drill holes in the black parts of your sign, approx 4 would suffice. place sign on roof and srew them in just a little. back them outand place silicone on holes in roof put sign back up.
placing screw on the black parts is easier to conceal. i prefer the dark green torx head deck screws from lowes those will be hidden well in the black areas of your sign.
they drill clean and wont rust
For it to be up to code, you need to screw it in. Use a few 3/8 self tappers, another thing you can do is drill some holes in the sign, counter sink stainless steel screws with a flat head and either cover the heads of the screws with vinyl or paint, alternatively you can use screw caps to cover the screw headers and gives it a stand off look.
Nothing was said about wood. It said aluminum sliding with a metal frame.
In that case drill a hole and use Timberlok screws.
If you can get behind it, use thread all and put a washer and nut on the opposite side.
I know nothing was mentioned but Ive been behind enuff of these facias to know in all of my cases there should be plywood behind this.
Should and is are two different things. I've found over a dozen that do not have any wood behind them. In those cases I generally have to cut a 2" X 12" piece of harden steel to act as a backer brace to prevent it from pulling through. Also a time when I went through 1/4" stainless steel, I broke 5 drill bits before getting my 6 holes drilled out and with self tappers, me and a buddy pulled very hard and it was 100% solid.
Now it's illegal in my state to rely solely on self tappers for electrical signs unless they are a minimum of 3/8" thickness and into a metal stud or frame and not sheet metal. However the best and preferred way is thread all secured by a wash and lock nut.
I never liked the way a sign looks screwed flat to a sloped mansard roof.
We make a couple triangles from slotted steel and paint them the same color as the roof.
These are bolted to the roof, thru bolts if possible, if not solidly screwed into whatever the studs are,
and sometimes if there is a major issues finding the studs or lining it up, toggle bolts.
We put some roof membrane on the back of the steel and silicone the penetrations.
Then mount the sign to that.