What You Need to Build a Raised Bed

A raised garden bed is a wooden or composite box you fill with soil and plant into. You can build one in a weekend with basic tools and materials you can buy at any home center. The simplest version uses four boards, four corner posts, a level, a drill, and wood screws—no special skills required.

The most common size is 4 feet by 8 feet by 12 inches deep, though you can make yours smaller or larger depending on your space and what you want to grow. Smaller beds (3 by 6 feet) are easier to reach into from both sides. Deeper beds (18 inches) work better for root vegetables like carrots and potatoes.

For materials, untreated cedar or composite boards last longest—cedar naturally resists rot, and composite boards won't rot at all but cost more. Pressure-treated lumber is cheaper and lasts 10 to 15 years, though some gardeners avoid it because of the chemicals. Pine and spruce are affordable but will rot in 3 to 5 years. Avoid railroad ties and old treated wood—they contain creosote, which can leach into soil.

Key Takeaways

  • A basic raised bed uses four boards, four corner posts, wood screws, and a level—you can assemble one in two to three hours with a drill and a saw.
  • Cedar and composite boards last the longest; pressure-treated lumber is cheaper and lasts 10 to 15 years; pine rots in 3 to 5 years.
  • The standard size is 4 by 8 feet by 12 inches deep, but smaller beds are easier to maintain and deeper beds work better for root crops.
  • You need to level the ground before assembly, and you should line the bottom with hardware cloth if you have burrowing pests like moles or voles.
  • Most raised beds sit directly on soil or grass; you do not need a foundation, but the ground should be reasonably flat and free of large rocks.

Choosing Your Location and Preparing the Ground

Pick a spot that gets at least six hours of direct sun for vegetables, or three to four hours for leafy greens. Check that water drains away from the area after rain—if water pools there, your bed will stay soggy. Avoid spots directly under trees, where roots will compete with your plants and leaves will shade the bed.

Once you have chosen the spot, clear away grass, weeds, and large rocks. You do not need to dig out the sod completely, but removing the top layer of grass makes assembly easier. Use a spade to scrape away the top 2 to 3 inches, or lay cardboard over the grass and let it break down over a few weeks before you build.

Use a level to check that the ground is reasonably flat. A slight slope is fine, but if one corner is much higher than the others, the bed will sit unevenly and water will pool in the low corner. If the ground is very uneven, you can dig out the high spots and fill the low spots with soil, or choose a different location.

Cutting and Assembling the Frame

Measure and cut your boards to length. For a 4 by 8 foot bed, you need two boards 8 feet long and two boards 4 feet long. If you do not have a saw, most home centers will make a few cuts for free or a small fee. Cut your corner posts to 18 inches long—they will stick 12 inches above ground and 6 inches below to anchor the bed.

Lay the boards and posts on the ground in the shape of your bed. Position each corner post so it sits inside the corner, with the boards on either side of it. This way the boards overlap the post slightly, which makes the joint stronger. Drill two pilot holes through each board into each post—pilot holes prevent the wood from splitting when you drive the screws in.

Drive 3-inch wood screws through the pilot holes into the posts. Use at least two screws per joint, or three if you are building a large bed. Deck screws or exterior wood screws work best because they resist rust. Once all four corners are screwed together, flip the frame over and check that it sits flat on the ground. If one corner rocks, shim it with a flat stone or adjust the ground underneath.

Lining the Bottom and Preventing Pests

If you have moles, voles, or gophers in your yard, line the bottom of the bed with hardware cloth—a metal mesh with half-inch holes. This stops burrowing animals from tunneling up into your bed. Lay the hardware cloth flat on the ground inside the frame, overlapping the seams by at least 6 inches, and staple it to the inside of the boards with a staple gun.

If you do not have burrowing pests, you can skip the hardware cloth. Some gardeners lay cardboard on the bottom to suppress weeds, though it will break down over one to two years. If you do use cardboard, overlap the pieces and leave a small gap around the edges so water can drain.

