How To Build a Cedar Sandbox

Completed cedar sandbox.
Completed cedar sandbox.

Kids love a sandbox, and building one is an easy project even for inexperienced DIYers. You can build this sandbox in a day, although it took us longer thanks to our two-year-old “helper” stealing our lumber to make block towers.

This tutorial will show you how to build a 5’ by 5’ sandbox with corner seats. The dimensions were a compromise – I planned a 4’ x 4’ sandbox to save lumber, but my husband wanted it at least 6’ x 6’ for what he called “maximum funnitude.” We settled on 5×5, and it has turned out to be the perfect size!

The leftover lumber was used for the corner seats, and it’s big enough for several children to actually climb inside and really get down to some serious fun.

As you’ll see, these instructions are easily customizable. You can personalize your sandbox by making it larger or smaller, leaving off the seats, or using different wood to make it your own!

Cedar lumber for sandbox.
Cedar lumber for sandbox.

Materials and Tools:

For this project, you will need:

Materials Needed:

  • 9 – 5/4” x 6” x 8’ cedar deck boards
  • 2 – 2” x 4” x 8’ cedar boards
  • 108 – 2” exterior deck screws
  • Penetrating waterproofing sealant
  • 24 – 50-lb bags of sand

Tools Needed:

  • Circular saw, miter saw, or handsaw
  • Drill with screwdriver and drill bits
  • Tape measure
  • Pencil
  • Framing square
  • Rafter square
  • Medium-grit sandpaper
  • Paintbrush
  • Post-hole digger and flat shovel

Cutting List:

  • 8 – 5/4×6 x 5’ (side walls)
  • 8 – 2×4 x 12” (side supports)
  • 4 – 2×4 x 24” (corner supports)
  • 4 – 5/4×6 x 11” (seats)
  • 4 – 5/4×6 x 22” (seats)

Step 1: Cut Lumber to Size

We actually had the boards cut to length for us at the lumber store, which made for a super-easy assembly. The only cuts we made at home were for the seats.

Sealing lumber using foam paintbrush.
Sealing lumber before assembling sandbox.

Step 2: Seal Cut Lumber

Paint the lumber, including the cut ends, with a penetrating waterproofing sealant, and allow to dry.

Step 3: Build Box Components

Lay four of the deck boards on a flat surface, and assemble them into a square. Make sure the boards lap over each other in the same direction on all four corners so the box has the same outside dimensions on each side.

Use a framing square to square up the corners, and secure each corner with two deck screws. Drill pilot holes to prevent splitting.

Repeat with the second four boards to make two identical boxes.

One half of the sandbox assembled.
One half of the sandbox assembled.

Step 4: Assemble the Sandbox

Flip one of the boxes over, and lay it on top of the other one, so that the corner joints alternate directions. Place the corners of the boxes on lumber scraps, so that you will have a nice level surface.

Square up the boxes by measuring diagonally from corner to corner and adjust until both of the diagonals measure the same length.

Step 5: Add Corner Supports

Attach a corner post to the inside of each corner of the sandbox.

Since the sandbox is being assembled upside down, check underneath to make sure the support post is flush with (or slightly below) the top of the sandbox. This is where the scrap lumber underneath comes in handy, so you have a level surface to press against.

Secure the support posts to the deck boards from the outside using two deck screws per board. Make sure to hold the corners tight so that they fit snugly.

Repeat on each of the other three corners.

Square aligning the side supports.
Squaring up the side supports before screwing.

Step 6: Add Side Supports

Attach two side supports to the inside of the sandbox on each side from the outside with deck screws. Use a square to align the side supports perpendicular to the sandbox, and make sure they’re flush with (or slightly below) the bottom edge, which will be the top of the sandbox.

Use four decking screws, two in each deck board, to attach the side supports to the sandbox.

Upside down assembled sandbox with corner supports.
Upside down assembled sandbox with corner supports.

Step 7: Mark Lumber for Seats

While the sandbox is upside down, slip two of the deck board scraps under a corner. Align the edge with the tip of the corner, and use the rafter square to make sure the boards are at a 45-degree angle to the sandbox sides.

Mark the outside edges of the boards using the sandbox as a template. Number the underside of the boards, and number the corner, so you’ll remember which seat goes with which corner.

In a perfect world, all of the corners would be identical, but marking them will account for any imperfections (and it’s OK if there are imperfections – it’s a sandbox, remember?).

Repeat with the remaining three corners.

Marking the scrap lumber for seats.
Marking the scrap lumber for seats.

Step 8: Cut and Seal Seats

Cut the seat pieces along the marked lines. Use sandpaper to lightly round the cut edges and remove any splinters. Paint them with your waterproofing sealant and set aside to dry.

Step 9: Prepare Site

Now, move the sandbox to the location it will be installed. Cut and remove any sod and tree roots, and level the surface using a flat shovel.

Step 10: Install Sandbox

Flip the sandbox over, and put it in place. Mark the corner posts on the ground and set the sandbox aside.

Use a shovel or post-hole digger to dig holes for the corner supports. If you’ve chosen not to bury the corner supports, skip this step and just make a level surface.

Leveling sandbox during installation.
Leveling sandbox during installation.

Put the sandbox corners in the holes, and adjust until the sandbox is flush with the ground and level in all directions.

Slip landscape fabric underneath the sandbox, lifting slightly to tuck it under so that the sides will weigh it down. We used two layers of fabric, one in each direction.

Gently push the landscape fabric away from the corners, and backfill your support holes. Tamp down the soil tightly. Then, gently slide the landscape fabric back into the corners.

Trim landscape fabric around the outside of the box, leaving 2” to 3” sticking out. If you like, you can staple the fabric to the outside of the box. We simply buried the edge of the fabric under the mulch.

Using screws to attach sandbox seat.
Using screws to attach sandbox seat.

Step 11: Install Seats

Take your numbered seat pieces and lay them on each sandbox corner, keeping the edges flush. Drill pilot holes and install the seats using deck screws. Repeat with the remaining corners.

Lastly, use your waterproofing sealant to touch up the seats, including the screw heads.

Filling sandbox with sand.
Filling sandbox with sand.

Step 12: Add Sand and Have Some Fun!

Now you’re ready for sand! We used 24 fifty-pound bags of play sand to fill the box about 2/3 full. Then just add kids, toys, and a cold drink for the workers, and you’ve got yourself a sandbox party!

Further Information


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