How to Build a Porch Railing Planter in 3 Hours

Measuring Your Railing Before You Buy Lumber

The planter mounts to the railing from below using two cleats that hook over the railing top. The cleat design must match your railing dimensions exactly, or the planter will either not fit or sit too loose and tip in wind.

Railing Width

Measure the flat top surface of your railing from outside edge to outside edge. A standard 2x4 railing measures 3.5 inches across the top. A 2x6 railing measures 5.5 inches. Some composite railings (Trex, TimberTech) measure 3.5 inches but have a rounded profile that requires a slightly different cleat notch. For composite railings with a 2x4 profile, subtract 1/4 inch from the cleat notch width to account for the rounded corners.

Railing Height

Measure from the top of the railing down to the bottom edge. A standard 2x4 is 3.5 inches tall; a 2x6 is 5.5 inches. The cleat hooks over the railing and extends down the outside face by 1.5 inches. The total cleat height equals the railing height plus 1.5 inches. For a 2x4 railing, the cleat is 5 inches tall. For a 2x6 railing, the cleat is 7 inches tall.

Railing Baluster Spacing

Check the spacing between vertical balusters (the pickets). Building codes require balusters spaced no more than 4 inches apart, but many older porches have wider spacing. If the gap between balusters exceeds 4 inches, the planter box itself (which is 7 inches deep) will bridge the gap and provide the required barrier. If your balusters are spaced wider than 6 inches, add a third cleat at the center of the planter for additional support.

The Cut List: One Board, Seven Pieces

Every piece for this planter comes from a single 1x8 cedar board at 6 feet long. A 1x8 cedar board at Home Depot or Lowe's costs $12 to $16. The actual dimensions of a 1x8 are 3/4 inch thick by 7.25 inches wide. This width becomes the height of the planter box (7.25 inches), which is deep enough for most annual flowers and shallow-rooted herbs.

Box Pieces

Cut two pieces to 24 inches for the front and back walls. Cut two pieces to 6.75 inches for the side walls (7.25 inches minus 2 times 3/4-inch board thickness equals 5.75 inches; add 1 inch for the bottom board overlap, making the sides 6.75 inches). Cut one piece to 6.75 inches for the bottom board.

Railing Cleats

Cut two pieces to 8 inches long from the remaining board length. These are the cleats that hook over the railing. The 8-inch length provides 4 inches of bearing surface on the planter bottom and 4 inches of hook depth over the railing. For a 2x4 railing, notch each cleat as follows: measure 3.5 inches from one end (matching the railing width), mark a line, and cut a notch 3/4 inch deep (matching the board thickness) on the bottom face of the cleat. The notch creates a step that hooks over the railing top. Use a jigsaw or handsaw to make the two cuts that define the notch, then chisel out the waste wood.

For a 2x6 railing, make the notch 5.5 inches wide instead of 3.5 inches. The cleat length remains 8 inches.

Verification

Before assembling, test the cleats on your railing. The notch should fit over the railing top with 1/8 inch of clearance on each side. If the fit is too tight, sand the inside of the notch with 80-grit sandpaper until it slides on easily. If the fit is too loose, glue a thin cedar shim to the inside of the notch and sand to fit.

Assembly: 45 Minutes from Boards to Box

Work on a flat surface. A workbench is ideal; a pair of sawhorses with a sheet of plywood on top works as well. The assembly uses exterior-grade screws and waterproof wood glue to create joints that resist moisture and hold up under the weight of wet soil (which adds 30 to 40 pounds to a 24-inch planter).

Step 1: Attach the Sides to the Bottom

Lay the bottom board (6.75 inches) flat on your work surface. Stand one side board (6.75 inches) on edge, flush with the bottom board edge. Apply a bead of Titebond III Ultimate Wood Glue ($9 per bottle) along the joint. Drive three 1-5/8-inch exterior deck screws through the side board into the bottom board edge, spaced 1 inch from each end and one in the center. Repeat for the second side board. Check that both sides are perpendicular to the bottom by measuring diagonally; both measurements should be equal.

Step 2: Attach the Front and Back

Apply glue to the ends of the side boards. Position the front board (24 inches) against the side board ends, flush with the bottom edge. Drive three 1-5/8-inch deck screws through the front board into each side board end, positioned 1 inch from the top and bottom edges and one in the middle. Repeat for the back board. The front and back boards overlap the side board ends, creating a simple but strong butt joint.

Step 3: Pre-Drill to Prevent Splitting

Cedar splits easily when screws are driven near the ends of a board. Pre-drill all screw holes with a 1/8-inch bit before driving screws. The pre-drilling adds 10 minutes to the project but prevents the frustration of a side board splitting at the last screw, which would require disassembly and a replacement piece.

Step 4: Attach the Railing Cleats

Position the cleats on the outside face of the back board, one cleat 2 inches from each end. The notch in each cleat faces downward and hooks over the railing. Drive four 1-5/8-inch deck screws through the cleat into the back board, two screws at the top and two at the bottom. The cleats should be flush with the bottom edge of the planter box. When mounted on the railing, the planter hangs from the cleats with the bottom resting on the railing top surface.

Drainage Holes and Liner Installation

Without drainage, water fills the planter to the soil surface within minutes of heavy rain. Roots submerged in standing water for more than 24 hours begin to rot. Proper drainage and a liner prevent both problems.

