What You Need to Build an Above-Ground Garden Bed

An above-ground garden bed is a wooden, metal, or composite box that sits on top of your existing ground. You fill it with soil and grow plants inside. The main difference from a raised bed is height: above-ground beds are typically 6 to 12 inches tall, while raised beds are often 18 inches or taller. Both work the same way, but above-ground beds are faster to build and cheaper to start.

To build one, you need four pieces of lumber (usually 2-by-6 or 2-by-8 boards), corner fasteners or screws, a drill or screwdriver, a level, and a saw if you are cutting boards to length. You do not need a foundation or concrete—the bed sits directly on grass or soil. The whole project takes two to four hours for a beginner.

The most common size is 4 feet by 8 feet, which is large enough to grow several vegetables but small enough to reach the center from either side without stepping inside. You can build smaller (3 by 6 feet) or larger (4 by 12 feet), but wider than 4 feet means you will have to step in to reach the middle, which compacts the soil.

Key Takeaways

  • Above-ground beds sit on top of existing ground with no digging required, making them faster to install than in-ground gardens.
  • Standard lumber (2-by-6 or 2-by-8 boards) costs less than composite materials and lasts 5 to 10 years before rot begins at the base.
  • Screw corners together with 3-inch deck screws or use metal L-brackets, both of which are stronger than nails.
  • Level the ground first so water does not pool in one corner and the bed does not settle unevenly over time.
  • You will need to refill the bed with soil every few years as it compacts and breaks down.

Choosing Your Lumber and Materials

Untreated pine or fir is the cheapest option and works fine for above-ground beds. A 2-by-6 board 8 feet long costs $8 to $15 depending on your region and lumber yard. Pine lasts about 5 to 10 years before the bottom boards begin to rot where they touch the soil. If you want longer life, cedar or redwood naturally resist rot and last 15 to 20 years, but cost two to three times as much per board.

Avoid pressure-treated lumber unless you are certain it is copper-based (not arsenic-based). Older pressure-treated wood can leach chemicals into soil where you grow food. Check the label or ask the lumber yard when it was treated.

Composite boards (wood fiber mixed with plastic) do not rot and last 25+ years, but cost $40 to $80 per 8-foot board. They are worth the price if you plan to keep the bed for decades or live in a very wet climate. Metal raised beds (galvanized steel or aluminum) are also rot-proof and cost $150 to $400 for a 4-by-8 kit, but they heat up faster in sun and cool down faster in cold weather, which can stress plants.

For fasteners, use 3-inch deck screws (galvanized or stainless steel) rather than nails. Screws hold corners together much more firmly and are easier to remove later if you need to rebuild. You will need 8 to 12 screws per bed, depending on how many you use at each corner. Metal L-brackets (also called corner braces) are another option—bolt or screw them to the inside of the corners for extra strength, especially if you are building a tall bed or using heavy soil.

Preparing the Ground and Leveling

Choose a spot that gets at least 6 to 8 hours of direct sun per day for vegetables, or 3 to 4 hours for leafy greens. Avoid areas under trees or next to buildings that cast long shadows. The ground does not have to be perfectly flat, but it should be reasonably level so water does not pool in one corner and the bed does not settle unevenly.

Clear away grass, weeds, and debris from the area where the bed will sit. You do not have to remove the sod completely—you can leave it in place and the bed will sit on top of it. However, if you remove the grass first, the bed will settle more evenly and you will have better drainage. Use a spade or sod cutter to strip away the top 2 to 3 inches of grass and roots.

Once the area is clear, use a 4-foot level to check the ground in both directions (lengthwise and widthwise). If one corner is more than 1 inch lower than the others, dig out soil from the high corners or add soil to the low corners until the ground is roughly level. This step prevents water from pooling and keeps the bed from rocking when you step on it.

Assembling the Frame

Lay out your four boards in a rectangle on the ground where the bed will sit. If your boards are longer than you want, cut them to length with a circular saw or hand saw before assembly. A standard 4-by-8 bed uses two 8-foot boards and two 4-foot boards.

Stand the boards on edge so they form a rectangle. The boards should touch at the corners. If you are using 2-by-6 lumber, the bed will be 5.5 inches tall (the actual width of a 2-by-6 is 5.5 inches, not 6). If you want a taller bed, use 2-by-8 boards (7.25 inches tall) or stack two layers of boards and screw them together.

