How to build a raised planter box from cedar for balcony gardening
Building a cedar raised planter box for your balcony is a satisfying weekend project that gives you fresh herbs, salad greens, or flowers in a compact space. Cedar is naturally rot-resistant and light enough for balcony use, making it a great choice for long-lasting planters. This guide walks you through the essential steps, materials, and measurements to make a sturdy, attractive box you can plant in within a day or two.
Step 1: Plan size and location
Decide the box dimensions based on balcony space and weight limits; a common comfortable size is 36 inches long × 12 inches wide × 12 inches deep, which holds about 3 cubic feet of soil and supports multiple plants. Check your balcony load capacity (often 40–60 lb/sq ft) and allow 10–15 pounds per cubic foot of moist potting mix to estimate total weight. Choose a spot with the sun your plants need and easy access for watering.
[Illustration: balcony layout with a 36x12x12 inch rectangle and sun exposure arrows]
Step 2: Gather materials and tools
Collect cedar boards: two 36×6 inch side boards, two 12×6 inch end boards, one 36×12 inch bottom slat or 3 slats to create a bottom, and optional 36×1.5 inch rim. Use 1×6 cedar (actual 0.75×5.5 inches) or 1×8 for deeper soil. You will also need 1¼-inch stainless steel or galvanized screws (20–30), exterior wood glue, landscape fabric, drill, tape measure, square, saw, sanding block, and a screwdriver. Buying cedar precut can save 30–60 minutes of cutting time.
[Illustration: pile of cedar boards, screws, drill, tape measure and saw on a workbench]
Step 3: Cut boards to size
Measure twice and cut once: trim the two long side boards to 36 inches and two end boards to 12 inches. If making a slatted bottom, cut three 12-inch bottom slats or one 36×12 inch board to fit inside the frame. Use a circular saw or miter saw; allow 1–2 hours for cutting and a little extra for sanding rough edges. Label each piece so assembly goes smoothly.
[Illustration: hands measuring and sawing cedar board with labeled cut pieces laid out]
Step 4: Assemble the outer frame
Glue and screw the corners: apply exterior wood glue to the mating faces, clamp or hold a 12-inch end to a 36-inch side at a right angle, then drive two 1¼-inch screws through the side into the end about 1 inch from each end. Repeat for the opposite end. Total assembly time about 20–30 minutes; glue adds strength while screws give immediate hold.
[Illustration: person using a drill to drive screws into a cedar corner joint with clamps]
Step 5: Attach the bottom and optional rim
Center and attach the bottom slats or single bottom board inside the frame with screws every 6–8 inches, leaving ¼–½ inch gaps between slats for drainage if slatted. If you want a finished look, attach a 1.5-inch rim across the top perimeter with screws. This step secures the box and creates the planting cavity; allow 15–30 minutes.
[Illustration: planter frame with bottom slats being screwed in and top rim pieces placed]
Step 6: Line and test drainage
Staple landscape fabric to the inside bottom and partially up the sides to retain soil while allowing water to escape; leave 6–10 drainage holes or ensure gaps in the slats. Test drainage by pouring 1–2 liters of water and checking that it exits freely within 5–10 minutes. Good drainage prevents root rot and keeps the planter light enough after watering.
[Illustration: hands stapling landscape fabric inside a cedar planter and pouring water to test drainage]
Step 7: Sand, finish, and cure
Sand exposed edges and corners with 120–150 grit sandpaper for a smooth finish and to remove splinters; total sanding time 20–30 minutes. Optionally apply one coat of clear exterior oil or a non-toxic, plant-safe wood finish and let it cure 24–48 hours before adding soil. Finishing protects cedar from weather and extends planter life.
[Illustration: person sanding planter edge and a small can of clear exterior oil nearby]
- Use potting mix with perlite or pumice for lighter weight and better drainage; aim for a mix density around 10–12 lb per gallon when moist.
- Line the bottom with a layer of crushed lightweight material (pumice or broken pots) 1–2 inches thick to reduce soil weight without blocking drainage.
- If you expect to move the planter, screw on two short cleats under the box to slide a hand truck beneath or add lightweight caster brackets rated for outdoor use.
- Pre-drill screw holes to prevent splitting—use a bit slightly smaller than screw shank and countersink for a neat finish.
- Place small rubber feet (¼–½ inch) under the planter to prevent direct wood-to-concrete contact and improve airflow under the box.
- Plant taller, heavier plants near the balcony rail and lighter trailing plants toward the front to balance weight distribution and aesthetics.
- Check building and balcony safety rules—some buildings restrict adding heavy planters or drilling into railings.
- Avoid overwatering; saturated soil dramatically increases weight and can exceed balcony load limits—always allow excess water to drain away.
- Cedar is rot-resistant but not rot-proof: do not use pressure-treated wood for food crops and avoid finishes with harmful chemicals that can leach into soil.
- Wear eye and hearing protection when cutting and drilling cedar; airborne dust and flying debris can injure you.
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