The short answer is: not necessarily, but it depends a lot on what your furniture is made of and how exposed it is. A rooftop deck is a tough spot for outdoor furniture because it gets hit by rain, sun, humidity, wind, pollution, and sometimes salt air if you live near the coast. If you leave furniture up there all year, those conditions will test it much more than a covered patio would.
Materials make the biggest difference. Powder-coated aluminum is one of the safest choices for an urban rooftop because aluminum itself does not rust like steel does. The powder coating adds a protective layer, though it can chip or scratch over time. Stainless steel can also work well, especially 316-grade stainless in coastal cities, but even stainless steel can show rust spots if it is scratched or exposed to salt and pollution for long periods. Regular steel, wrought iron, and cheap metal frames are the risky ones. Once their paint or coating fails, rust can start quickly.
Wood, HDPE, and teak are not going to rust, but they need their own kind of care. Wood can fade, crack, or grow mildew in damp rooftop conditions, while HDPE is very low-maintenance and handles weather well. If your furniture has metal hardware, hinges, or screws, check those too. Cheap hardware often rusts first and leaves stains on the furniture or deck.
To protect urban outdoor furniture on a rooftop deck, choose rust-resistant materials from the start, or at least make sure metal frames are well coated. Clean and dry the furniture regularly, rinse off pollution and bird droppings, touch up scratches before rust spreads, and use breathable covers rather than trapping moisture underneath. Good drainage matters, too, so water does not sit around the legs and joints. If you leave furniture out year-round, a little maintenance goes a long way, and choosing the right material will save you far more trouble than trying to rescue rusty steel later.