A freestanding cedar planter box is one of the most satisfying deck projects a homeowner can complete in a weekend. The result is a custom-fitted container — the right size for your space, the right depth for your plants — that costs a fraction of retail and lasts for years when built correctly. The build requires basic carpentry skills, a circular saw or miter saw, a drill, and a few hours of focused work.
This guide walks through a complete build for a 36x18x18-inch cedar planter box, which is a versatile mid-size that suits tomatoes, herbs, ornamental grasses, dwarf shrubs, and mixed annual displays. The dimensions can be scaled up or down using the same techniques. For context on how this size fits into a full deck garden plan, the deck planter style guide covers placement and arrangement principles.
Materials List
For a 36x18x18-inch planter box with 3/4-inch thick walls:
- 2 pieces cedar 1x8 at 36 inches (long sides, top course)
- 2 pieces cedar 1x8 at 36 inches (long sides, bottom course)
- 4 pieces cedar 1x8 at 16.5 inches (end sides, two courses each)
- 3 pieces cedar 1x4 at 17 inches (bottom slats with drainage gaps)
- 4 cedar 4x4 corner posts, 18 inches tall (or use doubled 2x4 cleats instead)
- Stainless steel 2.5-inch screws (deck screws work; exterior-rated is mandatory)
- Stainless steel 1.5-inch screws for bottom slat attachment
- Exterior wood glue (Titebond III or similar)
- 4 planter feet or rubber bumpers (to elevate box off deck surface)
- EPDM rubber membrane or 6-mil black poly sheeting for interior lining (optional but recommended)
- Staple gun with 1/2-inch stainless staples if lining with membrane
For lumber selection, choose western red cedar or eastern white cedar. Avoid using pressure-treated lumber for the soil-contact interior; if you use PT for structural posts or legs, line the interior before filling with soil. Cedar fence boards (1x6 or 1x8) are often the most economical cedar option at home centers.
Tools Required
- Circular saw or miter saw
- Drill with Phillips and countersink bits
- Clamps (4 to 6 bar clamps helpful)
- Speed square
- Tape measure
- Pencil
- Sandpaper (80 and 120 grit)
Step 1: Cut All Pieces
Cut lumber to length according to the materials list. Use a miter saw for clean, square cuts. If using a circular saw, clamp a straightedge guide to ensure straight cuts. Label each piece with masking tape as you cut to avoid confusion during assembly.
For the corner construction method described here, the end panels are set inside the long side panels. This means the end panels' interior dimension equals the outer box width minus twice the wall thickness. For an 18-inch-wide box with 3/4-inch walls, end panels are cut to 16.5 inches.
Step 2: Build the End Panels
Each end panel consists of two 1x8 boards stacked to create the 18-inch wall height, plus a vertical cleat on each edge to attach the long side panels. Cut four vertical cleats from 1x2 or 1x3 cedar at 17.5 inches. Glue and screw the cleats flush with the outer edges of the end panel boards. Pre-drill and countersink all screw holes to prevent splitting.
Allow the glue to cure before proceeding. Even with screws, glue dramatically increases joint strength and water resistance. Clamping during cure keeps joints tight.
Step 3: Attach the Long Side Panels
Stand the two end panels upright and attach the long side panels by screwing into the vertical cleats on the end panels. Work from bottom to top, checking for square with a speed square as you go. Drive screws every 6 to 8 inches along the cleat. Apply a thin bead of exterior glue along each joint before screwing.
After assembly, check that the box is square by measuring diagonals — if both diagonal measurements are equal, the box is square. If not, apply diagonal pressure to the longer corner until the measurements equalize, then clamp and allow glue to set.
Step 4: Install the Bottom Slats
Do not install a solid bottom — drainage gaps are mandatory. Cut three to five 1x4 cedar slats to span the interior width. Space them with 1/2-inch gaps between each slat to allow free drainage. Screw each slat to the bottom edge of the side panels or to a bottom frame. Alternatively, nail a 1x2 ledger strip to the inside bottom edge of all four panels and lay the slats on top of the ledgers.
