Short answer: in most jurisdictions, urban planter boxes are not banned outright on rooftop decks. However, they are rarely unregulated. Building codes generally treat them as landscape features, structures, or dead loads, and they must comply with structural, fire, egress, drainage, wind, and maintenance rules. A small movable planter with herbs or flowers is usually easier to approve than a large built-in bed with trees, trellises, or irrigation.
In the U.S., the International Building Code and International Residential Code set baseline rules, but local amendments, zoning, fire, and property-maintenance codes often matter more. The key issue is weight. Saturated soil is heavy: a 2 ft by 2 ft by 1 ft planter can hold about 4 cubic feet of soil, which may weigh 300 to 400 pounds or more when wet, plus the container and plants. That load is concentrated, not spread evenly. Many rooftop decks are designed for limited live loads, and older buildings may have less capacity. A structural engineer may be needed for large or fixed planters.
Fire and egress rules also apply. Planters cannot block required exits, stairs, fire escapes, standpipes, hydrants, or emergency access. Combustible materials, dry vegetation, and stored potting soil may be restricted near walls, vents, or openings. Some fire codes require clearances around rooftop equipment and parapets. Wind is another concern: lightweight containers can tip or blow off a roof, so tie-downs, wind screens, and weight distribution matter. Waterproofing and drainage are critical because overwatering or poor drainage can damage the roof membrane and cause leaks. Root barriers, saucers, and regular inspections help.
Most approvals depend on whether the planter is movable or built-in. Movable boxes under a certain size and weight may be treated like furniture, while built-in planters, pergolas, irrigation lines, and large trees usually require permits and possibly engineer-stamped drawings. Condo, co-op, HOA, and lease rules can be stricter than the building code. Before installing, check with the local building department, a structural engineer, and the property manager. In short, urban planter boxes are often allowed on rooftop decks, but they must fit the roof's structural capacity, fire and egress rules, drainage system, wind exposure, and local approval process.