For a concrete rooftop, I’d choose furniture that is lightweight, weather-resistant, and has smooth, protective feet rather than rough metal or hard plastic edges. Powder-coated aluminum is probably my first pick for frames because it is rust-resistant and usually has a clean, chip-resistant finish. Stainless steel is also a good option for hardware or frames if it is high quality, but I would avoid bare cast iron or uncoated steel because they can rust and leave marks. For tabletops, sealed teak, HDPE, or powder-coated metal work well; I would skip oiled wood, cheap unsealed wood, and rough stone-like surfaces because they can transfer oil or grit onto the concrete.
No furniture is completely scratch-proof on concrete, so the feet matter a lot. Look for chairs and tables with replaceable glides, felt pads, or wide rubber caps that are smooth and non-marking. Avoid small metal feet, rough concrete bases, and black rubber feet that can leave dark scuff marks, especially in heat. A good trick is to add an outdoor rug or a mat under the seating area and to lift furniture instead of dragging it. Wipe up spills quickly, rinse pollen and grit off the feet, and consider sealing the concrete if staining is a big concern. With the right materials and a little protection, urban outdoor furniture can look good on a rooftop without wrecking the surface.