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Roof Sheathing at Ridges

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Jim, the basic premise of a three-sided diaphragm is that it resists
lateral
forces by rotation.  There is no rotation of a simple-span roof
diaphragm of the type that has been discussed here.

A typical pitched roof diaphragm acts like two parallel beams with a
"fold" at the ridge.  This same phenomena was observed in 1960 APA tests
of folded plate roofs, which consisted of two sloped individual
diaphragms with chords connected at the ridge, in addition to chords at
the eave locations.  The ridge connections of the chords were designed
to resist shear forces acting in opposite directions on each side of the
diaphragm, along the ridge.

The diaphragm shear forces are uniformly distributed both across and
along the diaphragm, at each increment of length.  Therefore, the
diaphragm shear forces act not only parallel to the end walls, but also
in a perpendicular direction along the ridge, as well as in the field of
the diaphragm where the sheathing panels distribute loads in the field
of the diaphragm by rotation and bearing on one another. The diaphragm
shear forces are highest near the end shear walls, so that is the
critical location.  If the diaphragm aspect (L/W) ratio is low, or the
diaphragm is relatively short, or the wind/seismic forces are not high,
then the shear forces may be small enough that the unblocked sheathing
edge joint, and end-joint discontinuity in the roof framing at the
ridge, may be sufficient to resist these forces, at least in the design
range.  That is probably why most residential roofs sheathed with wood
structural panels do not have problems in wind or seismic events.  In
our tests, we found that lightly-constructed roof framing (e.g. typical
for manufactured homes) had a maximum shear capacity at the ridge --
shear force normal to framing -- of about 100 lb/ft.  Beyond that, sheet
metal "blocking" was needed over the sheathing at the ridge and stapled
through into the sheathing, to reinforce the ridge connection and
transfer shear forces from one side of the diaphragm to the other.  For
a discussion of this condition, see APA Report 146; a copy can be
ordered by contacting the APA Help Desk (help(--nospam--at)apawood.org). Examination
of the diaphragm during the test and after ultimate load was reached
plainly showed that the diaphragm shear forces really do act in opposite
directions at the ridge, with the greatest shear forces occuring near
the end shear walls.  At this location, failures in the plated truss
connections at the ridge occurred due to forces acting normal to the
trusses along the ridge.
John Rose
APA (retired)
Tacoma, WA


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