At each corner, drill two or three pilot holes (small starter holes) through one board into the end of the adjacent board. Use a drill bit slightly smaller than your screw diameter so the screw grips the wood. Then drive 3-inch deck screws through the holes. Screws should go at least 2 inches deep into the second board. Tighten all four corners, then use a level to check that the frame is square and level before you fill it with soil.

If you are using metal L-brackets instead of screws, bolt or screw the brackets to the inside of each corner, making sure the brackets are tight before moving on. Brackets add strength and make the bed easier to disassemble later if you need to move it.

Filling and Settling

Once the frame is assembled and level, you are ready to fill it with soil. Do not use garden soil from your yard—it compacts and drains poorly in a raised bed. Instead, use a soil mix made of topsoil, compost, and peat moss or coconut coir in roughly equal parts. A 4-by-8 bed that is 6 inches tall holds about 16 cubic feet of soil, which is roughly 1.5 cubic yards. Most garden centers sell soil by the bag or cubic yard.

Fill the bed to within 1 to 2 inches of the top. The soil will settle over the first few weeks as it absorbs water and compacts. You can plant right away, but expect to add more soil in a few weeks to bring it back to the original level. This settling is normal and happens every year, which is why you will need to add fresh compost or topsoil annually to keep the bed at the right depth.

Water the soil thoroughly after filling so it settles and you can see where it drops. Add more soil to bring it back to the desired level, then water again. The bed is now ready to plant.

Extending the Life of Your Bed

Untreated wood lasts longer if you keep it dry and protect it from direct contact with wet soil. Line the inside bottom of the bed with landscape fabric or cardboard before filling it with soil. This barrier slows rot and also prevents weeds from growing up through the bottom. Landscape fabric is better than cardboard because it lasts longer, but cardboard works fine for the first few years.

Do not let water sit against the outside of the boards. Make sure the ground slopes slightly away from the bed so rain runs off rather than pooling against the wood. If your area gets heavy rain, consider building the bed on a slight mound or adding a small drainage trench around it.

If you use untreated wood, inspect the bottom boards every few years. When you see soft spots or discoloration, it is time to replace those boards. You can remove the soil, unbolt or unscrew the damaged boards, and slide new ones in without taking apart the entire bed. Cedar and composite beds need less maintenance, but they cost more upfront.

Common Mistakes to Avoid

Do not use nails to fasten corners—they do not hold well and the bed will rack (twist out of square) when you step on it or fill it with soil. Screws or brackets are much stronger. Do not skip the leveling step; an unlevel bed will settle unevenly and water will pool in the low corner, rotting the wood faster.

Do not use garden soil from your yard as your fill. It compacts hard in a raised bed and drains poorly. Spend the extra money on a proper soil mix. Do not make the bed wider than 4 feet if you plan to reach into the center—you will end up stepping in the bed, which compacts the soil and damages plants.

Do not assume the bed is done after you fill it once. Soil settles and breaks down every year, so plan to add 1 to 2 inches of fresh compost or topsoil each spring. This keeps the bed at the right depth and replaces nutrients that plants have used.

Frequently Asked Questions

How deep should an above-ground bed be?

Most vegetables need 8 to 12 inches of soil depth. Shallow-rooted crops like lettuce and spinach can grow in 6 inches, but tomatoes, peppers, and root vegetables need at least 10 to 12 inches. If you are growing perennials like asparagus or rhubarb, go deeper—14 to 18 inches is better.

Can I build an above-ground bed on concrete or pavement?

Yes, but drainage will be poor because water cannot drain down into the ground. Drill or drill holes in the bottom of the bed frame, or line the bottom with landscape fabric with small slits cut in it to allow some water to escape. You may also need to water less frequently because the soil will stay wet longer.

Do I need a bottom for my above-ground bed?

No. Leaving the bottom open lets roots grow down into the soil below and allows water to drain away. If you want to keep burrowing animals like moles or gophers out, line the bottom with hardware cloth (metal mesh) before filling with soil. Cardboard or landscape fabric at the bottom slows rot but does not prevent it entirely.

How long does it take for untreated wood to rot?

Untreated pine or fir typically lasts 5 to 10 years before the bottom boards begin to soften and rot where they touch wet soil. Cedar lasts 15 to 20 years. Composite materials last 25+ years. The exact timeline depends on your climate—wet regions see faster rot than dry ones.

Can I stack two beds on top of each other to make a taller bed?

Yes. Screw or bolt the second bed frame directly to the first one, making sure all corners are tight and level. A stacked bed is heavier and puts more pressure on the bottom boards, so use screws or brackets rather than nails, and consider lining the inside with landscape fabric to slow rot.