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Re: 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, which acts like two parallel beams with a
"fold" at the ridge.  We saw this same phenomena in early tests of folded plate
roofs (1960s), which consist of two sloped individual diaphragms with chords
connected at the ridge, in addition to chords at the eave locations. 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 by rotation and bearing on one another). The 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 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 plf.  Beyond that, sheet metal "blocking" was applied
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, where failures in the truss connections at the ridge occurred due
to normal forces acting along the ridge.
John Rose
APA (retired)
Tacoma, WA

Jim Kestner wrote:

> With a 3 sided diaphragm, why would the attachment at 16" or 24" on center
> at the free edge be an issue? The fasteners at this location may be weak at
> the ridge but with a 3 sided diaphragm it needs to resist zero shear at this
> location anyway. There is no shear transfer along this free edge since all
> shear is taken back to the low side and the torsion is taken out by the end
> walls.
>
> Whether there is a continuous ridge vent creating a discontinuity or a two,
> sloped, unconnected, unblocked pieces of sheathing meeting at the ridge, it
> is essentially the same condition.
>
> What I am missing here?
>
> -----Original Message-----
> From: John Rose [mailto:jrose36(--nospam--at)earthlink.net]
> Sent: Tuesday, August 06, 2002 1:19 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Roof Sheathing at Ridges
>
> In testing pitched (sloped) roof diaphragms at APA, we found that the
> ridge joint was critical since both sheathing and framing end joint
> discontinuities occur at this location.  In tests of common (flat)
> unblocked diaphragms, the framing is continuous at the sheathing side
> joint.  Yes, the sheathing attachment can be standard, but remember that
> at the ridge, the unblocked edge has fasteners spaced 16" or 24" oc,
> typically.  The roof framing joint at the ridge is weak in resisting
> shear forces perpendicular to the framing. Therefore, additional framing
> and sheathing attachment may be required to resist these forces at the
> ridge, and transfer them to the other side of the diaphragm.
>
> I recall talking to an engineer in the SF Bay region about "creaking
> sounds" heard near the ends of a commercial building with a pitched roof
> diaphragm, on windy days.  We agreed that it was likely that the sounds
> were from the roof framing connection at the ridge, and sheathing
> fasteners working to resist high shear  forces at this location.
> Structural upgrading was suggested to reinforce the ridge connection to
> resist shear forces at the ends of the diaphragm.
> John Rose
> APA (retired)
> Tacoma, WA
>
> Jim Kestner wrote:
>
> >    Part 1.1    Type: Plain Text (text/plain)
> >            Encoding: 7bit
>
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