How to Build a Raised Garden Bed: Size, Lumber, Soil and Steps

A raised bed that works is a plain box 3 to 4 feet (90 to 120 cm) wide and 8 to 12 inches (20 to 30 cm) deep. Build it from wood that will not rot in a few seasons and set it straight onto loosened ground. Width is the measurement that matters most. If you cannot reach the middle from the path, you will eventually step in, and every footstep compacts the soil you paid to bring in.

The other decisions that matter are lumber that is safe beside food, what goes underneath and how much soil to order. A cut list and build sequence follow for the standard 4 x 8 foot (1.2 x 2.4 m) bed, a size that can grow most of the summer produce for one or two people.

Choose the site and size the bed

Give the bed at least 6 hours of direct sun, make it no wider than you can reach across, and leave paths wide enough for whatever you push down them. Most vegetables do best with 6 to 8 hours of sun. Put the bed within easy reach of a hose and, if you can, on a route you walk every day, because insects and disease are far easier to stop when you notice them early. A spot sheltered from strong wind helps too.

Width. At 4 feet (1.2 m), most adults can reach the center from either side, which makes it the most widely recommended width. A bed that can be worked from one side only, against a fence or wall, should be 2.5 to 3 feet (75 to 90 cm) wide. The classic mistake is going wider than that. The middle can then be reached only by standing in it, and foot traffic crushes the open structure that lets water drain.

Length. Length is limited only by convenience. Beds 4 to 12 feet (1.2 to 3.7 m) long suit most home gardens. Long runs are better broken up. Two 24-foot (7.3 m) beds with a 2-foot (60 cm) walkway between them beat one 50-foot (15 m) bed, because you can cross halfway instead of walking around the end. With wooden sides, boards 6 feet (1.8 m) or shorter are less prone to warping and breaking, so an 8-foot side needs a stake at its midpoint.

Paths. A 1-foot (30 cm) path is enough for feet alone. Allow 2 to 3 feet (60 to 90 cm) for a wheelbarrow or garden cart, and 3 to 4 feet (0.9 to 1.2 m) for a comfortable working aisle between framed beds.

How tall to build it

For vegetables on open ground, the frame needs to hold only 8 to 12 inches of soil (20 to 30 cm); build higher only for a specific reason. Most crops need a rooting zone of at least 6 to 12 inches (15 to 30 cm). In a bottomless bed their roots continue into the loosened native soil below, so the frame does not have to hold every inch they use. That is why a bed shallower than 12 inches (30 cm) should always be open at the bottom.

Every extra inch of height costs soil: on a 4 x 8 bed, each added inch takes another 2.7 cubic feet (75 L) of fill. Drip kits also work best on low-walled beds.

Three situations justify a taller bed:

• Hot, hard ground. On pavement or another very hot site, raise the bed 1 to 2 feet (30 to 60 cm) to lift roots away from the hot surface.
• Poor or contaminated soil below. A deeper bed gives roots a full root zone of clean soil; how to seal the bottom is covered in the section on what goes under the bed.
• Accessibility. A gardener in a wheelchair needs walls of 24 to 32 inches (60 to 81 cm), fitted to the person, and a bed no more than about 3 feet (90 cm) wide. For gardeners who simply find bending hard, 1 to 3 feet (30 to 90 cm) is the useful range.

Beds taller than 18 to 24 inches (45 to 60 cm) need true retaining walls with foundations and supports, which puts them outside a simple weekend build.

Choose lumber that will last

Use cedar heartwood, redwood or cypress that is all heartwood, or treated lumber labeled for ground contact. Untreated pine, fir, spruce and hemlock rot and break down within a few years of touching damp soil.

The key word is heartwood. The pale outer sapwood of essentially every species has low resistance to decay, and only the darker heartwood of durable species holds up. Second-growth lumber carries more sapwood than old-growth wood and is often less durable, which is why young-growth redwood and cypress rate a step lower in the table below. Most redwood and cypress sold today is young-growth, so choose boards that are heartwood all the way across. Expect shorter service from any species in direct ground contact and in warm, humid climates.

