectangle formed by two joists and
two of the T&G boards becomes a non-rectangular quadrilateral. This
only requires that the T&G boards slide relative to each other.
Plywood would resist this deformation much more than the individual
boards do.

It's safe to assume that the guy's framing DD's house didn't use 32"
long scrap to sheath the roof, so it's not just two _rafters_
involved, probably more like five or six or more. At the forces
required to make a, what?, 30' roof start to rack appreciably, the
whole roof would come off in a piece. Other failure modes would be
involved way before roof racking became an issue.

I understand your point about plywood adding to racking resistance,
but there's no indication in this instance - even if in a high wind
area - that the roof would benefit in any significant way from the
additional layer of sheathing. On the other hand DD's wallet would be
significantly lighter.

R