What You Need Before You Start

A raised bed is a wooden or composite box you fill with soil and plant into. Building one takes a weekend and costs between $50 and $200 depending on materials. You need four boards (usually 2-by-10 or 2-by-12 lumber), corner fasteners, a drill or screwdriver, a level, and a saw if you are cutting boards to length yourself.

The most common size is 4 feet by 8 feet by 12 inches deep, which fits two standard garden beds side by side and is straightforward to reach across. You can build smaller (3 by 6 feet) or larger, but anything wider than 4 feet becomes hard to reach the center without stepping into the bed. Depth matters: 12 inches works for shallow-rooted crops like lettuce and herbs; 18 inches suits tomatoes and peppers; 24 inches is best for root vegetables like carrots and potatoes.

Key Takeaways

  • Untreated cedar or composite boards last longer than pressure-treated pine, though pressure-treated is cheaper and safe for vegetables if made after 2004.
  • Corner brackets or lag bolts hold the frame together more reliably than nails, which work loose as the wood settles.
  • A level bed drains evenly; check all four corners and the center before filling with soil.
  • Landscape fabric on the bottom stops weeds and lets water through, but is not required if you are starting on bare ground.
  • You can build and plant the same day; the bed does not need to settle before use.

Choosing Your Materials

Untreated cedar is the gold standard. It resists rot naturally, lasts 10 to 15 years, and costs $40 to $60 per board. It is safe for all vegetables and looks finished. Redwood works the same way and costs about the same.

Pressure-treated lumber costs $15 to $25 per board and lasts 15 to 20 years. Boards treated after 2004 use copper-based preservatives and are safe for vegetable gardens (the older arsenic-based treatment is not). Many gardeners use pressure-treated successfully. If you are concerned, line the inside with landscape fabric to separate soil from wood.

Composite boards (wood fiber mixed with plastic) cost $50 to $80 per board, last 20+ years, and never rot or splinter. They are heavier to handle and overkill for most home gardens, but worth it if you plan to keep the bed for decades.

Avoid regular untreated pine and plywood. Pine rots in 3 to 5 years; plywood delaminates when wet. Do not use railroad ties or old utility poles — they are treated with creosote, which is toxic to food crops.

Assembling the Frame

Cut your boards to length if needed. For a 4-by-8 bed, you need two 8-foot boards and two 4-foot boards. Lay them flat on level ground in a rectangle and check that the corners are square by measuring diagonally from corner to corner — both measurements should be equal (for a 4-by-8 bed, both diagonals should measure about 8 feet 10 inches).

Fasten the corners with corner brackets (L-shaped metal plates bolted to the inside of the frame) or lag bolts (large screws that go through one board into the end grain of the next). Brackets are easier and more forgiving; lag bolts are stronger. Use at least two fasteners per corner. Drill pilot holes first to prevent splitting, especially in cedar.

If you are using nails, use 3-inch galvanized deck nails and drive at least four per corner, angled slightly inward. Nails will loosen over time as the wood settles, so plan to re-drive them every year or two. Screws (3-inch exterior-grade) work better than nails and are easier to adjust later.

Once the frame is together, set it in place where you want it and check that it is level. Use shims (thin wedges) under the low corners until all four corners and the center are level. This matters: a tilted bed will have wet spots and dry spots.

Preparing the Ground and Adding Landscape Fabric

Clear the area inside the frame of grass, weeds, and large rocks. You do not need to remove topsoil — the raised bed will sit on top of it. If the ground is very compacted, loosen it with a garden fork.

Landscape fabric is optional but useful. It stops perennial weeds and grass from growing up into your bed while letting water drain through. Lay it flat on the ground inside the frame, overlapping seams by 6 inches, and staple it to the inside of the boards. Cardboard works too and breaks down over time, which some gardeners prefer. If you skip fabric, you will pull more weeds the first year but will not have to replace it later.

Do not use plastic sheeting — it traps water and kills soil life. Do not use weed killer — it contaminates the bed.

Filling and Settling

Fill the bed with a mix of topsoil, compost, and peat moss or coconut coir. A common ratio is one-third of each, though you can adjust based on what you have. For a 4-by-8-by-12-inch bed, you will need about 32 cubic feet of soil, or roughly 1.2 cubic yards. Most garden centers sell soil by the bag (2 cubic feet per bag) or by the yard if you order delivery.

Fill the bed to about 2 inches below the rim. The soil will settle slightly over the first few weeks as it absorbs water and compacts. You can plant when ready — the bed does not need to sit empty. Water the soil well after planting to help it settle.

Add more soil in spring if it has settled noticeably. Over time, compost breaks down and the level drops; add 1 to 2 inches of compost each year to maintain depth.

Common Mistakes to Avoid

Building the frame too large makes it hard to reach the center without stepping in the bed, which compacts soil and damages roots. Stick to 4 feet wide or narrower. If you want a larger garden, build two beds side by side instead.

Using nails alone will leave you re-driving fasteners every season. Brackets or bolts hold much better. If you are on a budget, use screws instead of nails — they cost a few dollars more and save frustration.

Skipping the level check creates drainage problems. A bed that slopes even slightly will have a wet corner and a dry corner. Spend 10 minutes with a level before you fill it.

Overfilling the bed makes watering messy and soil spills over the edge. Leave 2 inches of freeboard (empty space between soil and rim).

Frequently Asked Questions

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

Yes. The grass will die under the bed as it is shaded and compressed. It will break down over the first year and add organic matter to the soil below. You do not need to remove it, though doing so speeds the process and gives you a cleaner start.

How deep does a raised bed need to be?

Twelve inches is the minimum for most vegetables. Lettuce, herbs, and shallow-rooted crops do fine at 12 inches. Tomatoes, peppers, and beans need 18 inches. Root vegetables like carrots and potatoes want 24 inches. Deeper is always better if you have the budget and materials.

Do I need to line the bottom of the bed to keep out pests?

Landscape fabric stops weeds and grass from growing up into the bed but does not stop burrowing pests like moles or voles. Hardware cloth (a metal mesh) stops them but is expensive and overkill for most gardens. If burrowing pests are a problem in your area, ask local gardeners what they use.

What is the best wood for a raised bed?

Untreated cedar lasts 10 to 15 years and is safe for all vegetables. Pressure-treated lumber (post-2004) lasts longer and costs less. Composite boards last 20+ years but cost more. All three work; choose based on budget and how long you want the bed to last.

Can I build a raised bed in fall and plant in spring?

Yes. Building in fall gives the soil time to settle and compost to break down over winter. You can also build and plant the same day — the bed does not need to sit empty. Either way works.