Structural U's are roll formed of galvanized steel or lightweight, heavy-duty aluminum. Although strong, these sturdy U's add a minimum of weight to the roof, mansard, or fascia structure. The hat sections are used for the main frames and the installation rails. The U channels are used for crossbars and connectors between the hats. The structural U and hat sections or sub girt can be used effectively for framing and equipment screens. The design of actual framing should be reviewed and certified by a local engineer.