Your concern about weight on a rooftop deck is very valid and smart. Excess weight can stress the structure. The good news is that several planter box materials are designed specifically to be lightweight and ideal for elevated spaces.
The lightest options typically include:
Fiberglass Planters: These are exceptionally strong yet very light. Modern manufacturing allows for realistic stone or wood finishes without the hefty weight.
Foam or Polyethylene Planters: Often made from molded foam or recycled plastics, these are incredibly lightweight, durable, and weather-resistant. Look for "lightweight resin" or "poly planters."
Fabric Grow Bags: Made from breathable geotextile fabric, these are the lightest option when empty and remain relatively light even when filled with soil.
Metal Planters (Thin-Gauge): Planters made from thin, powder-coated aluminum or corten steel can be relatively light, especially in smaller sizes.
For the absolute lightest setup, combine these planters with a lightweight potting mix (avoid dense garden soil). Look for mixes with perlite, vermiculite, or coir. Also, consider the weight distribution—placing heavier items over load-bearing walls or columns is a best practice.
By choosing the right lightweight planters and soil, you can create a vibrant urban oasis on your deck without compromising safety or peace of mind.