Start with the frame: wood, composite, or metal

A raised bed is a bottomless box that sits on the ground. The frame holds soil in place and keeps it separate from the earth below. You can build one from untreated wood boards, composite lumber, galvanized metal, or composite composite kits sold pre-cut.

Untreated wood is the cheapest option and easiest to work with. Use cedar, redwood, or pine boards at least 2 inches thick and 6 to 12 inches tall. Avoid pressure-treated wood, which contains chemicals that leach into soil over time. Untreated wood lasts 5 to 10 years before rot sets in, depending on your climate.

Composite lumber (wood fiber mixed with plastic) costs more but lasts 15 to 20 years and requires no maintenance. Galvanized metal or steel frames last even longer but conduct heat, which can warm the soil too much in summer. Pre-made kits from garden suppliers come with all pieces and fasteners, though they cost the most per square foot.

Key Takeaways

  • A raised bed frame can be built from untreated wood boards, composite lumber, or metal, each with different costs and lifespans.
  • The frame should sit directly on bare ground with no bottom, so soil roots can reach down and water can drain.
  • Assemble the frame with corner brackets, wood screws, or bolts depending on your material, and check that corners are square before filling.
  • Fill the bed with a mix of topsoil, compost, and aged manure or peat moss, not garden soil alone, which compacts over time.
  • Water the bed thoroughly after filling and let it settle for a few days before planting, as soil will compact and drop.

Measure and choose your location

Raised beds work best in full sun—at least 6 to 8 hours of direct sunlight per day for most vegetables. Choose a spot that is relatively level and away from tree roots, which will compete with your vegetables for water and nutrients. If your ground slopes, you can build the bed level by digging into the high side and piling soil on the low side.

Standard bed sizes are 4 feet by 8 feet or 4 feet by 4 feet, which are straightforward to reach across from either side and fit standard lumber lengths. You can make a bed any size, but anything wider than 3 feet becomes hard to reach the center without stepping on the soil. Mark out your space with string or chalk before you start building.

Assemble the frame with fasteners

Lay your boards on the ground in a rectangle and check that the corners are square by measuring diagonally from corner to corner—both diagonals should be equal. If they are not, adjust the boards until they match.

For wood frames, use corner brackets (L-shaped metal plates bolted to the inside of two boards) or wood screws (3-inch exterior-grade screws driven through one board into the end grain of the next). Brackets are faster and stronger; screws are cheaper. Drill pilot holes before driving screws to prevent the wood from splitting. Use at least two fasteners per corner.

For composite or metal frames, follow the kit instructions, which usually involve bolting corner pieces together. Tighten all fasteners firmly so the frame does not rack (twist out of square) when you fill it with soil.

Prepare the ground underneath

Set the assembled frame in place on bare soil. Do not lay cardboard or landscape fabric on the bottom—the bed needs direct contact with the earth so roots can grow down and water can drain. If you have a serious weed or grass problem, you can lay cardboard under the frame before assembly, but cut it away once the frame is in place so it does not block drainage.

If the ground is very compacted, loosen the top 2 to 3 inches with a garden fork. This helps water and roots penetrate into the native soil below. You do not need to dig deep—the raised bed itself lifts your growing space above the problem soil.

Fill the bed with the right soil mix

Do not use garden soil or topsoil alone. Garden soil compacts heavily when confined in a raised bed and drains poorly. Instead, mix three ingredients: topsoil (40 percent), compost (40 percent), and aged manure or peat moss (20 percent). This blend drains well, holds moisture, and provides nutrients for the first season.

For a 4-by-8-foot bed that is 10 inches deep, you will need roughly 26 cubic feet of soil mix. Buy bagged soil from a garden center or order bulk soil delivered by the cubic yard, which is cheaper for large beds. Mix the ingredients in a pile on a tarp before filling, or layer them in the bed and mix with a garden fork as you fill.

Fill the bed to within 1 to 2 inches of the top. The soil will settle over the next few days, dropping 1 to 3 inches depending on how much you compress it while filling.

Water and let the bed settle

After filling, water the entire bed thoroughly until water runs out the bottom. This settles the soil and removes air pockets. The bed will look full at first, but soil compacts as it absorbs water.

Wait 3 to 5 days before planting. During this time, the soil will continue to settle. You may need to add more soil mix to bring the level back up. Once the soil has stopped dropping noticeably, you are ready to plant.

Common mistakes to avoid

The most common error is using garden soil or potting mix alone. Garden soil is too dense and will compact into a brick-like layer within a season. Potting mix is too light and drains too fast, drying out in hot weather. The three-part blend described above balances drainage, water retention, and nutrients.

Another mistake is building the bed too tall without reinforcing the corners. A bed taller than 12 inches filled with wet soil exerts enormous outward pressure on the corners. If you want a tall bed, use corner brackets or bolts, not just screws, and consider adding a center support board for beds longer than 6 feet.

Do not seal or paint the inside of a wood frame. Paint can flake into the soil, and sealers prevent the wood from breathing, which speeds rot. Untreated wood is safe for vegetables and will last long enough without treatment.

Frequently Asked Questions

Can I build a raised bed on top of grass or weeds?

Yes, but the grass will not disappear—it will decompose slowly underneath and can create air pockets as it breaks down. Mow the area short, lay cardboard over the grass under the frame, then remove the cardboard once the frame is in place so water can drain. Alternatively, remove the sod with a shovel before setting the frame.

How deep should a raised bed be?

Most vegetables need 8 to 12 inches of soil. Shallow-rooted crops like lettuce and spinach do fine in 6 inches, while carrots, beets, and tomatoes need 10 to 12 inches. If you are growing root vegetables, go deeper—18 inches is ideal for carrots and parsnips.

What wood should I avoid?

Avoid pressure-treated wood, which contains copper or arsenic compounds that leach into soil. Do not use railroad ties, which are soaked in creosote. Cedar and redwood are naturally rot-resistant and safe. Pine is cheaper and works fine if you accept it will need replacing in 5 to 7 years.

Can I use a raised bed in winter?

Yes. In cold climates, a raised bed warms up faster in spring because the soil is above ground and exposed to more sun. In winter, you can grow cold-hardy crops like kale, spinach, and garlic, or leave the bed fallow and let the soil rest. Cover the bed with a tarp to keep it from getting waterlogged.

Do I need to replace the soil every year?

No. Add 1 to 2 inches of compost to the top each spring to replenish nutrients and organic matter. After 3 to 5 years, the original soil will have broken down significantly, and you may want to remove the top 6 inches and replace it with fresh mix. The bottom layers can stay in place.