Most raised beds do not need one. Utah State University Extension draws the line at depth, and any bed shallower than 12 inches (30 cm) should be left open at the base. Roots then run straight down into the ground under the frame. Raised bed bottoms earn their place only when that ground is something roots cannot enter, which is far fewer gardens than the liner aisle suggests.
Root access to subsoil is the reason that 12-inch rule exists. The boards are not the whole root zone. They are the top of it, and roots keep pushing into whatever sits underneath, so a 10-inch (25 cm) frame on open ground gives you more room than the tape measure says. Around 1 foot (30 cm) of soil depth is the usual target for a productive vegetable garden, and open ground hands you the rest for free.
Deeper beds change the stakes, not the answer. The 12-inch mark is a floor rather than a ceiling, and a bed built taller than that already holds the soil depth a vegetable garden wants. It still does better with an open base on usable ground, because walling the roots off buys you nothing. Add a bottom only when the ground below is the problem.
Set that same 10-inch bed on a patio slab and the math flips. Roots hit the concrete and stop there, so the only soil the crop ever gets is the soil you shoveled into the frame yourself. Ten inches turns into a hard ceiling. You either build the sides deeper or you stick to shallow-rooted crops and live with it.
A raised bed on concrete is one of the few cases where a layer does belong under the soil. The same goes for asphalt and for ground you have reason to think is contaminated. The material for that job is geotextile barrier fabric, laid between the bed soil and the hard surface below. A metal trough is its own case, because water sits in the base with nowhere to go until you add drainage holes. Those are the only raised bed bottoms worth paying for.
The frame itself is optional as well. A mound with sloped sides raises the rooting area the same way a boxed bed does, and there is no enclosure to build and no base to argue about. Gardeners skip past that option because it looks less tidy in photos. It asks for compost, nitrogen and shovel work instead of boards.
Check what sits under the spot before you spend a cent on lining it. Push a spade in and see what happens. If the blade goes down into real soil that has tested clean, leave the base open and let the roots have it. Raised bed bottoms are a fix for a bad surface rather than a standard build step. Liner choice is a separate question, and the short answer is that fabric goes in for a reason, not out of habit.
One thing an open base will not do is water the bed for you. A raised bed garden has to be irrigated, and drip or trickle irrigation is the most efficient way to run it. Plan that part before the first seed goes in, because a bed with no water line stops being productive in the first hot week of summer.
Dig a test hole where the bed will sit. Soil a spade goes into, and a lab test comes back clean on, means you can leave the bottom open and save the fabric for a slab.
Read the full article: Raised Garden Bed Specs Backed by Research
