Raised Bed on Concrete: Depth, Design & Container Alternative

Building a raised vegetable bed on an existing concrete slab. Minimum soil depth and layout advice.

Depth Recommendation

• 12″ (1 ft) is the consensus minimum for most annual vegetables.
• 18″ preferred for tomatoes, carrots, deep-rooted perennials.

Frame Construction

• Use 2×12 or stacked 2×6 cedar/redwood; anchor to concrete with Tapcon screws.
• Drill ½″ weep holes every 4 ft along bottom edge for drainage.
• Line bottom with landscape fabric (not plastic) to retain soil but allow water exit.

Space-Saving Alternative: Grid of Fabric Pots

• 1-ft³ (7–10 gal) fabric grow bags placed directly on concrete.
• One crop per bag: tomato, pepper, kale, herbs, etc.
• Higher plant density than rows; better root aeration; movable.
• Fill with same 50/30/20 mix (topsoil/compost/coir).

Soil Volume Calculation

4×8 ft bed × 12″ = 32 cu ft = 1.2 cu yd. For 10 bags × 10 gal = 13.4 cu ft = 0.5 cu yd.

Fabric bags on concrete often outperform framed beds in heat management and root health.