How to Build and Install a Window Box Planter That Won't Leak

Sizing Your Window Box Correctly

The window box should extend 2 to 3 inches beyond the window frame on each side. Measure the width of your window trim (not the glass, the trim). A standard double-hung window in a 1980s-era suburban home measures 30 to 36 inches across the trim. Add 4 to 6 inches to that measurement for the box length. For a 34-inch trim width, build the box at 40 inches long. This proportion looks balanced and provides enough soil volume for healthy root growth.

The depth of the box matters more than most people realize. Shallow boxes under 6 inches deep dry out within hours in summer heat, requiring daily or twice-daily watering. A box that is 7 to 8 inches deep holds enough soil moisture to go 2 to 3 days between waterings. The box described here is 7.5 inches deep externally (6.5 inches of soil depth after accounting for the bottom board thickness and drainage layer).

The front height of the box should be 7.5 inches, and the back height should be 6.5 inches. This 1-inch slope directs water toward the front drainage holes and away from the wall. The box width (front to back) is 7 inches, which provides adequate root space for most annual flowers and trailing plants like petunias, bacopa, and sweet potato vine.

Cut List and Materials

All lumber is 1x6 western red cedar (actual dimensions: 3/4 inch by 5.5 inches). You need one 6-foot board for a 40-inch box. Cedar at Home Depot or Lowe's costs $4.50 to $6 per linear foot for 1x6 common grade, so a 6-foot board runs $27 to $36.

Board Cuts

From one 6-foot (72-inch) 1x6 cedar board, cut the following pieces:

• Front board: 40 inches (the long visible face)
• Back board: 40 inches (against the wall, 1 inch shorter in height after ripping)
• Bottom board: 38.5 inches (fits inside the front and back boards)
• Two end boards: 5.5 inches each (the actual width of a 1x6 board)

The back board needs to be ripped (cut lengthwise on a table saw or with a circular saw and guide) from 5.5 inches wide to 4.5 inches wide. This 1-inch difference creates the slope from front to back. If you do not have a table saw, ask the lumber yard to rip it for you; most charge $1 to $2 per cut.

Hardware and Waterproofing

• 1-5/8-inch exterior deck screws (Grip-Rite brand, $8 per box of 100): 24 screws needed
• 2-inch exterior deck screws: 8 screws for bracket attachment
• Window box brackets (Arrow CP60 or equivalent, $12 per pair at hardware stores): rated for 50 lbs each
• 6-mil plastic liner (HDX brand, $7 for a 10x25-foot roll): one piece cut to size
• 1/2-inch galvanized roofing nails ($5 per box): 12 nails for liner attachment
• 1/2-inch drill bit for drainage holes
• Small gravel or perlite ($4 for a 2-quart bag): drainage layer

Total Cost Breakdown

Cedar board: $27 to $36. Hardware and waterproofing: $36 to $41. Total: $63 to $77. If you buy cedar from a local lumber yard instead of a big-box store, the price drops to $18 to $24 for the board, bringing the total to $54 to $65. Brackets are the single most expensive component; if you build simple wooden cleat brackets from scrap 2x4, you can reduce the total to under $40.

Tools You Need

A cordless drill/driver handles every fastening task. A circular saw with a finishing blade (40-tooth or higher) makes clean cuts in cedar. A jigsaw is not required for this project since all cuts are straight. A tape measure, speed square, pencil, and clamps (two 6-inch Irwin Quick-Grip clamps at $9 each) round out the essential tools.

For the waterproof liner, a utility knife and a pair of scissors cut the 6-mil plastic. A 1/2-inch spade bit or drill bit creates the drainage holes. Sandpaper (120-grit) smooths the cut edges. A level (24-inch torpedo level, $8) is essential for bracket installation. A stud finder ($15) locates wall studs for secure mounting. Safety glasses and work gloves complete the list.

Building the Box: Step-by-Step

Assembly takes about 1 hour. Work on a flat surface. Cedar is soft and easy to work with, but it splinters if you force dull blades through it.

Step 1: Cut and Rip All Boards

Cut the front board to 40 inches. Cut the back board to 40 inches, then rip it to 4.5 inches wide. Cut the bottom board to 38.5 inches. Cut two end boards to 5.5 inches. Sand all cut edges with 120-grit sandpaper to remove splinters. The end boards need no additional cutting because the actual width of a 1x6 board is 5.5 inches, which matches the interior width of the box (the front and back boards are each 3/4 inch thick, so the interior width is 7 inches minus 1.5 inches equals 5.5 inches).