Drilling Drainage Holes

Drill six 3/8-inch holes through the bottom board, spaced evenly in two rows of three. Use a spade bit or twist drill bit. Sand the holes smooth on the inside face to prevent sharp edges from cutting the liner. For a 24-inch planter, six holes provide adequate drainage for typical rainfall. If you live in an area with heavy summer thunderstorms (more than 2 inches of rain per hour), drill eight to ten holes.

Installing the Liner

Cut a piece of 4-mil polyethylene sheeting ($5 for a 10x25-foot roll at Home Depot) to 25 inches long by 20 inches wide. Press the liner into the planter, folding the corners. Staple the liner to the top edge of the planter with a staple gun, keeping the liner 1/2 inch below the rim so it is not visible from outside. Punch holes through the liner with a pencil to align with each drainage hole in the bottom board.

The liner slows the rate at which water drains through the bottom, keeping soil moist longer between waterings. Without a liner, water runs straight through the drainage holes in 30 seconds, and soil dries out between morning and evening on hot days. With the liner, drainage takes 2 to 3 minutes, and soil stays moist for 12 to 24 hours longer.

Finishing: To Seal or Not to Seal

For a planter that will hold edible plants (herbs, strawberries, lettuce), leave the cedar unfinished. Any finish or sealant on the interior surface can leach into the soil and be taken up by plant roots. Cedar's natural oils provide sufficient rot resistance for 8 to 12 years without any finish.

For ornamental plantings only, you can extend the planter's life to 15 years by applying a coat of Thompson's WaterSeal ($12 per quart) to the exterior surfaces only. Apply with a brush, let it penetrate for 15 minutes, and wipe off excess. Thompson's WaterSeal is a silane/siloxane water repellent that does not form a film; it penetrates the wood fibers and causes water to bead on the surface instead of soaking in. Reapply once per year in early spring.

Do not use exterior paint or polyurethane on a planter box. Paint peels from cedar within two years because the wood's natural oils prevent paint adhesion. Polyurethane cracks as the wood expands and contracts with moisture, and the cracks trap water against the wood surface, accelerating rot rather than preventing it.

Filling and Planting: What Fits in 1.5 Cubic Feet

The interior volume of the finished planter is approximately 22 inches long by 5.75 inches wide by 6.5 inches deep, which equals roughly 1.5 cubic feet. A 2-cubic-foot bag of Miracle-Gro Moisture Control Potting Mix ($13) fills the planter with some leftover. Do not use garden soil; it compacts in containers and blocks drainage.

Plant Selection for Railing Planters

Trailing plants work best in railing planters because they cascade over the edges and soften the rectangular shape. Petunias (6 plants per planter), calibrachoa (6 to 8 plants), bacopa (4 to 6 plants), and sweet potato vine (2 to 3 plants) all perform well in 6.5 inches of soil depth. For a mixed planter, combine one tall plant in the center (dracaena spike or upright geranium), three mid-height plants on each side (petunias or marigolds), and two trailing plants at the edges (sweet potato vine or licorice plant).

For herbs, plant rosemary, thyme, oregano, and chives in a single planter. These four herbs have compatible water and sunlight needs (full sun, moderate water) and will produce enough for weekly cooking through the entire growing season. Basil also works but requires more frequent watering and may crowd the other herbs in a 24-inch planter.

Watering Schedule

In full sun during summer, a 24-inch railing planter needs water every 1 to 2 days. In partial shade or cooler weather, every 3 to 4 days. Stick your finger 1 inch into the soil; if it feels dry at that depth, water until it runs from the drainage holes. During heavy rain, the drainage holes prevent waterlogging. During dry spells, the liner slows moisture loss enough that the planter does not need daily attention.

Weight Limits and Safety Considerations

A wet 24-inch cedar planter filled with soil and plants weighs 35 to 45 pounds. This weight is well within the capacity of a standard porch railing, which is designed to support a 200-pound lateral load per building code. However, the planter must not extend more than 4 inches beyond the outside face of the railing. Overhang beyond 4 inches creates a lever arm that can tip the planter off the railing in high wind or if someone leans on it.

The cleat design described here positions the planter centered over the railing with 2 inches of the planter bottom resting on the railing top surface. This center-of-gravity placement prevents tipping under normal conditions. In hurricane-prone areas (coastal Florida, Gulf Coast), remove the planter from the railing and set it on the porch floor during storms with sustained winds above 60 mph.

Inspect the cleats and screws once per month during the growing season. Cedar is a soft wood, and screw holes can enlarge over time as the wood swells and shrinks with moisture. If a screw spins freely when you test it, remove it, squirt a toothpick coated in Titebond III into the hole, break the toothpick off flush, and re-drive the screw into the repaired hole. This fix takes 2 minutes and restores full holding strength.

Moving Forward

This planter design scales up or down to fit any railing width. For a 36-inch planter, buy an 8-foot board instead of a 6-foot board and adjust the front and back pieces to 36 inches. For multiple planters along a long porch railing, build several 24-inch planters spaced 12 inches apart. The total material cost for three planters is $40 to $50 using two 8-foot boards. Each planter takes 2.5 hours to build after the first one; the learning curve on the cleat notching and assembly means the second and third planters go significantly faster than the first. With cedar's natural rot resistance and proper drainage, these planters will last 10 to 12 years on a covered porch, or 7 to 9 years on an exposed railing with direct rain and sun.