I’d start by treating theft resistance as part of the design brief, not just buying the heaviest chairs you can find. For a rooftop lounge, the best approach is usually a mix of fixed pieces and furniture that can be locked down without looking institutional. Anchor benches and tables to the deck or pavers with tamper-resistant bolts, or use heavy concrete or steel planters as anchors for seating and shade structures. Modular sofas with locking connectors work well too, because you can keep the layout flexible while making it difficult to walk off with individual pieces.
I’d also avoid loose, lightweight chairs, freestanding cushions, and small side tables unless you have a lockable storage room. Instead, choose commercial-grade aluminum or steel frames with powder-coated finishes, marine-grade stainless hardware, and integrated storage where possible. Swivel chairs can be bolted down, umbrellas should have weighted bases and locks, and lighting or heating should be hardwired or securely mounted. Cushions are the weak point, so use fitted, fastenable cushions or store them when the lounge is closed.
Hard-to-steal doesn’t have to mean ugly. Fixed banquettes, built-in planters, tethered picnic-style tables, and sculptural concrete seating can look intentional and high-end. Just check weight load, wind uplift, drainage, and fire-code clearance with your building engineer before anchoring anything. The goal is furniture that feels comfortable and flexible for residents but inconvenient to remove without tools, time, and attention.