Matt,

The underlaymnet in your photo is PolyStick, from PolyGlass Corp. The underlayment is not rated for permanent exposure, so it needs to be covered.

The exposed nail/screwhead on those tiles means the headlap is too short. It should be a minimum of 3 inches.

Obviously, loose tiles (or no fastener) need to be corrected.

The specs on nail size relate to Uplift, sheathing thickness, and other factors. I believe one 10d nail or #8 screw is allowed in most areas around here.

Check out the Tile Roofing Institute's specs, back of the guide, here:

http://www.monierlifetile.com/techni...l_GuideWEB.pdf

Dom.