How to fasten new OSB roof deck thru rigid insul to original sheathing? 1

[wedostr2]

We have an existing wood framed vaulted ceiling roof that is poorly insulated. We are going to install new rigid insulation, R-38 to bring it to current codes, above the original roof sheathing and cap that with a new layer of OSB. We plan to fasten through the new construction into the original sheathing with long washer-headed screws, but I cannot find any information to use in determining the appropriate fastener spacing. Any help would be appreciated.

 

[Ron]

This would constitute a "site-engineered" system and you won't likely find any standard data for it.

I would determine the pullout resistance of the fasteners by field testing as intended to be installed, fasten through the original sheathing into the primary structural members, considering the tributary area of each fastener, and applying appropriate uplift loads to the roof. I would also use a FS of at least 3 since the fasteners are in tension.