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Re: Conventional Lumber Diaphragms

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At 09:23 AM 9/18/99 -0700, John Rose/APA  wrote:
>As regards wood structural panel sheathing over spaced boards for
re-roofing, I believe it has become common practice to generate some
diaphragm capacity and strengthen roof. It also allows other types of
roofing such as asphalt comp. shingles or tiles to be installed as roofing,
instead of wood shakes which were common in Southern CA.  Spaced board roofs
would have no diaphragm capacity.
>
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Without disagreeing, my sense of it from involvement with many reroof jobs
is that the priorities are the other way around: Roofers use new wood panel
sheathing for their own convenience whenever they strip old roofing down to
skip sheathing boards and wood shakes/wood shingles are not the replacement
roofing.  

Tiles require lines of nails at tile-imposed modules, and usually require
metal or 1x2 wood cleat strips above the sheathing surface, again on a
module special to the tile type. Felt underlayment wants solid support, as
do comp shingles. Often, more skip sheathing boards could infill between
existing ones to satisfy reroofing needs, but at increased labor and
nuisance. Hence the panel overlay.

Diaphragm benefits appear to be an incidental advantage that isn't the
roofer's motivation. They invariably propose sheathing panel overlay on
their own initiative, but fight against sheathing panels if wood shakes are
to be applied, even in new construction.

Then building departments intervene to require panel edges be ripped to
occur over existing boards, and ask the engineer to specify the intended
shear nailing, when shear purposes are satisfied by conventional
construction nailing schedules and existing features that remain.

Some engineers naively suppose the panel overlay is primarily used to
satisfy a new, code-required lateral forces analysis, and have testified as
experts that negligence was committed by engineers who only undertook to
review (to satisfy local bldg dept policy) existing roof framing for
vertical load adequacy for the new roofing.

What's next? Demanding installation of newly engineered diaphragm chords and
collectors according to 97 UBC coefficients when comp roofing replaces wood
shakes?

Charles O. Greenlaw SE   Sacramento CA