Okay, I have a mansard roof to build. Rafters are 8J16s x 12′. It is to be sheathed with 3/4 cdx, then 2″ Z-clips and 2″ rigid insulation and then 1/2″ Dense Deck. Question: What size/ gauge screw do I use to screw down the layers of sheathing. They need to be self-tapping and bugle/ countersunk heads. I would also prefer that they are square drive. I think the gauge should be #10. I can only find #8’s. Are #8’s big enough? Wait, I lied. I did find self-tapping #10’s but they have wings which are supposed to break off when hitting the steel. Nope. Called the manufacturer and was told they are for HEAVY gauge steel framing. 16 gauge is too light. Riiiight. Tried grinding the wings off first, but the shank wasn’t long enough to tap a proper threaded hole. Gotta buy them this weekend. So far I am ready to go with #8’s by default. Should somebody stop me?
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I suggest you try that question here
http://forums.delphiforums.com/breaktimeclass
For sheathing, #8 is just fine. If you are worried about their holding capability (for instance, high wind or seismic areas), you would use more of them - there are specified numbers of fasteners for edge and field in these zones.