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Re: Roof Construction Advice

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I agree with 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.


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).


Adrian, MI

On Fri, 3 Feb 2006 Jnapd(--nospam--at) 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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