On Apr 7, 11:42 am, Wayne Whitney wrote:
On 2008-04-07, RicodJour wrote:

On Apr 7, 12:09 am, Wayne Whitney wrote:

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.

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.

Right, I was referring to the whole system deforming, so that "each"
rectangle deforms, together.

Which doesn't happen. Deformation happens locally and then spreads
when the initial resistance is overcome. By the time the whole roof
is involved you've got one hell of a big and airy skylight.

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.

The longer the ridge is, the lower the resistance to lateral forces
will be.

Maybe you should review that sentence. How can the resistance be
lower with a longer ridge? More rafters are involved, more points of
attachment, more moments resisting racking. It can't be lower. If
you meant proportionally lower, well, I don't buy that either as the
roof is probably of a piece, but for argument's sake, exactly how
short/narrow of a house do you think DD lives in?

As for uplift, that's a great point--increase the racking
resistance enough and you can be sure that the uplift failure mode
will dominate.

You can always reinforce something to make something else the weak
point. That's not the issue. The issue is whether it's money well
spent and whether it's a real risk.

So _if_ the OP is in a high wind or seismic area, and
_if_ the OP wishes to take the opportunity to strengthen the roof
structure, then it would also make sense to add hurricane clips to
resist uplift.

IFF (if and only if) that is the case, the hurricane clips should be
the first thing installed.

R