Start with the frame: wood, composite, or metal

A raised bed is a box that sits on the ground and holds soil. The frame is what holds everything together. You have three main choices: untreated wood (usually cedar or pine), composite boards (wood fiber mixed with plastic), or galvanized metal.

Untreated wood is the cheapest option and easiest to cut to size. Cedar lasts 10 to 15 years before rotting; pine lasts 5 to 10 years. Never use pressure-treated wood for vegetables—the chemicals can leach into soil. Buy boards 2 inches thick and 8 to 12 inches tall. A 4-by-8-foot bed needs four boards: two 4-foot pieces and two 8-foot pieces.

Composite boards cost two to three times more than wood but last 20+ years and don't rot. They're heavier and harder to cut, so bring them to a home center to cut to length before you leave. Metal raised beds (usually galvanized steel) are the most expensive, last indefinitely, and heat up faster in spring—useful in cold climates but a problem in hot ones.

Key Takeaways

  • A raised bed frame needs four corner supports (wood stakes or metal brackets) and boards screwed or bolted together, not nailed.
  • Dig out grass and weeds from the ground where the bed will sit, then level the soil before placing the frame.
  • Line the bottom with hardware cloth if gophers or moles are a problem in your area, but skip it if they aren't.
  • Fill the bed with a mix of topsoil and compost—not pure compost, which compacts and drains poorly.
  • A 4-by-8-foot bed 12 inches deep needs about 32 cubic feet of soil, which is roughly 1 cubic yard.

Assemble the frame with corner supports

Lay your boards on a flat surface and arrange them into a rectangle. The corners are where the frame fails if you don't reinforce them. You need corner supports—either wooden stakes driven into the ground or metal L-brackets bolted to the boards.

For wooden stakes: Cut 2-by-2 posts 18 to 24 inches long (they'll go 6 to 12 inches into the ground). Place one at each corner, inside the rectangle. Drill pilot holes through the boards into the stakes, then drive 3-inch wood screws through the boards into the stakes—at least two screws per corner, one near the top and one near the middle. Use a power drill; hand-driving takes forever and the screw will bend.

For metal brackets: Buy corner brackets rated for outdoor use (galvanized or stainless steel). Bolt them to the boards with bolts, washers, and nuts. This method is stronger but requires a drill and wrench. Don't use nails—they pull out as the wood shifts and the bed will collapse.

Prepare the ground and level the frame

Set the assembled frame where you want it. Mark the outline with spray paint or flour, then remove the frame. Dig out all grass, weeds, and roots from inside the marked area. Use a shovel to scrape down any high spots and fill in low spots with soil. The goal is a level surface—use a 2-foot level to check.

Place the frame back down and check it again with the level, this time on all four sides. If one corner is high, you can dig out soil underneath. If one corner is low, add soil under it. A frame that's not level will hold water unevenly and look wrong. Spend five minutes getting this right.

If you're in an area with burrowing pests (gophers, moles, voles), now is the time to line the bottom. Unroll hardware cloth (a metal mesh with 1/4-inch holes) across the bottom of the frame and staple it to the inside of the boards. This keeps rodents from tunneling up into your bed. If you don't have these pests, skip this step—it's extra work and expense for no benefit.

Fill the bed with soil and compost

Never fill a raised bed with pure compost. Compost compacts as it breaks down, drains poorly, and becomes hard to work with by mid-season. Instead, use a mix: about 60 percent topsoil and 40 percent compost. For a 4-by-8-foot bed 12 inches deep, you need roughly 32 cubic feet of soil total—that's about 1 cubic yard, or 27 cubic feet (you'll have a little left over).

Order topsoil and compost from a landscape supply company and have them delivered. Buying bags from a garden center works but costs much more and creates a lot of plastic waste. Dump the soil and compost into the bed and mix them together with a shovel or garden fork. Break up any clumps. The mix should be loose and crumbly, not packed down.

Water the soil lightly after filling. It will settle over the first week or two. You may need to add more soil to bring it back to the top of the frame. This is normal.

Common mistakes to avoid

The most common mistake is using nails instead of screws. Nails work for a few months, then the wood shrinks and the nails back out. The frame falls apart. Use screws every time.

The second mistake is filling with pure compost or pure topsoil. Pure compost is too rich and compacts; pure topsoil is too dense and drains slowly. The 60-40 mix works because it holds moisture without staying soggy and gives plants the nutrients they need without burning them.

The third mistake is not leveling the frame. A tilted bed looks bad and water pools on the low side, drowning plants there while the high side dries out. Spend the time to get it level.

The fourth mistake is making the bed too wide. If you can't reach the center from either side, you'll step into the bed to weed, compacting the soil. Keep beds 3 to 4 feet wide so you can reach across.

Sizing and placement tips

Standard raised beds are 4 feet wide by 8 feet long by 12 inches deep. This size is straightforward to build, holds enough soil to grow a real garden, and fits through most gates. If you have a small space, 3-by-6 feet works fine. If you want to grow root crops (carrots, beets, potatoes), go 18 inches deep instead of 12.

Place beds where they get at least 6 hours of direct sun for vegetables, or 3 to 4 hours for leafy greens. Avoid placing them under trees—falling leaves clog the bed and tree roots compete with your plants. Orient long beds north-south so the sun hits both sides evenly.

Leave at least 2 feet between beds so you can walk and work. If you're building multiple beds, this spacing also lets you run a hose or wheelbarrow between them.

Frequently Asked Questions

Do I need to put cardboard or newspaper on the bottom?

Only if you're building on top of grass or weeds and want to kill them without digging. Lay cardboard (no tape or staples) across the bottom, overlap the pieces by 6 inches, and wet it down. It will break down over a season and add organic matter to the soil. If you've already dug out the grass, you don't need it.

Can I use treated wood or old pallets?

No. Pressure-treated wood contains chemicals that can leach into soil and vegetables. Pallets are often treated with pesticides or have absorbed fuel and chemicals from shipping. Stick with untreated cedar, pine, or composite boards.

How deep does a raised bed need to be?

Twelve inches is the minimum for most vegetables and herbs. Lettuce, spinach, and herbs need 8 to 10 inches. Tomatoes, peppers, and root crops need 12 to 18 inches. Deeper beds hold more water and give roots more room, but they also cost more to fill and are heavier to move.

What if my soil is clay or rocky?

A raised bed solves this problem—you're not digging into the bad soil, you're building on top of it. The hardware cloth or cardboard bottom keeps roots from going down into clay. Just make sure the frame is level and the soil mix is loose and well-draining.

Can I build a raised bed on concrete or pavement?

Yes, but the bed will dry out faster because water drains away underneath instead of soaking into the ground. You'll need to water more often. Make sure the concrete is level or the bed will tilt. Avoid placing it where runoff from the roof or driveway will flood it.