My name's Kel.
Most Building Codes require an 8 penny nail to hold roof decking. An 8 penny is 2 1/2" long.
Mind if I ask why you like aluminum nails?
They tend to bend easily and are difficult to remove.
I'd recommend a cement coated sinker. They're easy to drive by hand and have more holding power.
Have I answered your question?
Hi I am Dr.Fix-It, Your online expert ready to assist you...
Bill, I am in the west coast and if would like my response please make note FOR DR.FIX-IT so others know to whom the question was directed... as only expert with the accepted answer is compensated for the assistance provided.
For 1/4", 3/8", 1/2" plywood panels can use 6d nails and for 5/8", 3/4", 1" plywood would use 8d nails.
Here are some good galvanized nails that have spiral for better holding grip > http://www.homedepot.com/p/Grip-Rite-2-1-2-in-8D-Hot-Galvanized-Spiral-Shank-Deck-Nails-5-lb-Pack-8HGSTPD5/202308532
Well let me know how the project is progressing.... thanks
Galvanized nails are good in case get some moisture do not rust as easy, as far as length on the flat roof just need to keep the wood held in place, longer nails ensures a more secure hold, while winder head ensure easier nailing and keeps the wood from pulling through... You will not have much uplift or gravity pull on the panels as lays flat on the roof surface with the rubberized membrane to seal the seams and roofing to provided the weather proof top player to protect from the element.
reference > http://homeguides.sfgate.com/sheath-flat-roof-30897.html