Heartwood groupings from the USDA Forest Products Laboratory Wood Handbook. Sapwood of every species belongs in the bottom row.

Material choice drives cost. Cedar lasts but costs more than copper-treated lumber; for more on building with it, see cedar raised garden beds. At spring 2024 retail prices in Alabama, the frame for a 4 x 8 bed took two 16-foot pressure-treated 2x12 planks ($62 to $84, for a bed 12 inches or 30 cm deep), four 16-foot composite deck boards ($265 to $330) or 22 standard 8 x 8 x 16 concrete blocks ($50 to $57).

Other materials

• Composite boards last a long time, do not warp or bow, and hold their color, but come in a limited range of sizes.
• Galvanized steel is a popular choice. Zinc can leach from very acidic soil, below pH 5; read the warning below before buying a metal kit.
• Brick and pavers cost more than wood and can raise soil pH.
• Concrete block or stacked stone under 2 feet (60 cm) tall needs no mortar; offset the joints from one course to the next.
• Landscape timbers are held in place by drilling a hole every 4 feet (1.2 m), and 6 to 8 inches (15 to 20 cm) from each end, then driving rebar through into the ground.
• Pallets stamped HT (heat treated), KD (kiln-dried) or DB (debarked) are acceptable. Never use one stamped MB, for methyl bromide, whatever other stamps it carries.
• Reclaimed lumber is a gamble: materials sold for ornamental use may not be suitable next to food.

Treated lumber, CCA and railroad ties

Copper-based treated lumber sold today, labeled UC4A or higher, is acceptable for food beds; old CCA lumber and creosote railroad ties are not. The use-category code on the label tells you where a board is rated to go, and boards rated only for decking are not meant for soil.

American Wood Protection Association use categories, as printed on treated-lumber labels.

Residential treated lumber now uses alkaline copper quat (ACQ) or copper azole (CA), which contain copper but no arsenic or chromium. Some copper does leach into the soil, but detectable increases stay close to the boards and the amounts are too low to be a biological hazard. Copper exposure from ACQ lumber has been assessed and is not expected to harm adults or children. To limit contact, line the inside faces of the boards. Heavy polyethylene blocks contact best but holds water against the wood; landscape fabric drains better, and either one extends the life of the sides. Keep any liner to the sides only. Certified organic programs may not allow treated lumber, so check your rules before you buy.

Chromated copper arsenate (CCA) dominated treated lumber from the late 1970s until manufacturers voluntarily stopped making it for homeowner use at the end of 2003. It is now limited mainly to industrial uses and is not sold at retail lumberyards. The risk is salvaged wood: old deck boards, fencing or a neighbor's leftovers may well be CCA, and they do not belong around food crops. With older CCA, the bigger hazard is people, especially children, absorbing arsenic through their skin from handling it.

Railroad ties are treated with creosote, a mixture of more than 100 chemicals that includes known carcinogens and has no approved residential use. Plants that touch creosoted wood can be injured or killed.

Build a 4 x 8 bed: cut list, hardware and steps

Six 8-foot 2x6 boards, a short 4x4 and a box of 4-inch exterior screws make an 11-inch-tall 4 x 8 bed with almost no waste. Two 2x6 boards stacked give 11 inches (28 cm) of height because lumber is sold by nominal size, and the actual board is smaller than its name.

Actual dimensions of dry dimensional lumber (USDA Forest Products Laboratory Wood Handbook). A single 2x12 gives a bed 11-1/4 in (29 cm) tall.

Cut list

Worked example. The corner blocks and stakes sit in soil, so they need the same decay resistance as the walls. The end pieces sit inside the long sides, so each is 48 inches (122 cm) minus two 1-1/2-inch board thicknesses. Outside size 48 x 96 in (122 x 244 cm); inside 45 x 93 in (114 x 236 cm).

Stakes that hold a wall should be twice the bed's height with half their length in the ground, which is where the 22-inch (56 cm) figure comes from. If you would rather the corner blocks double as anchors, cut them 22 inches (56 cm) long from an 8-foot 4x4 instead. For 2x8 walls, set decay-resistant wooden stakes on the inside face.

