Anyone
have information on typical roof construction? Will there be a 2x8
facia on the gable, under the siding?

Round here, fascia construction is a 2x4 nailed to the ends of the
rafter tails, with a 3/8x6 piece of decorative trim on top.

You'll hit the 2x4 no matter where you put in a screw (as long as you
stay in the upper 3.5 inches), but it's better to hit the rafter tails
so you can use really long screws. There'll be a tail on either side of
the peak. Look for nails in the decorative fascia for a clue to the
exact location.

Some higher-end houses have actual 2x6 or 2x8 boards with no decorative
layer. I would still hit the rafter tails.

--
Steve Bell
New Life Home Improvement
Arlington, TX USA