Step 2: Drill Drainage Holes in the Bottom

Mark five evenly spaced drainage holes along the centerline of the bottom board. Position the first hole 4 inches from the left end, the last hole 4 inches from the right end, and space the remaining three holes evenly between them. Drill each hole with a 1/2-inch bit. These holes allow excess water to escape. Without them, soil becomes waterlogged within hours and roots suffocate.

Step 3: Assemble the Sides and Bottom

Lay the front board (40 inches, full 5.5-inch width) flat on your work surface. Place one end board (5.5 inches) perpendicular to the front board at the left end. The end board should be flush with the top edge of the front board and extend downward. Apply a bead of Titebond III wood glue ($7 per tube) along the joint. Drive three 1-5/8-inch deck screws through the front board into the end board, spaced evenly. Repeat for the right end board.

Now attach the back board (40 inches, ripped to 4.5 inches wide) to the end boards. The back board sits 1 inch lower than the front board, creating the slope. Align the bottom edges of the front and back boards flush with each other; the top of the back board will be 1 inch below the top of the front board. Drive three screws through each end board into the back board.

Step 4: Install the Bottom Board

Slide the bottom board (38.5 inches) into the box from the top. It rests on the inner edges of the front and back boards, 3/4 inch up from the bottom edge. The bottom board should sit flush against the inside face of both end boards. Drive two 1-5/8-inch screws through each end board into the bottom board ends. Check that the box sits flat on your work surface without rocking. If it rocks, the bottom board is not seated properly; remove it and reposition.

The Waterproof Liner System

This is the step that prevents wall stains. The liner serves two functions: it keeps soil moisture away from the cedar (extending the box life from 5 years to 12 to 15 years), and it creates a channel that directs overflow water to the front of the box rather than letting it seep out the back.

Cutting and Fitting the Liner

Cut a piece of 6-mil polyethylene sheeting 42 inches long by 16 inches wide. The extra length (2 inches longer than the box) allows the liner to fold up over the front and back edges. The width (16 inches) wraps around the bottom and up both sides with 2 inches of overlap at the top.

Press the liner into the box, starting at the back. Fold the front corner like wrapping paper: press the liner flat against the back, fold the excess at the corners inward, then bring the liner down the front. The liner should cover the entire interior surface. Trim the top edge so the liner sits 1 inch below the rim of the front board (this hides the plastic from view when planted). At the back, fold the liner over the top edge of the back board and staple it in place with galvanized roofing nails.

Creating the Drainage Channel

Poke holes through the liner with a pencil to align with each of the five drainage holes in the bottom board. Then, cut a 1-inch slit in the liner at the front edge, directly above the bottom board, at the lowest point of the slope. This slit allows water that accumulates between the liner and the bottom board to escape through the front rather than pooling at the back. This single detail is what prevents the brown water stains on siding.

Adding the Drainage Layer

Add a 1-inch layer of small gravel or perlite to the bottom of the lined box. This layer separates the soil from the drainage holes, preventing soil from washing out while still allowing water to flow through. A 2-quart bag of perlite ($4) is enough for a 40-inch box. Perlite is lighter than gravel, which matters when the box is mounted on brackets with a weight limit.

Mounting the Box to the Wall

Improper mounting causes more window box failures than any other factor. The box must be attached to structural framing (studs or masonry), not just to siding. Siding alone cannot support 40 to 60 lbs of wet soil, plants, and the box itself.

Finding Studs

Use a stud finder to locate wall studs behind the siding. In most homes, studs are spaced 16 inches on center. Mark stud locations with painter's tape. The brackets must attach to at least one stud each. For a 40-inch box, two brackets positioned 8 inches from each end will typically land on studs. If they do not, use a third bracket in the center.

Bracket Installation

Position the brackets so the top of the bracket arm sits 1 inch below the bottom of the window trim. This placement allows the box to sit directly under the window without blocking the trim. Use a 24-inch level to ensure both brackets are at exactly the same height. A 1/8-inch difference between brackets causes the box to sit crooked, which looks wrong and causes uneven water distribution.

Attach each bracket with two 3-inch exterior lag screws (included with Arrow CP60 brackets) driven into the wall studs. Pre-drill 1/8-inch pilot holes to prevent splitting the studs. Tighten the lag screws with a socket wrench until the bracket sits flush against the siding. Do not overtighten; the screw heads should be snug but not digging into the siding material.

