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Re: Plywood Diaphragms

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> RDAHLMANN wrote:
>
> > I have an existing building (cerca 1945) with a wood roof, but no ligitamite
> > diaphragm. The structure is in Seismic 2. We are remodeling this structure and
> > need to bring it up to code. We are adding a plywood diaphragm on top of the
> > existing framing.
> >
> > The contractor wants to leave the existing roofing (1/2" thick roofing) in
> > place. I am looking for opinions on increasing the nail size and allowing them
> > to do this. The plywood will not carry any gravity loads. Has anyone seen any
> > pullover problems or anything else which may suggest problems with this?
> >
> > Thanks
> >
> > Richard Dahlmann, P.E.
> >
>
>  Mr. Dahlmann,
> Leaving the existing roofing material between the existing decking and the new
> plywood sheathing creates a fire hazzard.  Local building departments (south
> central Washington) require that the roofing be removed prior to adding new
> sheathing.  Then if the existing  structure is able to carry the additional dead
> load we have applied plywood sheathing over existing decking.  If the decking is
> attached to the roof framing members and is solid decking, adding plywood over the
> top can be considered as a blocked diaphragm.  Nailing provides transfer of shear
> from sheet to sheet and does not require that the sheathing be attached directly
> to the framing members provided that the chord forces are accounted for.
>
> Jill T. Shuttleworth, P.E., S.E.
> Sunnyside, WA