Rigid Insulation over Roof Sheathing

[jdgengineer]

We have a project that will have 1 1/2" rigid insulation installed above the 5/8" structural roof sheathing. An additional layer of 1/2" plywood is proposed above the rigid insulation for attachment of asphalt shingles. All wood construction.

How is the upper layer of plywood typically attached? Would you put in 2x flat sleepers in the rigid insulation layer to attach the upper plywood to or would it be more common to use long nails (or preferably screws) for attachment of the upper layer of sheathing directly into the roof rafters below? We are in an area with no snow load and moderate wind loads (110 mph). I'm assuming the only real concern is wind uplift on the upper sheathing and we will consider the lower sheathing layer to be our structural diaphragm.

 

[XR250]

I typically call out long screws

 

[jdgengineer]

Thanks XR250. 4" screw with 1 3/8" embedment?

 

[BridgeSmith]

"...attachment of the upper layer of sheathing directly into the roof rafters below?"

Even if the location of the rafters is known (which would require alot of measuring, marking and transferring the markings, wouldn't it?), it would still be difficult to accomplish hitting the rafters consistently through almost 3" of material. I would think it unnecessary, unless your governing code requires it. The 5/8" roof sheathing will be attached to the rafters, correct? I would just use 3" screws into the roof sheathing at the necessary spacing, and don't worry about where the rafters are. I don't know what the design manuals would say, but in my personal experience with screws into 5/8" OSB, the pullout capacity is greater than the strength of the screw.