48″ long window box plans

Parts

Cut list

Kerf 1/8″ included. Every cut runs edge to edge.

Steps

• Tape measure
• Speed square
• Pencil
• Drill/driver
• Clamps
• Safety glasses
• Hearing protection
• Dust mask
• Circular saw or miter saw
• Stud finder
• Level
• 1/2″ spade bit
• 3/16″ drill bit
• Socket wrench with a 7/16″ socket
• Staple gun
• Stepladder
• Sander, 120 and 180 grit

Underlined words have a short explanation: hover or tap them. All of them are on the glossary page.

1. 1Before you start

   Read every step once, start to finish, before you buy or cut anything. Each step names the parts it uses by letter; the parts list gives their sizes.

   Set up somewhere flat with good light. A pair of sawhorses with a sheet of plywood on top makes a fine work table.

   Wear safety glasses whenever a tool is running, hearing protection for saws, and a dust mask for sanding and for cutting treated wood or sheet goods. Tie back long hair and loose sleeves. Keep children and pets out of the work area.

   • Tip At the store, pick straight boards: hold each one up to your eye and look down its length. Skip boards that bow, twist or have big knots near the ends.
   • Tip Measure twice, cut once. Most mistakes are a misread tape, not a bad cut.

2. 2Cut and label the parts

   Cut every part on the cut list. Cut the longest parts first: if one goes wrong, it can still become a shorter part.

   To mark a cut, hook your tape on the end of the board, make a small V with its point exactly at the measurement, and draw a square line through the point with a speed square. Put an X on the waste side.

   Cut on the waste side of the line, so the blade's kerf (about 1/8″) comes out of the scrap, not your part.

   Write each part's letter in pencil on a face that won't show, and stack the parts by letter.

   Sand the faces before assembly while they're flat and easy to reach: 120 grit, then 180.

   ✓ Lay the parts out and count them against the parts list. Measure a few: if one is off by more than 1/16″, recut it now.

   STOP. Let the saw reach full speed before it touches the wood, keep both hands clear of the blade's path, and never reach under a board you're cutting. Support long boards on both sides of the cut so the blade doesn't pinch.

3. 3Pick the boards and the screws

   Cedar for the window box: look for boards with mostly reddish-brown heartwood. Pale yellow-white streaks are sapwood, which rots years sooner; keep those streaks on the inside or skip the board.

   Sight down each board at the store like a rifle barrel. Skip boards that twist, bow more than about 1/4″ over 8′, or have big loose knots near where you will cut or screw.

   Use exterior screws only: coated deck screws (labelled for treated lumber, "ACQ compatible") or stainless steel. Plain steel reacts with the tannins in cedar and leaves black streaks running down from every screw. Indoor wood screws and drywall screws rust and snap outdoors.

   For food plants, both cedar and modern treated wood (labelled CA or ACQ, not the old green CCA, which hasn't been sold for homes since 2004) are widely used. If you want to be extra careful, staple landscape fabric inside so soil doesn't touch the wood.

   • Tip Buy one extra board of the most-used size. A split or a mis-cut then costs you nothing but a few dollars.
   • Tip Cedar splits easily: predrill every screw within 2″ of the end of a board with a bit about the size of the screw's inner shank.

   ✓ Every board is reasonably straight, and the screw box says exterior, coated for treated lumber, or stainless.

4. 4Drill the drainage holes

   Draw a line down the middle of the bottom (B). Mark a hole every 6″ along it, the first and last about 3″ from the ends: 7 per line, 7 in all.

   Clamp the bottom on a scrap board and drill each mark with a 1/2″ spade bit, straight down. The scrap underneath stops the wood from tearing out where the bit comes through.

   Without drainage holes, rain fills the box like a bucket and the roots rot within days.

   • Tip Let the spade bit's point break through, then flip the board and finish the hole from the other side for an even cleaner hole.

   ✓ 7 clean holes you can see daylight through, none closer than 2″ to an end.

5. 5Join the ends to the bottom

   Stand an end (C) on the bench with its grain running up and down. Butt the end of the bottom against it, with the bottom's underside 1/2″ above the end's lower edge. A 1/2″ scrap under the bottom holds it there.

   Spread waterproof glue on the bottom's end, then predrill and drive three 1 5/8″ screws through the end into it.

   Repeat at the other end.