You do not need a solid bottom—the whole point of a raised bed is that water drains through the soil and into the ground below. A solid bottom or plastic liner will trap water and rot your plants' roots.

Filling Your Bed With Soil

Once the frame is assembled and lined, fill it with soil. Most gardeners use a mix of topsoil, compost, and peat moss or coconut coir in roughly equal parts. This blend drains well, holds nutrients, and gives plants room to grow. You can also buy bagged raised bed soil mix at garden centers, though it costs more than mixing your own.

For a 4 by 8 by 12 inch bed, you will need about 32 cubic feet of soil, which is roughly 1 cubic yard. A cubic yard of topsoil weighs about 1.5 tons, so if you are buying in bags, expect to buy around 50 to 60 bags of 40-pound soil. Delivery from a landscape supplier is usually cheaper than buying bags if you need more than half a cubic yard.

Fill the bed to about 2 inches below the top of the boards. The soil will settle over the first few weeks, so you may need to add more. Water the bed thoroughly after filling to help the soil settle and to remove air pockets.

Securing the Bed and Preventing Warping

Wooden raised beds can warp or bow outward as the soil pushes against the sides, especially if the boards are thin or the bed is very large. To prevent this, install a support strap or corner brace on the outside of the bed. A straightforward option is to run a metal band or cable around the outside of the frame and tighten it with a turnbuckle. Another option is to install metal L-brackets on the inside corners, which reinforce the joints.

For a standard 4 by 8 foot bed with 12-inch-deep boards, you usually do not need extra bracing if you used 2-inch-thick boards and 3-inch screws. If your boards are thinner or your bed is deeper or larger, add bracing to be safe. Composite boards are stiffer than wood and rarely need bracing.

Maintaining Your Raised Bed Over Time

Check the boards and screws once a year, usually in spring before planting. Look for soft spots in the wood, rust on the screws, or gaps opening up at the corners. If a screw is loose, tighten it. If a board is rotting, you can replace just that board without rebuilding the whole bed—unscrew the corners, slide the old board out, and slide a new one in.

If you used pressure-treated lumber, it will last 10 to 15 years before the wood starts to fail. Cedar lasts 15 to 20 years. Composite boards last 20 to 30 years. You can extend the life of wooden boards by sealing them with a water-resistant stain every few years, though this is optional and does not prevent rot forever.

Top off the soil every spring with 1 to 2 inches of compost. The soil settles and compacts over time, and plants remove nutrients, so adding compost keeps the bed productive and maintains the soil level.

Frequently Asked Questions

Do I need to put anything on the bottom of the raised bed?

Only if you have burrowing pests like moles or voles. If you do not, you can fill the bed directly on the ground. Some gardeners lay cardboard to suppress weeds, but it is not required and will break down over time anyway.

Can I use treated lumber for a vegetable garden?

Pressure-treated lumber is safe for vegetable gardens. Modern pressure-treated wood uses copper-based preservatives, not arsenic. The risk of chemicals leaching into soil is very low, especially after the first year. Cedar and composite are safer if you are concerned, but treated lumber is a common and affordable choice.

How deep does a raised bed need to be?

Twelve inches is the minimum for most vegetables and herbs. Root vegetables like carrots, potatoes, and parsnips need 18 inches or deeper. Shallow-rooted plants like lettuce and spinach can grow in 8 to 10 inches. Deeper beds also hold more moisture and stay cooler in summer.

What size raised bed should I build?

Four by eight feet is the standard because it is straightforward to reach the center from either side without stepping into the bed. If you have limited space, a 3 by 6 foot bed or even a 2 by 4 foot bed works fine. Avoid beds wider than 4 feet if you cannot reach the center comfortably.

Can I build a raised bed on top of concrete or pavement?

Yes, but you will need to line the bottom with landscape fabric or hardware cloth to prevent roots from growing down, and you will need to water more often because the bed cannot drain into the ground. A solid surface also makes it harder to anchor the bed, so use longer corner posts or add weight to keep it stable.