Hollow steel is a cold-rolled P-design profile that is about 2mm thick and is hollow inside. It’s what you see online with prices. Mostly intended for secondary structures such as sheds, barns, pool houses and homes where the door or window won’t be the main or exterior. Solid steel is a 5mm thick hot-rolled profile featuring an “L” design that is the timeless slim narrow look every designer is after! Also available to be thermally broken for cold climates.