My pole barn, with aluminum foil insulation, metal roof & ceiling, metal walls, is about 100' from the house, and the signal was real weak. I got one of the Hawking USB antenna network adapters you can get at Best Buy, hung it from the ceiling and it raised the signal strength from <5 to ~24, which is enough to maintain a link. Hawking also has a directional antenna network adapter in stock, too, as one of the other members stated.
But as the others said, the best thing to try is take a laptop in the farthest point behind the most metal, and see what the signal strength is like. It doesn't take much to connect; it just may affect the speed from time to time. Outside weather may weaken it as well; I notice when it rains heavy the signal strength is down.
For what its worth
-Scott