Fasteners

Screw the boards into a wooden block or a metal corner plate inside each corner using 4-inch (10 cm) decking screws. Wet, copper-treated wood corrodes ordinary steel, and 300-series stainless steel has held up well in both ACQ and copper azole lumber. If you use metal brackets, make the screws from the same metal as the brackets so the two do not corrode each other. Cedar, redwood and cypress are rich in extractives that react with the iron in plain steel and leave blue-black stains, often the morning after rain or dew; stainless, aluminum or hot-dipped galvanized fasteners reduce the staining.

On sloping ground

The top of the frame still has to sit level so the soil surface is flat. Run the long sides across the slope, along the contour, which keeps digging to a minimum, then dig the uphill side into the ground until a level laid across the frame reads true. On a longer slope, step a row of level beds down the hill like stairs. If the downhill wall would end up more than 18 to 24 inches (45 to 60 cm) high, it has become a retaining wall and needs a proper foundation.

Step by step

1. Mark the outline. Drive a stake at each corner of a rectangle 4 feet by 8 feet (1.2 by 2.4 m) and run string between them, so you can check sun, path widths and hose reach before you dig.
2. Clear and loosen the ground. On former lawn, remove the turf first: slide a spade just under the grass roots, about 1 inch (2.5 cm) deep, a couple of days after a soaking rain, or rent a sod cutter for a large area. Then loosen the native soil 6 to 12 inches deep (15 to 30 cm) with a spade or tiller, working only when it is dry enough to crumble; roots and drainage both depend on this layer.
3. Cut the boards. Follow the cut list. Turn each board so its heartwood face points into the bed, which makes any warp bow outward at the middle rather than inward.
4. Assemble the frame. Screw the long-side boards into the corner blocks first, then fit the end pieces between them and screw them into the same blocks, at least two screws per board end.
5. Lay rodent mesh if needed. Where gophers, voles or mice dig, spread hardware cloth across the whole footprint so the walls will sit on its edges and nothing can come up from below.
6. Set, square and stake the frame. Lower the frame into place and measure both diagonals; when they match, the box is square. Check it with a level, digging the uphill side in on a slope, then drive a 22-inch stake halfway into the ground inside the middle of each long side to stop the 8-foot boards bowing.
7. Fill and blend. Add your soil blend in stages and fork the first few inches into the loosened ground beneath, so there is no sharp boundary for water and roots to stall at.
8. Water to settle. Soak the bed thoroughly. The mix will sink as it settles, so top it back up before planting.
9. Lay drip or soaker lines. Before the plants and mulch go in, run drip or soaker hose on the soil surface in lines about 2 to 3 feet (0.6 to 0.9 m) apart. Run it at 7 to 10 psi (48 to 69 kPa).

What goes under the bed

On open ground, nothing goes under the bed except wire mesh where burrowing animals are a problem. Set the frame directly on the loosened native soil so roots and water move straight down. A barrier underneath limits root growth and does nothing that loosening the soil does not do better.

Two popular underlays cause trouble. Cardboard laid to smother grass keeps water from soaking into the ground until it breaks down. Remove it, along with any tarp, and kill the turf ahead of time instead. Plastic sheeting on the bottom stops drainage altogether and should never be used there. On a patio or other hard surface, use a water-permeable landscape fabric as the base.

Mesh sizes for rodent barriers under raised beds.

The one case for a sealed bottom is contaminated ground. On urban lots or anywhere with an uncertain history, find out what the site was used for and test the soil before building. If it is contaminated, lay an impermeable barrier across the bottom so roots cannot reach the soil below, and drill drainage holes low in the walls.

Fill it: soil mix and how much to buy

Fill with loam or sandy-loam topsoil blended with compost, about two parts topsoil to one part compost by volume. Workable recipes run from about 10% to 50% compost, always blended with real topsoil rather than compost or potting mix alone.

Raised-bed fill recipes from five state extension services. Sandy, low-organic soils, common in Florida, can take the higher compost shares.

