What You Need to Build a Raised Bed

A raised planter bed is a wooden or composite box that sits on top of your existing ground. You fill it with soil and grow plants in it. To build one, you need four sides (usually made from boards), four corner posts or brackets to hold them together, a level surface to set it on, and soil to fill it. Most people build them 2 to 4 feet wide, 4 to 8 feet long, and 6 to 12 inches deep, though you can adjust those dimensions to fit your space.

The most common materials are untreated cedar or composite boards (which last longer than pine), 4x4 posts cut into corner pieces, wood screws or galvanized bolts, a power drill, a level, and a saw. If you do not own these tools, you can rent a drill and saw from a tool rental shop for a day for less than the cost of buying them. Cedar costs more upfront but resists rot for 10 to 15 years; pine is cheaper but may need replacement in 5 to 7 years.

Key Takeaways

  • A raised bed is a wooden box you assemble on level ground and fill with soil; the simplest design uses four boards and corner brackets screwed together.
  • Cedar or composite boards last much longer than pine, and the extra cost upfront saves you from rebuilding in a few years.
  • The bed must sit on level ground or water will pool on one side and drain away from the other, killing plants unevenly.
  • You can assemble a basic 4-by-8-foot bed in two to three hours with a drill, a level, and a saw, or without a saw if you have the boards cut at the lumber yard.
  • Filling the bed with soil is the heaviest part of the job; a 4-by-8-foot bed 12 inches deep holds about 32 cubic feet of soil, which weighs roughly 1,600 pounds.

Choosing Your Location and Checking the Ground

Pick a spot that gets at least six hours of direct sun per day if you are growing vegetables, or four hours if you are growing leafy greens or herbs. Avoid low spots where water pools after rain, and stay away from tree roots that will compete with your plants for water and nutrients. Once you have chosen the spot, use a level to check whether the ground is actually level. Place the level on the ground in two directions (north-south and east-west) and note where it tips.

If the ground slopes more than half an inch over the length of your bed, you have two choices: dig out the high side and fill in the low side to level it, or accept that one side will have shallower soil than the other. Most people choose to level the ground because uneven soil depth causes uneven watering—the shallow side dries out faster and the deep side stays wet longer. Leveling takes an hour or two with a shovel and a wheelbarrow, but it prevents problems later.

Cutting and Assembling the Frame

Decide on your bed dimensions. A 4-by-8-foot bed is a standard size that fits most yards and is straightforward to reach across from either side. If you want a different size, write it down before you go to the lumber yard. Ask the yard to cut your boards to length if you do not have a saw at home—most yards do this for free or for a small fee, and it saves you time and the cost of renting a saw.

Lay the four boards on the ground in a rectangle and check that the corners are square by measuring diagonally from corner to corner. If both diagonal measurements are the same length, the corners are square. If they differ by more than a quarter inch, adjust the boards until they match. Once the frame is square, attach the corners. The simplest method is to use corner brackets (L-shaped metal pieces) and galvanized wood screws. Drill pilot holes first—small holes that guide the screw and prevent the wood from splitting—then drive the screws through the bracket into both boards.

If you prefer not to use brackets, you can cut 4x4 posts into 12-inch pieces and attach one to the inside corner of each joint. Drill pilot holes and drive three screws through each board into the post. This method is stronger but takes longer because you have to cut the posts and position them precisely. Either way, check that the frame is still square after you tighten all the screws.

Setting the Frame Level on the Ground

Move the assembled frame to your prepared spot and set it down. Place your level across the frame in both directions and check for high and low spots. If one corner is higher than the others, you can shim it up with wood shims (thin wedges) or dig out soil underneath to lower it. Work slowly—you only need to move the frame a fraction of an inch at a time. Once the frame is level, step inside and press down on each corner to make sure it does not rock.

If the ground is very uneven, you may need to dig out a shallow trench around the perimeter of the frame so it sits flush with the soil. This prevents gaps that let water and soil escape. Once the frame is level and stable, you are ready to fill it.

Filling the Bed with Soil

Calculate how much soil you need by multiplying length times width times depth in feet, then dividing by 27 to get cubic yards. A 4-by-8-foot bed 12 inches deep needs about 1.2 cubic yards of soil. You can order this from a landscape supply company and have it delivered, or you can buy bags of potting soil or garden soil from a garden center and fill the bed yourself. Bagged soil is more expensive per cubic foot but easier to handle if you do not have a driveway for a delivery truck.

Spread the soil evenly across the bed and use a rake to smooth it. Fill the bed to within an inch of the top—soil settles over time and will drop another inch or two in the first month. Water the soil thoroughly after filling; this helps it settle and reveals any low spots you can top off. Do not plant for at least a few days after filling; let the soil settle first.

Protecting the Wood from Rot

Cedar and composite boards resist rot naturally, but you can extend their life by keeping them dry. Do not paint or seal the inside of the bed with anything toxic to plants—the soil will absorb it. If you want to seal the outside, use a plant-safe wood stain or leave it bare. Avoid pressure-treated lumber (wood soaked in chemicals to resist rot) because those chemicals can leach into the soil over time, especially in vegetable beds.

Every few years, check the corners and the bottom edges of the boards for soft spots or discoloration. If you see rot starting, you can replace just that board without rebuilding the whole bed. Unscrew the corners, slide out the damaged board, and slide in a new one. This is much easier than building from scratch, which is why choosing durable wood upfront saves work later.

Frequently Asked Questions

Can I build a raised bed on top of grass without removing it first?

Yes, but the grass will decompose underneath and create an air gap that settles over time. You will end up with a sunken bed in a year or two. It takes only an hour to strip the grass with a shovel, and it prevents settling. If you want to skip that step, lay cardboard under the frame first—it kills the grass and adds organic matter as it breaks down.

Do I need a bottom for the raised bed?

No. An open bottom lets roots grow down into the native soil below and lets water drain away. If you have a gopher or mole problem, you can staple hardware cloth (a metal mesh) to the underside of the frame before setting it down, but most people leave the bottom open.

How deep should the bed be for vegetables?

Most vegetables need 8 to 12 inches of soil. Root crops like carrots and beets need at least 12 inches. Leafy greens and herbs can grow in 6 inches. Tomatoes and peppers do better with 12 inches or more. If you are unsure, build it 12 inches deep—it works for almost everything.

What is the best wood for a raised bed?

Cedar lasts 10 to 15 years and costs more upfront. Composite boards (made from wood fiber and plastic) last 15 to 20 years but cost even more. Pine is the cheapest but rots in 5 to 7 years. Avoid pressure-treated wood in vegetable beds because the preservatives can leach into soil. Cedar is the best balance of cost and durability for most people.

Can I build a raised bed on a patio or deck?

Yes, but you need drainage holes or the soil will stay waterlogged. Drill several half-inch holes in the bottom boards before assembly, or set the frame on spacers (small blocks of wood) so water can drain underneath. Without drainage, roots rot and plants die.