Drill four to six 1/2-inch holes through the bottom slats as additional drainage insurance. Even with gaps, holes ensure that water does not pool around any plant stems.
Step 5: Add Planter Feet
Elevating the planter off the deck surface is critical. Direct contact traps moisture, promotes rot in the planter, and damages the decking below. Attach rubber planter feet, furniture glides, or 2-inch-square cedar blocks at each corner of the bottom. A 1/2-inch to 1-inch elevation provides adequate airflow. For heavier planters, use adjustable leveling feet that allow you to compensate for an uneven deck surface.
Step 6: Interior Lining (Recommended)
Lining the interior with EPDM rubber membrane dramatically extends the planter's lifespan by preventing direct soil-to-wood contact. Cut the membrane generously — enough to cover all four interior walls and the bottom, with several inches of overlap at the top edge. Staple the membrane to the top interior edge using stainless staples every 3 to 4 inches. Cut a 1-inch drainage slit over each drainage hole in the bottom slats.
If EPDM is not available, 6-mil black polyethylene sheeting works as a lower-cost alternative. It is less durable than EPDM but will still significantly extend wood life compared to unlined construction. Avoid clear poly, which allows light penetration that promotes algae growth against the wood.
Step 7: Finishing the Exterior
Sand all exterior surfaces with 80-grit paper, then 120-grit for a smooth finish. Round over sharp edges and corners with a sanding block. Apply a penetrating exterior wood oil or cedar stain to all exterior faces. Avoid film-forming finishes like polyurethane, which peel in outdoor conditions. Two coats with a 24-hour dry time between coats provide good initial protection.
Do not finish the interior if you have lined it with membrane — the finish adds no benefit and some solvents may interact with the membrane material. If you choose not to line the interior, apply a wood preservative to the interior faces before filling with soil.
Step 8: Fill and Plant
Use a quality potting mix rather than garden soil. Garden soil compacts in containers, reducing drainage and root oxygen. A good starting mix for a large planter is 60 percent potting mix, 20 percent perlite, and 20 percent compost. The deck planter soil mix guide provides detailed formulations for different plant types, including tomatoes, herbs, and ornamentals.
Do not overfill. Leave 2 inches of headspace below the top edge to allow for watering without overflow and to keep soil from washing out in rain.
Frequently Asked Questions
What screws should I use for outdoor cedar construction?
Use stainless steel or hot-dipped galvanized deck screws for all outdoor cedar projects. Standard zinc-plated screws corrode quickly in moisture and leave rust streaks on the wood. Stainless steel is more expensive but does not corrode at all. For most planter builds, 2.5-inch #8 deck screws are the right size for 3/4-inch lumber.
How deep should a planter be for tomatoes?
Tomatoes need a minimum of 12 inches of soil depth, with 18 inches ideal for indeterminate varieties. The 18-inch-deep planter in this guide meets that requirement. Use at least a 5-gallon soil volume per tomato plant — a 36x18x18-inch planter can support two determinate tomato plants or one large indeterminate variety.
Can I use fence boards instead of dimensional lumber?
Yes. Cedar fence boards (1x6 dog-ear pickets or flat-cut boards) are often the least expensive way to buy cedar at a home center. They are typically rougher in texture and have more variation than dimensional lumber, but they work fine for planters. You may need to stack three boards per wall course to achieve the desired height rather than two.
Do I need to treat the inside of a cedar planter?
Lining with EPDM or poly sheeting is the most effective protection. If you choose not to line, applying a wood preservative (like Copper Green or End Cut) to all interior surfaces before filling adds years to the planter's life. Avoid products containing arsenic, which is prohibited in modern wood preservatives but still found in some older stock.
How long will a DIY cedar planter last?
A well-built cedar planter with stainless hardware, interior lining, and periodic exterior refinishing (every 2 to 3 years) should last 15 to 20 years. Without lining and with no maintenance, expect 7 to 10 years before significant rot develops. The joints and bottom are always the first areas to fail — inspect annually and address soft spots early.