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Re: Plywood Diaphragms

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I agree with Nels; the existing roofing should be removed. This will allow for a
smoother, even nailing surface so the new roof deck won't have high spots at panel
edges that could later cause roofing leaks.  Also, as Nels points out, the
intermediate layer of roofing would create a soft layer that would create more
nail slip at applied lateral load on the fasteners.

The existing roof sheathing boards should be inspected for deterioration and
replaced as needed, then new plywood or OSB roof sheathing can be installed over
the existing roof deck.  The roof deck connections at the perimeter (boundary) to
framing, ledgers, etc. is especially important.  Larger (longer) nails are needed
to provide proper penetration through the existing roof deck board sheathing into
the framing at the boundary.  Install the panels at a 45 degree angle to the
existing sheathing boards, so that all panel edges will be located off the joints
in the board sheathing; also the adjoining panels should be installed at 90
degrees to each other (ashlar fashion, end to side) to equalize diaphragm shear
forces.  The roof sheathing panels can be nailed to the existing board decking
which then acts as blocking to stiffen/strengthen the diaphragm.  Oregon State
Univ. conducted a diaphragm test in the 1970s to study diaphragm action when
plywood sheathing was nailed to a straight-sheathed lumber diaphragm (1-1/2 in.
thickness); the report is available from APA (contact APA's Help Desk at
253-620-7400).  This research was the basis for the provisions in the UBC that
allow this type of construction with diaphragm values as set forth in Table
23-II-H of '97 UBC.  If sheathing is only 3/4 in. thick, I'd suggest using
deformed shank nails for the sheathing attachment; or better yet nail the roof
sheathing panels through the board sheathing to framing below.  Mark the framing
locations on the existing sheathing with chalk lines to line up where the nails
should be placed on the new sheathing.
John Rose, APA (Tacoma, WA)

NRoselund wrote:

> Richard Dahlmann,
> My understanding of the idea is to install a new plywood diaphragm over
> existing roofing, forming a roofing-sandwich between the new diaphragm and the
> old roof structure?
> If I have that right, I don't like it.  How will the installer know what the
> nails are being driven into: a sheathing board near a board edge, the space
> between sheathing boards,  close to (or off) the end of a board at a butt
> joint over a rafter, or well into a sound sheathing board with the nail well
> centered in a rafter?  It seems that most nails are likely to have inadequate
> penetration.  Nail values assigned by the Code are based on proper
> penetrations, as in UBC Table 23-I-G.
> Appendix I (eye) of the NDS for Wood Construction illustrates the six 2-member
> yield modes considered in developing the strength values for connectors in
> wood -- all have the the connected wood members directly in contact.  I think
> that an assembly with a 1/2" gap of roofing would be an untested kind of
> assembly with unknown strength values.
> Nels Roselund
> Structural Engineer