The short answer
Yes. The University of Maryland Extension says raised beds on hard surfaces are fine if they are at least 8 inches deep for leafy greens, beans and cucumbers, and 12 to 24 inches deep for peppers, tomatoes and squash. With no soil below, every root is in the fill, so Washington State University Extension’s advice for that case is a bed at least 15 to 18 inches deep.
What to fill it with
Maryland’s mix for hard surfaces is compost and a soilless growing mix in equal parts. Topsoil can make up to 20% of the mix, but only in beds at least 16 inches deep. The soil mix calculator has this recipe as a preset.
Raised bed soil mix calculator
Open the calculatorLet the water out
Water must leave the bed. The University of Minnesota Extension suggests lining the bottom with a material that lets water through, such as landscape fabric, rather than plastic. Do not add a gravel layer: an Ask Extension answer for planter beds on a patio says gravel causes a perched water table that keeps water in the soil. On a surface you want to protect, plan where the drainage water goes before filling.
What a filled bed weighs
Weight is the question on a deck, balcony or roof. Soil at 2,200 lb per cubic yard, one supplier’s figure for its screened topsoil, weighs about 81.5 lb per cubic foot; a lighter garden mix at 1,500 lb per cubic yard, another supplier’s upper figure, weighs about 55.6 lb. A bed filled 12 inches deep puts that much on every square foot it covers, before water and plants. The University of Maryland Extension notes that a 20-inch container of moist potting mix and plants can weigh 100 pounds.
We cannot tell you what your deck or roof can carry. Have the surface assessed for the filled weight before you build; a planter’s own load rating does not say what the surface underneath can take. The weight calculator turns your bed’s volume into pounds.
| Soil depth | At 2,200 lb per cubic yard | At 1,500 lb per cubic yard |
|---|---|---|
| 8 in | 54 lb | 37 lb |
| 12 in | 81 lb | 56 lb |
| 18 in | 122 lb | 83 lb |
| 24 in | 163 lb | 111 lb |
Elevated planters
A planter on legs with its own floor keeps the soil off the surface and spreads the weight onto its feet. UGA Extension describes planters 10 inches deep, raised on legs or blocks, as a way to save soil. The catalogue lists elevated planters with their planting depth.
References
- Growing Vegetables in Raised Beds, University of Maryland Extension
- Raised Beds: Will They Benefit Your Vegetable Garden? (FS075E), Washington State University Extension
- Raised bed gardens, University of Minnesota Extension
- Planter beds on top of a patio (question 926805), Ask Extension (Cooperative Extension)
- Growing Vegetables in Containers and Salad Tables, University of Maryland Extension
- Raised Garden Bed Dimensions (Circular 1027-4), University of Georgia Cooperative Extension