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How to build a 4x8 cedar raised garden bed

A 4 × 8 ft cedar raised bed built from six 8-foot 2 × 6 boards, one 4 × 4 post and 40 stainless steel screws: the cut list, the steps and the soil it takes.

The plan at a glance

This bed measures 96 × 48 inches on the outside and stands 11 inches tall: two 2 × 6 boards high on every side. The long boards are used whole, the end boards fit between them, and a 4 × 4 post sits inside each corner and at the middle of each long side. The inside is 93 × 45 inches.

The University of Georgia Extension notes that a 4 × 8 ft bed is most easily built with 8-foot lumber, which is why this plan uses 8-foot boards with no joins along the sides.

96 in long boards (outside)Inside: 93 × 45 in45 in end boardsOutside: 96 × 48 in · 4 × 4 posts at the corners and midpoints
Top view, drawn to scale from the cut list. Each wall is two 2 × 6 boards high.

Cut list and materials

A 2 × 6 board is 1½ × 5½ inches when you buy it dry, and a 4 × 4 is 3½ × 3½ inches, the dressed sizes the American Wood Council lists. The lengths below use those sizes.

Cut list for one 4 × 8 ft bed
PartCut fromLengthPieces
Long side boards2 × 6 × 8 ft, used whole96 in4
End boards2 × 6 × 8 ft, two cut from each board45 in4 (from 2 boards)
Corner and middle posts4 × 4, at least 6 ft11 in6
ScrewsStainless steel exterior screws, 3 in40

Six 8-foot boards in total. Each board you cut into end pieces leaves a 6-inch offcut.

Choosing the wood and screws

Western red cedar heartwood is in the decay-resistant group in the USDA Forest Products Laboratory’s Wood Handbook, which says resistant heartwood generally gives satisfactory service above ground. Lower grades of cedar include sapwood with no natural resistance to decay, so pick boards with as much of the darker heartwood as you can.

How long the bed lasts varies: the University of New Hampshire Extension says cedar or redwood beds can last 10 years or more, while Washington State University Extension notes that untreated wood, even cedar, often begins to decay within a few years. The best wood guide compares the options, including treated lumber.

Use stainless steel screws. The Western Red Cedar Lumber Association recommends stainless steel for outdoor cedar projects and says hot-dipped galvanized will do; plain steel reacts with cedar’s tannins and stains the wood.

Build it

  1. Choose a level spot and mark a 96 × 48 inch rectangle. Leave room for paths around it: UGA suggests 4 feet between beds for carts, wheelbarrows and wheelchairs.
  2. Cut two of the 2 × 6 boards into four 45-inch end boards, and cut the 4 × 4 into six 11-inch posts. Check each cut with the square.
  3. Build each long side: lay two 96-inch boards flat, edge to edge. Place a post on them 1½ inches in from each end, leaving room for the end boards, and one post centered at 48 inches. Drive two screws through each board into each post, 12 screws per side.
  4. Stand the two long sides up, posts facing in, so the boards are 45 inches apart on the inside.
  5. Fit two 45-inch boards at each end, between the long boards and against the corner posts. Drive two screws through each end of each board into the post, 16 screws in all.
  6. Measure both diagonals from corner to corner on the outside. They should both be about 107.3 inches; push the frame until they match, then check it is level.
  7. If gophers or voles dig in your garden, fasten hardware cloth across the bottom before you set the bed down. UC IPM advises hardware cloth or ¾-inch mesh poultry wire under raised beds for gophers; Colorado State University Extension gives ¼-inch hardware cloth for voles and pocket gophers.
  8. Set the bed in place and fill it.

The middle post keeps each 8-foot side in two spans of about 4 feet. UGA recommends a vertical support about every 4 feet for boards only 1 inch thick; these boards are 1½ inches thick and the plan keeps the support anyway.

How much soil it takes

Filled 10 inches deep, the depth UGA gives as the minimum for most crops, the inside of this bed takes 23.8 cubic feet of soil once the posts are taken out: 0.88 cubic yards, or 16 bags of 1.5 cubic feet. That leaves an inch at the top of the 11-inch walls for mulch.

Open this bed in the soil calculator to change the depth or add more beds, or pick a mix in the soil mix calculator.

What this plan does not cover

This is a plan for a bed on open ground. It is not an engineered design and makes no load rating. For a bed on a patio, deck or roof, read the hard-surface guidance and have the surface assessed for the filled weight first.

References