A raised bed on concrete needs a clear drainage route, enough root space and a structure suitable for its filled weight. Unlike an open-bottom bed on soil, it cannot rely on roots extending into the ground below. Plan the base and growing medium before filling it.
Check the surface and the load
Identify whether the concrete is a ground-level slab or part of a balcony, roof or other suspended structure. For a suspended surface, have the load capacity assessed for the wet bed, people and other objects. A visual inspection of the slab is not enough to establish that capacity.
Keep access routes, building drains and outlets clear. Consider where drainage water will go and whether soil particles or moisture could stain the paving or affect nearby finishes. If the surface is shared or rented, check the applicable property requirements before installation.
Provide an actual way for water to leave
A planter with a solid base needs drainage outlets designed for that planter. An open-bottom frame on paving also needs a route for excess water to escape; a frame sealed tightly to an uneven slab may trap water. Check the design rather than assuming the gap beneath it will be adequate.
Do not line the bottom with an unperforated waterproof membrane and then fill it as an ordinary vegetable bed. That can create a reservoir with no controlled outlet. Mesh may help retain material in some designs, but mesh itself does not create drainage where no outlet exists.
A layer of gravel is not a universal way to improve drainage within a contained growing medium. See Maryland Extension’s container-growing guidance for drainage and medium selection.
Choose depth for the crop and planter
Allow usable growing depth, not merely outside wall height. Space occupied by a false base or an unneeded rubble layer does not count as root space. Taller crops may also need secure support that the planter is designed to accommodate.
The University of Maryland’s raised-bed guidance discusses beds over hard surfaces and differing crop-depth needs. Select crops and depth together rather than promising that one shallow frame will suit everything. A deeper container also adds substantial material and wet weight.
Use a medium suited to the design
A contained bed can need a container-style growing mix or an appropriate raised-bed blend, depending on its construction. Do not reject potting mix categorically or fill a deep planter with dense garden soil without considering drainage and weight.
Ask the bed and growing-medium suppliers for compatible recommendations. Fill gradually, water to settle the mix and check that outlets remain clear. Leave enough space below the rim to water without washing material over the side.
Monitor moisture and drainage after planting
Beds over paving can dry faster than beds connected to garden soil. Check moisture in the root zone, especially during heat or wind, and adjust irrigation to the plants and medium. A wet surface or dry crust alone may not represent conditions below.
Inspect drainage after watering and rain. Correct standing water, blocked outlets or escaping soil before they become recurring problems. Keep nearby paving safe to walk on.
For choosing a frame and arranging access, see building an urban raised bed. The slab installation still needs its own base, drainage and load assessment.
