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Re: Roof Construction Advice

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Irv:

It has to do with how the plywood would be potentially attached to the
rest of the structure (i.e. load path from your supposed diaphragm to the
vertical lateral system).  The T&G will be directly attached to the
supporting structure including the vertical lateral system, while the
plywood will be attached at the end of an X inch (depending on how much
EPS foam you have) long screw/nail that is "cantilevered" from the
supporting structure.  Since hte connection of the T&G is likely a
"stiffer" connection, that is where the load will likely go.

An analogy is a SIP with two skins.  The typical attachment of the SIP to
the supporting structure is by way of a long nail or panel screw.  This
screw will be installed downward from the roof surface, through the
exterior skin, through the EPS foam, through the interior skin, and into
the supporting structural member.  So, now imagine the screw without hte
SIP.  It is a cantilever sticking out of hte structural member.  If you
push on the screw down close to the structural member (i.e. where the
interior skin would be), then the screw will not deflect much under a
load.  Apply the same load to the top tip of the screw (i.e. where the
exterior skin would be) and it will deflect quite a bit.  This means that
now when the SIP is there, most of the load will go to where the stiffness
is...i.e. down on the screw/nail near the structural support member or
where the interior skin is.

In you case, the T&G is like the interior skin and the plywood is the
exterior skin.

Regards,

Scott
Adrian, MI


On Sat, 4 Feb 2006, IRV FRUCHTMAN wrote:

> Joe & Scott
> Thanks for the advice.
> Why is it unlikely that the plywood would not be an
> effective diaphragm?
> Irv
>
>
> --- Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu> wrote:
>
> > Irv:
> >
> > I agree with Joe...it is highly unlikely that the
> > plywood would be able to
> > effectively act as your diaphragm.  What you are
> > describing is rather
> > similar to a Structural Insulated Panel (SIP) even
> > though it is not a SIP.
> > When SIPs are used as a roof system including as a
> > roof diaphragm, it is
> > the interior SIP skin that "does the work".  In this
> > manner, the T&G in
> > your case will be like the interior skin of a SIP.
> >
> > As to nailing of the plywood to the 2x4s, I don't
> > see the deformation of
> > the EPS foam being a huge issue, especially if they
> > are using a nail gun.
> > If they are doing it hand, I still don't see it
> > being a problem, but
> > certainly reserve the right to be wrong.
> >
> > If I may suggest (which also is slightly a "plug"),
> > why not just use SIPs
> > instead of the EPS foam plus 2x4s plus plywood?  You
> > could do a true
> > structural SIP if you wanted (i.e. a skin of OSB
> > adhered to each side of
> > EPS foam) or a "non-structural" SIP of just one skin
> > of OSB on the top
> > side of the EPS foam.  It would minimze the amount
> > of nailing that you
> > might have to do and eliminate some lumber (you
> > would still have splines
> > at the panels joints, but likely not nearly as many
> > 2xs as what you are
> > likely to have with what you describe).  Plus, if
> > you do a "true"
> > structural SIP, then you will have an interior skin
> > of OSB right on top of
> > the T&G that you could use as the diaphragm...if you
> > don't want/can have
> > the diaphragm to be the T&G.
> >
> > Joe:
> >
> > As to screws, I don't know what he specifically
> > plans to use, but he
> > certainly can use "SIP" screws.  These are roughly
> > 1/4" diameter screws
> > that come in lengths of upto about 15".  This is
> > what we typically use to
> > attach SIP panels together.  The other option is to
> > use "SIP" spikes
> > (really long nails).
> >
> > Regards,
> >
> > Scott
> > Adrian, MI
> >
> >
> > On Fri, 3 Feb 2006 Jnapd(--nospam--at)aol.com wrote:
> >
> > > Irv:
> > >
> > > The 2x T&G is your diaphragm the upper plywood is
> > to cover insulation  and
> > > attach the roofing material. The 2x4 below plywood
> > is backing for  nailing.
> > > What is the screw type and embedment required in
> > purlins?
> > >  What is the wind uplift loads on roof ?  The
> > screws should  resist this
> > > uplift load so you don't loose the plywood in a
> > storm.  Purlins  need to resist
> > > uplift load also.
> > >
> > > Joe  Venuti
> > > Johnson & Nielsen Associates
> > > Palm Springs,  CA
> > >
> > >
> >
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