Why not potting soil? Bagged potting mix is built for pots and dries out too quickly in a bed, and soilless mixes are costly and can run short of nutrients over time. Why not all compost? Too much compost dries out fast, can shed water once dry, and builds up phosphorus and salts, especially when manure is part of the mix.

Mix, do not layer. Distinct layers of different soils stall water and roots at each boundary, so blend the fill and keep it uniform from top to bottom. Unfinished woody material such as fresh wood chips needs extra nitrogen. For every inch of it, add 1/4 pound (0.11 kg) of actual nitrogen per 100 square feet (9.3 m²). Fertilizer weight is not nitrogen weight: ammonium sulfate is 21% nitrogen, urea 46% and blood meal 12%, so that inch takes about 1.2 pounds (0.54 kg) of ammonium sulfate, just over 1/2 pound (0.25 kg) of urea or about 2 pounds (0.95 kg) of blood meal. Without it, microbes breaking down the wood take nitrogen the plants need.

How much to buy. Multiply length by width by depth in feet to get cubic feet, then divide by 27 for cubic yards. Bulk topsoil sold by the yard costs far less than bags; in spring 2024 it ran $1.07 to $2.22 per cubic foot in Alabama, roughly $29 to $60 a yard. The cubic yard calculator does the math for odd shapes.

Computed from outside dimensions at 27 cubic feet per cubic yard; 1 cubic foot is 28 L and 1 cubic yard is 0.76 m³. A level 5-gallon (19 L) bucket holds about 0.67 cubic foot, so roughly 40 buckets make a cubic yard.

The first season and every year after

A new bed dries out fast, so water deeply and often the first summer, then top up the soil each spring and test it every 2 to 3 years. Raised beds warm earlier in spring than in-ground soil and lose water faster. Mulch and row covers can move planting up by as much as 2 weeks compared with an in-ground garden, though organic mulch spread too early keeps the soil cool.

Watering. Give at least 1 inch (2.5 cm) of water a week when it does not rain; on a 4 x 8 bed that is about 20 gallons (75 L). A new mix drains so freely in its first year that it may need up to 2 inches (5 cm) a week, and in hot spells you may water every other day. An inch of water wets about 12 inches (30 cm) of well-drained soil, so a slow soak reaches the whole root zone. Push a finger 2 to 4 inches (5 to 10 cm) into the soil and water when it is dry at that depth. Drip and soaker lines beat overhead sprinklers, which wet the foliage and spread disease.

Planting. Raised beds let you plant closer than in rows. Set tomatoes and summer squash on 24-inch (60 cm) centers; cabbage, potatoes and corn 1 foot (30 cm) apart; and snap beans, peas and onions 4 to 6 inches (10 to 15 cm) apart. The plant spacing calculator turns those distances into plant counts for your bed. Vining cucumbers take up far less of the bed on a cucumber trellis run along one long side. Once the soil has warmed, spread a 2- to 4-inch (5 to 10 cm) layer of mulch around young plants.

Rotation. Keep the tomato family moving. Wait 3 to 4 years before replanting tomatoes, peppers, eggplants, potatoes or tomatillos in the same bed, which is one good reason to build two or three beds rather than one large one.

Each year. The organic part of the mix breaks down and shrinks, so add 1 to 2 inches (2.5 to 5 cm) of fresh blend every spring. Test the soil every 2 to 3 years and feed based on the results, aiming for a pH of 6.0 to 6.5. Over winter, cover an empty bed with 2 inches (5 cm) of pine straw, shredded leaves or compost, or sow a cover crop. For planting methods and seasonal care, see raised bed gardening, and if you plan several beds, you can lay out beds and paths to scale in Sow's garden design studio.

When a new bed is not working

Most raised-bed problems trace back to the build: the width, what sits underneath, or what went in as fill. Match the symptom to its cause before adding fertilizer or water.

Common raised-bed faults, their usual causes and the fix for each.

A bed whose soil level drops an inch or two each year is behaving normally. That is the compost breaking down, and the spring top-up replaces it.