Your concern about rooftop weight is very valid and responsible. The weight of a filled urban planter box is not a single number; it depends on several factors. Primarily, it's the weight of the wet soil, which is significantly heavier than dry soil.
A critical calculation is for the soil. A cubic foot of dry potting mix weighs about 40 pounds. When saturated with water, that can easily double to 80 pounds per cubic foot. Therefore, a common planter box measuring 4 feet long, 2 feet wide, and 1.5 feet deep holds 12 cubic feet of soil. When wet, that soil alone can weigh between 480 and 960 pounds, plus the weight of the planter box itself (wood, metal, or plastic), which can add 50-150 pounds.
Before placing any planters, you must consult a structural engineer or your building manager to determine your roof's live load capacity, usually measured in pounds per square foot (psf). Distribute weight by placing heavier planters near load-bearing walls or columns. Always use a high-quality potting mix (lighter than garden soil) and ensure excellent drainage to prevent waterlogging, which adds unnecessary weight. Use saucers with caution, as they hold water and increase the load. Starting with smaller, shallower planters is a safer way to begin your rooftop garden journey. Never guess with structural weight—professional advice is essential for safety.