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Wayne Whitney Wayne Whitney is offline
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Default Roofing Question

On 2008-04-07, RicodJour wrote:

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.

Engineering practice is to treat conventional roof rafter and ceiling
joist construction as if it were a simple flat diaphragm (joists plus
sheathing), as far as calculating its lateral load resistance. That
is certainly easier to visualize and analyze.

For lateral force applied in a given direction, the only part of the
structure that can carry that force to the foundation are the walls
parallel to the force. So the diaphragm has to carry the lateral
force to the end walls parallel to the force. The diaphragm is
modelled as a deep beam spanning between those end walls. The longer
the distance between those walls, the greater the span of this "beam",
and the less the resistance.

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

Absent an analysis of a conventionally framed roof with T&G board
sheathing, that seems quite reasonable, that the uplift capacity is
the weakest link. Uplift on conventional roof framing puts both the
sheathing-rafter nailing and the rafter-plate toe nailing in
withdrawal, so that probably won't work very well. :-)

Yours, Wayne