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A raised bed sitting on open soil or lawn does not need drainage holes — it drains through its open bottom into the ground below. Drainage holes are only required when the bed has a solid floor or sits on concrete, asphalt, a deck or a rooftop, where water has nowhere to go. In that case, plan on roughly one half-inch hole per square foot of floor, or better, raise the frame on 1-inch spacers so the whole base perimeter drains. What almost never helps, in either case, is a layer of gravel in the bottom.
Which situation are you in?
| Bed sits on | Holes needed? | What to do |
|---|---|---|
| Lawn or open soil | No | Leave the bottom open; fork the ground below first |
| Compacted clay or driven-on ground | No, but | Break up 6–8 in of subsoil before filling |
| Concrete, patio or asphalt | Yes | 1 half-inch hole per sq ft, or lift on 1-in spacers |
| Wood deck | Yes | Lift on sleepers and drain to a gutter or off the edge |
| Rooftop or balcony | Yes | Drain to a tray or a floor drain; check load limits |
| Elevated planter box with a floor | Yes | Usually pre-drilled; verify holes are open |
Why does gravel in the bottom make drainage worse?
Because of a soil physics effect called a perched water table. When fine-textured soil sits above a coarse layer, water does not cross the boundary until the fine layer above is nearly saturated — capillary forces hold it in the small pores rather than releasing it into the large ones. The result is a saturated zone sitting directly above the gravel, exactly where you did not want it. Extension horticulturists have been correcting this piece of folk advice for decades; the same principle is why potting a plant over a layer of pot shards does nothing useful. Uniform mix all the way down drains better than layered mix.
How do you drain a bed on concrete or a deck?
Two approaches, and the second is better if you have the option.
- Drill the floor. Half-inch holes on roughly 12-inch centers, biased toward the low side if the slab slopes. Cover the inside with quarter-inch hardware cloth so the mix does not wash through, and add a thin layer of coarse compost — not gravel — as a transition.
- Lift the whole bed. Set the frame on 1 to 2-inch pressure-treated sleepers, bricks or composite shims so there is a continuous air gap at the base. The gap cannot clog the way a hole can, it lets air reach the underside of the frame (which extends the life of a wooden bed considerably), and it keeps the deck boards below from staying permanently wet.
On a deck or rooftop, weight is the constraint people forget. Saturated bed mix runs roughly 70 to 100 pounds per cubic foot, so a 4-by-8 bed 12 inches deep can exceed 2,500 pounds when wet. Check the structure before you build, and place beds over joists or bearing walls rather than mid-span.
Does the mix matter more than the holes?
Usually, yes. Most waterlogged raised beds are not short of holes; they are full of the wrong material. Straight topsoil, unscreened native soil, or fine compost with no coarse fraction compacts into a dense block that holds water regardless of what the bottom of the bed looks like. A bed mix that drains reliably is roughly half screened topsoil or garden soil, a third finished compost, and the balance a coarse aerating component such as pine bark fines or coarse sand. The proportions and reasoning are in what is the best soil mix ratio for a raised bed.
The bottom line
On soil: no holes, no fabric, no gravel — just fork the ground beneath before you fill. On anything solid: drill half-inch holes on 12-inch centers or lift the frame on spacers, and mind the weight on decks. Then get the mix right, because that is what most drainage complaints actually come down to. For the hard-surface case in full see can you put a raised bed on concrete and for the fabric question see should you line a raised bed with landscape fabric. Our best elevated planter box roundup covers boxes with floors, where drainage design is built in.
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