What's inside
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A wooden window box is a manageable first outdoor project: it needs straight cuts, basic fastening, and a few drainage details that prevent soggy soil and early rot. This plan makes a box about 30 inches long, 8 inches deep, and 8 inches high. Adjust the length to fit your window, but keep the box slightly shorter than the space between trim so it can be installed and removed without scraping.
Contents
Materials and sizing
Use naturally rot-resistant cedar or redwood if the box will sit outdoors year-round. Pressure-treated lumber also works, but it can be rough and wet when new; let it dry before painting or sealing, and use fasteners rated for treated wood. For a budget build, untreated pine is acceptable if it is kept painted, drains freely, and can be replaced after a few seasons.
For a 30-inch box, buy two 1-by-8 boards at least 30 inches long for the front and back, one for the bottom, and a 1-by-8 board to cut the ends. Actual 1-by-8 lumber is usually about ¾ inch thick and 7¼ inches wide, so the finished box will be about 7¼ inches high. The bottom fits between the front and back; with ¾-inch-thick sides, cut it 28½ inches long. Cut two ends 7¼ inches square.
A cedar 1-by-8 board costs more than pine, but it can be worthwhile where the box stays exposed to rain. For fasteners, choose exterior-rated screws, such as stainless steel or coated deck screws, about 1⅝ inches long. Avoid drywall screws, which can corrode and snap outdoors.
Tools and material trade-offs
You’ll need a saw, drill/driver, tape measure, pencil, and a ⅛-inch drill bit for pilot holes. A handsaw is fine for four straight cuts; a circular saw makes repeated cuts quicker but is not essential. Sandpaper or a sanding block helps soften sharp edges. For finishing, use an exterior paint or stain compatible with the wood.
| Choice | Good for | Trade-off |
|---|---|---|
| Cedar or redwood | Unpainted boxes and frequent rain | Costs more; soft wood dents easily |
| Untreated pine with exterior paint | Low-cost builds and sheltered windows | Needs sound paint coverage and upkeep |
| Plastic nursery liner | Easy seasonal plant changes | Must fit without blocking drainage or trapping water |
A liner is optional. A plastic window-box liner can keep wet soil from sitting against the wood, but only use one with drainage holes that line up with the box holes or can be drilled through. A sealed, hole-free liner turns the box into a water tank.
Cut and assemble the box
1. Cut the front and back to 30 inches. Cut the bottom to 28½ inches and the ends to 7¼ by 7¼ inches. Check each end against your actual lumber width before cutting; board widths vary. If your stock is narrower or wider, size the end panels to match the front and back.
2. Sand splinters and rough edges. Set the front and back on a flat surface, with the bottom edge of each board aligned to the bottom of an end panel. Apply a thin bead of exterior construction adhesive at the joints if desired; screws alone are adequate for a small box.
3. Drill two pilot holes through each end of the front and back, spaced about 1½ inches apart and centered over the end panel. Drive 1⅝-inch exterior screws. Pilot holes matter in cedar and pine: screws driven close to an edge without them can split the board.
4. Fit the bottom between the sides, flush with their lower edges. Drill pilot holes through the front and back into the bottom every 6 to 8 inches, then screw it in place. Add two screws through each end into the bottom. Keep the box square by measuring diagonally from corner to corner; the two measurements should be within about ⅛ inch.
Drainage and finish
Drill five ⅜-inch holes through the bottom, spaced evenly along its length and centered across its width. These provide an exit for excess water; tiny holes can clog with roots and soil. If the box will rest against a flat sill or shelf, add two small ¼-inch-thick wood strips underneath, running front to back. The gap helps water escape instead of pooling under the box.
Do not put a layer of gravel in the bottom. It takes up planting depth and does not fix blocked drainage. Use a free-draining potting mix, and keep the holes clear of paint, liner material, and debris. If water remains in the box after a heavy watering, check the holes before adding more soil or changing the mix.
Paint or stain the outside and end grain before planting. End grain absorbs water quickly, so give it careful coverage. Avoid coating the inside with an unverified wood preservative where soil and edible plants will be in contact with it. Let the finish cure according to its label, usually at least a day or two, before filling the box.
Mounting and seasonal care
A filled box is heavy. Wet potting mix and plants can push a 30-inch box well past 40 pounds, so do not rely on small decorative brackets or screws into trim alone. Use sturdy brackets attached to framing or masonry with appropriate hardware, and follow the bracket maker’s load rating. If you cannot locate solid support, place the box on a stable sill instead of hanging it.
Keep the box a little below the window so water cannot run into the frame. Water until excess drains, then check that it exits freely. In autumn, remove dead plants and clear the holes; in freezing climates, empty a waterlogged box before a hard freeze to reduce splitting. Recoat painted pine when the finish starts peeling, not after bare wood has been exposed for a full season.