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Re: Roof Sheathing at Ridges

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Shear distribution in a wood structural panel diaphragm (e.g. simply supported by end shear walls) is maximum at the ends of the diaphragm, and varies linearly to zero at the mid-length, when the structure is subjected to uniform load as in wind loading.  Also, the shear distribution is uniform across the width of the diaphragm, for diaphragms which are fastened with mechanical fasteners such as nails, screws or staples.  Therefore, the maximum diaphragm shear occurs at the ends of the diaphragms, and also along the ridge adjacent to the ends.  This is the critical area as regards the effect of sheathing
and roof framing discontinuity at the ridge.  Ridge blocking and sheathing nailing requirements would be most necessary near the diaphragm ends, and can be placed at alternate spans between roof framing to allow space for ventilation in the unblocked spans (thre ridge vent can be continuous, but the ventilation occurs only in every other span). Along the ridge, the blocking can be spaced out even further (such as every third span), or eventually omitted altogether, as the diaphragm shear forces reduce (to zero) toward the mid-length of the diaphragm.
John Rose
APA (retired)
Tacoma, WA

MSSROLLO(--nospam--at)aol.com wrote:

> I guess I am missing something in this discussion about why the ridge would be critical in a roof diaphragm.  I am assuming the eaves on both side are where the forces eventually go to.  It would seem the ridge of a gable roof would not need to transfer shear unless all the load was being transfered to one side only.
>
> This would be similar to Roof X-bracing in a metal building where the loads are assumed to be near zero at the center of the roof and accumulate in the rods as you proceed to the eaves.  The rods in the center are generally smaller than the ones near the eaves since the rod load increases as you near the eave.  The only time this is not true is when we are using what is called "one-sided" bracing where one eave is assumed zero because there is not any wall bracing present and then all load must transfer to the other wall.  In these cases, the center rods do not have a lower force than the unbraced eave.
>
> Ron Martin
> Tuscaloosa, AL
>
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