Yes, you can absolutely use urban planter boxes on a flat, gravel-covered roof, but it requires careful planning and consideration of several critical factors to ensure success and safety. The gravel surface actually provides some benefits for this application, though it also introduces specific challenges you must address.
The most crucial consideration is weight capacity. You must determine your roof's load-bearing limit, accounting for the planters, soil (especially when saturated with water), plants, and people maintaining them. Consult a structural engineer or your building's specifications. Generally, distribute weight evenly and place heavier planters near load-bearing walls rather than the roof center.
Drainage is paramount. While gravel allows water to flow through, your planters must have adequate drainage holes to prevent waterlogging. Ensure the roof's drainage system remains unobstructed by gravel displacement. Place planter feet or a protective layer beneath boxes to prevent gravel from blocking drainage holes and to protect the roof membrane.
Waterproofing protection is non-negotiable. The gravel provides some protection, but planters and watering can compromise the waterproof membrane underneath. Use protective trays under planters and regularly inspect for damage. Avoid direct contact between planter material and roof membrane.
For plant selection, choose shallow-rooted, drought-tolerant varieties like succulents, herbs, or ornamental grasses that thrive in containers. Lightweight soil mixes reduce overall weight. Consider smaller, modular planters for easier weight management and arrangement flexibility.
With proper planning for weight, drainage, and protection, your gravel roof can successfully support a vibrant urban garden, transforming underutilized space into a productive green oasis.