   Why 1/2″ up? Water dripping off the box then falls from the board edges instead of sitting under the bottom and rotting it.

   ✓ Both ends are square to the bottom (check with a speed square) and at the same height.

6. 6Add the front and back

   Lay the box on its back and set the front (A) on top, flush with the ends and their lower edges.

   Glue the edges it touches, then predrill and drive 1 5/8″ screws through the front into the bottom's edge every 6″, and three into each end.

   Turn it over and fix the back the same way.

   • Tip Wipe glue squeeze-out with a damp rag right away; dried glue blocks stain and paint.

   ✓ The box measures 48″ by 8 3/4″ outside, its diagonals across the top match, and it sits flat on a bench.

7. 7Line the inside

   Staple landscape fabric to the inside of the front, back and ends, from the bottom up to 1″ below the top edge. Lay a piece over the bottom too.

   The fabric keeps wet soil off the wood so the box lasts longer, and water still runs out of the holes.

   ✓ The holes are covered by fabric, not by plastic, and water poured in runs out underneath.

8. 8Protect it from the weather

   Decide how you want the window box to look in five years. Bare cedar and bare pressure-treated wood both weather to silver-grey and still last many years; doing nothing is a real option.

   To keep the colour, use an exterior oil or semi-transparent stain: brush it on, let it soak in 15 minutes, then wipe off whatever is still shiny. Renew it every one or two years. Avoid clear varnish or polyurethane outdoors: sun cracks the film, water gets under it, and it peels.

   For a solid colour, prime with an exterior primer and use two coats of exterior latex paint. Paint end grain twice: it soaks up water like a straw.

   Leave the inside of anything that holds soil for food plants bare. Finish only the outside faces and the top edges.

   • Tip Finish on a dry day above 50°F with no rain forecast for 24 hours.
   • Tip Oil-soaked rags can catch fire on their own as they dry. Lay them flat outside to dry or keep them in a sealed metal can of water.

   ✓ The finish is dry to the touch everywhere, with no wet or shiny puddles in corners.

9. 9Find the studs and fix the brackets

   Below a window, the wall has cripple studs about 16″ apart, and a stud at each side of the window. Find them with a stud finder from inside, or look for the nail lines in the siding. Mark each on the outside with painter's tape.

   Choose 3 studs in a row, centred under the window, for the 3 brackets 16″ apart. Mark a level line for the top of the brackets: the box top usually sits 2–4″ below the window sill.

   Hold each bracket on its stud mark, level, and predrill through the siding into the stud with a 3/16″ bit. You should feel the bit bite into solid wood after about 1″.

   Drive two 1/4″ × 3″ galvanized lag screws with washers through each bracket into the stud with a socket wrench. Snug them up; overtightening crushes the siding.

   • Tip Run a bead of exterior caulk around each lag screw where it enters the siding so rain can't follow it into the wall.

   ✓ Every lag screw pulled tight into wood (none spins loose), and a level across the bracket tops reads level.

   STOP. Do this only on a ground-floor window you can reach from a short, steady stepladder on firm, level ground. For second-storey windows, hire it out. Never hang a box on vinyl siding, trim or plastic anchors alone: only screws into the studs hold the weight of wet soil.

10. 10Set the box and fill it

    Set the empty box on the brackets. Slide a composite shim between the back of the box and the wall at each bracket so there is a 1/4″ air gap: the siding and the box back can then dry after rain.

    Drive the screws that came with the brackets up through the holes in each bracket arm into the box, then pull the shims out.

    Fill with potting mix: about 1.0 cu ft. Plant, then water until it drips from the holes. Soaked, the box weighs about 108 lb.

    ✓ The box doesn't move when you push and pull on it, there's a gap behind it you can slide a shim into, and water drips from the holes.

11. 11Finish

    Ease every sharp edge and corner with 120-grit sandpaper so nobody gets a splinter.

    Sand all over with 180 grit, always along the grain (the direction the wood fibres run). Scratches across the grain show through stain.

    Wipe off the dust with a barely damp rag and let it dry.

    Outdoors, use an exterior finish: a penetrating deck oil or exterior stain, or primer and exterior paint. Leave cedar bare to weather silver if you like. If it holds soil for food plants, leave the inside unfinished.

    Let it dry for the time on the can before using it.

    • Tip Try your finish on a scrap of the same wood first: stain looks different on every board.