Raised Garden Bed Construction: Materials, Sizing, and Soil Mixing
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Raised beds put plants in better soil than what is on the ground, improve drainage, reduce back strain, and define garden spaces. The construction is basic carpentry — a rectangular frame filled with soil. But the details matter: material choice determines how long the bed lasts, sizing determines what you can grow, and soil mix determines how well things grow. Here is what works.
Material Options
Cedar and redwood are the traditional choices. They resist rot naturally without chemical treatment. A cedar bed lasts 8 to 15 years depending on climate. Cost: $3 to $6 per board foot. These are the premium option.
Pressure-treated lumber is cheaper ($1 to $2 per board foot) and lasts 15 to 20 years. Modern ACQ and MCA treatments are approved for garden use — they do not contain the arsenic (CCA) that was banned in 2003. However, some gardeners still prefer to avoid treated lumber for food gardens.
Concrete blocks are permanent, fireproof, and cost about $1.50 per block. They make excellent beds but are heavy and hard to move. The hollow cores can be filled with soil for planting herbs. No fasteners needed — gravity holds them in place.
Corrugated galvanized steel (stock tanks or purpose-built garden beds) lasts 20+ years, looks sharp, and transfers heat to the soil in spring for earlier planting. Cost: $100 to $300 for a 4x8-foot bed. The steel rusts eventually but galvanized coating extends life significantly.
Sizing
Width: 4 feet maximum. You need to reach the center from either side without stepping into the bed. If the bed is against a wall or fence, limit width to 2 to 3 feet since you can only access from one side.
Length: any practical length, but 8 feet is the most common because standard lumber comes in 8-foot lengths. Longer beds should have intermediate support posts to prevent the sides from bowing outward under soil pressure.
Height: 10 to 12 inches for most vegetables. 18 to 24 inches for root crops like carrots, for accessibility (less bending), or for sites with contaminated native soil where you want a full soil volume barrier. Taller beds need thicker lumber (2x12 instead of 2x6) or stacked courses.
Leave 18 to 24 inches of walkway space between beds. Narrower paths are uncomfortable to work in and make it hard to maneuver a wheelbarrow for soil amendments.
Construction
For a simple 4x8-foot bed using 2x12 lumber: cut two 8-foot boards (sides) and two 45-inch boards (ends, accounting for the overlap). Stand them on edge, clamp the corners, and drive three 3-inch exterior-grade screws through the side boards into the end grain of the end boards.
Add a 4x4 post in each inside corner, screwed to both the side and end boards. This reinforces the corners against the outward pressure of the soil. For beds longer than 8 feet, add a mid-span post on each long side.
If using 2x6 boards, stack two courses and connect them with screws or internal corner brackets. Offset the seams between courses for rigidity — the top board's joint should not align with the bottom board's joint.
Level the frame before filling. Set the frame in position, check for level with a 4-foot level on all sides, and shim under low corners with gravel. A bed that is not level fills unevenly — water pools at the low end and the high end dries out.
Soil Mix
Do not fill a raised bed with native soil — it compacts, drains poorly, and defeats the purpose of raising the bed. The classic raised bed soil recipe: 60 percent topsoil, 30 percent compost, 10 percent perlite or coarse vermiculite.
For a 4x8-foot bed that is 12 inches deep, you need about 1 cubic yard of soil mix. Buying bulk (delivered by the cubic yard from a landscape supply company) is significantly cheaper than buying bags from a home center. Bagged soil costs 3 to 5 times more per cubic foot.
Mel's Mix (from the Square Foot Gardening method) is 1/3 peat moss (or coco coir), 1/3 vermiculite, and 1/3 blended compost. It is lightweight and drains fast. The downside: it dries out quickly and needs more frequent watering.
Amend the soil annually. Compost decomposes and the soil volume drops 1 to 2 inches per year. Top-dress with 2 inches of compost each spring before planting.
Drainage and Ground Contact
Do not put landscape fabric or plastic on the bottom of the bed. These materials impede drainage and prevent roots from reaching native soil. Roots from established plants will grow through the bottom of a raised bed into the ground below — this is a feature, not a problem.
If your native soil has poor drainage (heavy clay), improve the drainage under the bed by removing 2 to 3 inches of native soil and replacing it with gravel before setting the bed frame.
If the bed is on concrete, a solid surface, or contaminated soil, line the bottom with hardware cloth (to keep burrowing pests out) and ensure the bed is deep enough to hold a full root zone — at least 18 inches for most vegetables.
Gopher and mole protection: staple 1/2-inch hardware cloth across the bottom of the frame before filling. This keeps burrowing animals from entering the bed from below while still allowing water drainage.
Frequently Asked Questions
How much does it cost to build a raised bed?
A 4x8-foot cedar bed costs $80 to $150 in lumber. Pressure-treated costs $40 to $80. Soil fill costs $30 to $80 depending on whether you buy bulk or bagged. A complete bed with soil runs $100 to $250 for wood construction, or $150 to $350 for metal.
Do I need to treat or seal the wood?
Cedar and redwood need no treatment — their natural oils resist rot. Pressure-treated lumber is already treated. Untreated pine or fir will rot in 2 to 3 years of ground contact. You can line the inside of any wood bed with heavy plastic sheeting to reduce wood-to-soil contact, but this also reduces drainage through the sides.
Can I place a raised bed over grass?
Yes. Lay cardboard over the grass before setting the bed frame. The cardboard smothers the grass and decomposes over a few months. Fill the bed with soil on top of the cardboard. By the time the roots grow through, the grass underneath will be dead.