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Re: Another related roof diaphragm question

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Albert,
When I was at APA, we considered such a construction situation.  We developed a
detail for it, which basicially consisted of nailing beveled cant strips to the
flat top chord of the trusses or rafters, to provide a solid bearing and nailing
surface for attachment of the wood structural panel roof sheathing.  Otherwise,
there is great liklihood that the surface of the roof sheathing will be uneven
over the supports.

Usually, hip rafters are oriented up the slope, so their nailing surface is
flush with the sheathing.  Hip trusses, on the other hand, typically have a flat
bearing top surface, so cant strips would be required in that case to provide a
flush bearing surface for the roof sheathing that is sloped up the hip portion
of the roof.  A detail should be provided on the design, so the builder can
correctly cut and install the cant strip.

 From a structural standpoint, the gap formed between the sloped roof sheathing
and a flat top chord or rafter (if cant strips are not used) would have the
effect of cantilevering the sheathing fasteners, thereby reducing their lateral
and withdrawal resistance.

For more information, direct your question to the APA Help Desk
(help(--nospam--at)apawood.org), and they will send you an excerpt from APA literature or
correspondence that addresses this issue.  Hope this helps.
John Rose
APA (retired)
Tacoma, WA

Albert Meyer wrote:

> As I was reading the wood frame roof diaphragm Q&A a question came to mind.
> When you have a hip roof which is constructed with plated trusses you will
> typically have common trusses at the interior of the roof and as you go
> through the hip area you have a hip set roof truss up to a point where a hip
> set girder truss is placed 4' or 6' in from the end wall of the building.
> Monoslope trusses are then used to complete roof framing and are attached to
> the hip set girder truss.
>
> The hip set trusses step down toward the girder truss, but the top chords
> are not beveled.  I have seen a number of builders install 2x4 flat purlins
> (the purlins are probably nailed to the truss top chords with (2) 10d or 12d
> gun nails at each truss top chord) perpendicular to hip set trusses to have
> a flat surface to attach the roof sheathing to, but I know this isn't always
> done.  I realize that I haven't considered this in my designs and you could
> have two sheet edges fall over the top corner of the truss top chord
> necessitating toenailing both sheet edges to the top chord of the truss if
> purlins, sloped blocking between trusses or bevel cut blocking attached to
> the top chord of the truss is not used.
>
> Has anyone considered how this affects the diaphragm strength?  Does anyone
> have any standard details they use for this condition?  Mr. John Rose, do
> you know if any research has been done or standard details have been
> developed for this condition?  Thanks in advance for your comments.
>
> Albert J. Meyer, Jr., P.E.
> Martin Engineering
> 238 North 22nd Street
> Philadelphia PA 19103-1004
> (215) 665-8570
> (215) 561-5064 Fax
>
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