Doors and Windows - Window casing on upper windows

My installer has connected a sheet of aluminum between the top of a new brick mould window and up about 8 inches to the soffit. The effect is in some homes where they don't put bricks above the upstairs windows they just cap the space he capped the space but he put nothing behind it. He didn't screw the aluminum into anything because it is floating off the wall sheathing by about 2". He used silicon as an adhesive at the Brick mould the side two brick walls as it runs to the soffit and finally at the soffit. When I told him why not build up wood behind it he said he didn't need to and that would weaken the finish. I said the gap behind will cause the metal to travel back and forth like a bellows and the silicone will fail. He said no it will last for a long time.

Is he just lying.

I would have built it out with wood so that it was solid behind it. But once that silicone sets up, he is probably right that it will stay put. Silicone is quite tenacious that way.

But what he said about it "weakening the finish" is a crock. Either that or I just don't understand what he meant.