How to Build a Tree Fort

Day-to-Day Timeline
Day 1:  Build the framing (Steps 2–12).
Day 2:  Add the decking and railing (Steps 13–19).

Mark the bark about 17 inches below the finished deck height on one side of the tree. Use a 3-inch self-feed bit to bore into the trunk 2 inches past the bark layer. Drill a second hole directly opposite the first.

Using a 1 1/8-inch auger bit, drill 2 inches into the middle of the first hole; make sure it’s level. Then use a 1-inch auger bit to drill another hole 4 1/2 inches deep to accept the threads of the tree bolt. Slip the pipe bracket that will support the framing onto the tree bolt, cap it with a hex nut, as shown, and twist the threaded end into the hole with a 1 7/8-inch socket wrench (or a pipe wrench). Repeat the process on the opposite side.

Make a beam by joining a pair of 2x10s with construction adhesive and 16d ring-shank nails. Slide the beam onto the pipe bracket, as shown, and center it. Attach the bracket to the beam with 4-inch structural screws. Level the beam using a 4-foot level, then brace it by driving 2-inch deck screws through 2x4s or deck boards and into each end of the beam. Repeat the process for the beam on the opposite side of the tree.

Attach joist hangers a few inches inside the ends of the beams with 1 1⁄2-inch joist hanger nails. Make a short beam with more 2x10s to fit in the hangers. Nestle the header in the joist hanger and nail it in place with more joist hanger nails, as shown. Repeat at the opposite ends of the beams.

A specialty bracket and 4x6 timber support the end of the beam assembly. Use a rafter square to hold the bracket at a 45° angle to the tree, as shown. Pull the tape measure from the metal tab to the underside of the header to get a rough length. Repeat this step on the other side of the tree.

Start by making a mark on the board face 2 1⁄4 inches up from the bottom edge of the 4x6. Rest the shoulder of the rafter square against the bottom edge with the 45° angle facing the board end. Draw a 45° line from the mark to the bottom edge. Flip the square over with its shoulder against the opposite edge and the angle facing the end. From the mark, make a second line along the angle to the top edge, creating an angled L, called a bird’s mouth, on the face of the timber. Start the cut along the lines with a circular saw, then finish with a reciprocating saw, as shown. Now measure from the corner of the L and cut the timber to length.

The knee-brace bracket rests in a slot cut into the end of the 4x6. To make the joint, use the rafter square to mark the center of the end of the 4x6. Then use a circular saw or chainsaw to cut a 3⁄8-inch-wide channel in the timber as long as the tab on the bracket. Make a second knee brace following these same steps.

Hold the knee-brace bracket against the face of the 4x6 at the slotted end and mark the two bolt holes. Use a 5⁄8-inch bit to drill through the timber. Slip the bracket into the slot, hammer the lag bolts through, as shown, then add washers and nuts. Tighten the nuts with an adjustable wrench. Repeat for the second brace.

Toenail 2x8 joists 16 inches on center to the carrying beams with ring-shank nails to create an 8-by-10-foot framework. Use hurricane ties to reinforce the joist–beam connections. Lift the header about an inch with a 4x6 and a bottle jack—you’ll remove it to put tension on the brace once it’s installed. Hold the brace in place with the bird’s mouth biting the header. Drive a pair of 6-inch structural screws through the outside of the header and into the 4x6, as shown.

Hold the end of the knee-brace bracket to the tree. Using the metal tube as a guide, drill an 8-inch-deep hole into the tree with a 1 1⁄4-inch auger bit. Thread a 15-inch galvanized lag bolt into place and tighten it with a socket wrench. Leave a few inches between the bracket and the bolt head to accommodate tree growth. Now lower the jack and repeat the process on the other side of the tree.

Place a piece of full-length decking 3 inches from the tree, to allow for growth, and hammer ring-shank nails through it into each joist. Work out toward the edge of the framework, using the same nails as spacers between boards. Stop about a foot from the joist ends.

Now work in the other direction, installing the boards interrupted by the tree. Scribe the ends to follow the contour of the bark, as shown. Cut the marks with a jigsaw, and attach the boards, again leaving 3 inches for growth. Install all the interrupted pieces, then continue laying full-length boards, stopping a foot shy of the joist ends. With the decking in place, trim the ends of the boards with the jigsaw, creating a natural edge.

Tip: To accurately scribe the boards that run into the tree trunk, hold the compass so its legs are parallel with the joint in the boards.

For the corner posts, nail 2x8 blocking about 8 inches in from the joist ends of the first and last joist bays on the long sides. Add blocking for two field posts evenly spaced between the corners. The rail posts are made from straight branches 4 to 6 inches in diameter. Make corner posts by cutting a 7 1⁄4-inch-long notch into one side of a branch with a reciprocating saw, as shown.

Position a corner post with the notch against the joist and shoulder on the decking, and drive a pair of 10-inch structural screws through the post and into the blocking, as shown. Repeat with the remaining corners and field posts for the long sides. Now notch the last deck boards to fit around the posts and install them.

On the short sides, plan for one field post between the corner posts. Notch all four sides of the field posts with a reciprocating saw. Hold a post in place on top of the decking, mark around the notch, then cut a mortise with a jigsaw. Drop the field post in the hole, as shown. Use 10-inch screws to attach the post to the side of the joist.

Use a reciprocating saw to cut the field posts about 3 feet tall and notch their tops for a half-lap joint. Find one long top rail that spans corner to corner and notch its ends to complete a half-lap joint with the posts. Then notch the rail’s underside where it meets the field posts with shallow saw cuts and a chisel. Drive deck screws through the top rail and into the posts. Attach the bottom rail in sections between the corner and field posts.

Miter the ends of 2-inch-diameter branches to make balusters. Drill pilot holes through the mitered ends and into the top rail and attach with deck screws, as shown. Screw the other end of the balusters into the bottom rail. For safety reasons, space the balusters about 3 1⁄2 inches apart. Attach the cargo netting to the fort with eye hooks and secure the other end using stakes.

Tip: Use a scrap piece of 2x4 to help space the balusters consistently.