Hanging the Box

Lift the box onto the brackets. The back of the box rests on the bracket arms, and the front hangs freely. The 1.5-inch gap between the back of the box and the wall (created by the bracket depth) allows air circulation and prevents moisture buildup against the siding. Secure the box to the brackets with 1-inch pan-head screws driven up through the bracket arm into the bottom of the box. Two screws per bracket, four total, hold the box in place.

Soil and Plant Selection for Window Boxes

Fill the box with a lightweight potting mix, not garden soil. A 40x7x6.5-inch box holds approximately 1.3 cubic feet of soil. A 2-cubic-foot bag of Miracle-Gro Moisture Control Potting Mix ($14) fills the box with some left over. This particular mix contains water-retaining crystals that extend the time between waterings by 30 to 40 percent, which is valuable for a window box that dries out faster than ground-level planters.

For a sunny south-facing window, plant trailing petunias (Supertunia Vista series, $6 per 4-inch pot at garden centers), calibrachoa (Million Bells, $5 per pot), and sweet potato vine (Marguerite or Blackie varieties, $5 per pot). Space plants 6 inches apart in a single row. A 40-inch box holds six to seven plants comfortably. For a shady north-facing window, use impatiens, begonias, and fuchsia, which tolerate 2 to 4 hours of direct sun.

For an edible window box, plant basil, thyme, and trailing rosemary. These herbs thrive in the 6.5-inch soil depth and the reflected heat from the wall. Pinch basil stems back every 2 weeks to prevent flowering and encourage bushy growth. Rosemary and thyme are drought-tolerant once established and only need watering every 3 to 4 days in summer.

Seasonal Maintenance

In spring, remove old soil and roots, replace the perlite drainage layer, and refill with fresh potting mix. Reusing soil from the previous season introduces pathogens and depleted nutrients. A new bag of potting mix ($14) is a small investment for healthy plants. Inspect the brackets and lag screws; tighten any that have loosened over winter. Check the liner for cracks or tears; replace it if you see any damage (a new piece of 6-mil plastic costs $2).

In fall, before the first frost, remove annual plants and cut back any perennials to 2 inches above the soil. If you live in USDA Hardiness Zones 6 or colder, remove the box from the brackets and store it in a garage or shed. Freezing water expands inside the wood joints and can split the cedar. In Zones 7 and warmer, the box can stay mounted year-round with winter-hardy plants like pansies, ornamental kale, and creeping juniper.

Every 2 years, apply a coat of Penofin Verde exterior wood preservative ($28 per quart) to the outside of the box. This penetrating oil contains rosewood oil and UV inhibitors that slow the graying process without forming a film that could peel. Apply with a brush, wait 20 minutes, and wipe off excess. One quart covers four to five window boxes.

Weight Capacity and Safety

A fully planted 40-inch window box weighs 45 to 60 lbs: 15 lbs for the cedar box, 10 lbs for soil, 5 lbs for perlite, and 15 to 30 lbs for plants and water weight after a thorough watering. The Arrow CP60 brackets are rated for 50 lbs each, so a pair supports 100 lbs, which provides a comfortable safety margin. If you use heavier materials (gravel drainage instead of perlite, wet garden soil instead of potting mix), the weight can exceed 80 lbs. In that case, add a third bracket in the center.

For brick or masonry walls, use Tapcon masonry anchors (1/4-inch by 2-3/4-inch, $12 per box of 25) instead of lag screws. Drill into the mortar joints, not the brick face, for the strongest hold. Pre-drill with a 3/16-inch masonry bit, then drive the Tapcons with an impact driver or hammer drill set to drill mode. Masonry installations take 30 minutes longer than wood-frame installations due to the harder material.

What Makes This Box Different

The sloped bottom, waterproof liner with drainage channel, and bracket-mounted gap between box and wall are the three details that separate a window box that lasts 12 to 15 years from one that stains your siding in two summers. The material cost of $30 to $50 (or under $40 if you build wooden brackets) is comparable to a store-bought box, but the construction quality is substantially higher. Store-bought boxes use thin PVC or composite materials that warp in heat and crack in cold. Cedar holds up to temperature cycling, and the replaceable liner means the box itself never sits in direct contact with wet soil. Build it once, maintain it with a $14 bag of soil each spring, and it will outlast every store-bought option on the market.