How to Build a Treehouse

Choose a Tree

Due to their beauty and strength, deciduous hardwoods like oak, maple, beech, apple, and ash have long been used for building treehouses. Evergreens such as Douglas Fir, pine, spruce, and hemlock work well, too. Ensure that the tree is healthy, straight, and as close to vertical as possible.

Lower branches should be trimmed away to the desired height of the platform. Then, clear another 6 feet of branches above the platform height.

Mark the Post Locations

Using the tree as the center point of the treehouse, mark four locations for the post footings in a square shape. Post footings will be 8 feet from each other and equidistant from the tree. Mark these locations by pounding four wood stakes into the ground.

Dig the Post Footer Holes

With the post-hole digger, dig four holes. Dig to a depth required for your area so that each post extends below the frost line. Bed the bottom of each hole with 3 inches of gravel.

Then, place a cardboard form tube in each hole. Cut each form tube down so that about 2 inches of the tube extends above ground level.

Pour the Footers

Mix the concrete. Fill each form tube until the concrete is level with the top of the tube. Smooth the concrete so that it is flat.

With the concrete still wet, set a 5/8 inch diameter J-bolt into each concrete footing. The J-bolt should extend 3/4 inch to 1 inch above the concrete.

Mount the Posts

Install a post base to each of the four footers with the included washer and nut that attach to the embedded J-bolt. Set each piece of 6-by-6-inch lumber atop a post base. Plumb with a level and then nail pieces into place with 16d nails.

Temporarily secure the post by tacking two 2-by-4-inch pieces of lumber to each post to form a triangle brace with two free arms resting on the ground.

Build the Outer Section of the Floor Frame

Using four 2-by-10-inch pieces of wood, create a square that will form the outside perimeter (or rim joist) of the floor frame. The frame should be 8 feet long by 8 feet wide.

Cut two of the 2-by-10-inch boards so that each one is 7 feet by 9 inches long. The other 2-by-10-inch boards remain 8 feet long. Nail the four boards together with 16d nails on end to form the square.

Mount the Floor Frame

With two people on ladders, raise the floor frame above the level of the tops of the posts. The floor frame should now be wrapped around the four posts. Slide down the floor frame to the desired floor height.

Temporarily tack the frame into place on each post with 16d nails. The nail heads should stay exposed for easier removal.

With the cordless drill, make two pilot holes at each post through the 2-by-10-inch frame and into the post. Follow up by screwing the lag bolts and washers into each pilot hole with a ratchet wrench. Remove the temporary 16d nails with the claw end of the hammer.

Install the Knee Braces

Remove the temporary 2-by-4-inch board braces as these will be used to create permanent cross braces. These can either be short knee braces that extend from one post to the floor frame or they can be longer diagonal braces that extend from one post to an adjacent post.

To create knee braces, cut four of the 2-by-4-inch boards in half so that you have eight 2-by-4-inch boards that are each 48 inches long. Place a 2-by-4-inch board in the desired position of the knee brace (45-degrees, extending from the rim joist to the post), then mark the cuts from the back with the pencil.

Cut with the miter saw and test it out on the treehouse. If it works, use this as a template for cutting the remaining three knee braces.

Drill pilot holes, then attach one knee brace per post with the 1/2-inch lag bolts. To avoid lag bolts from one brace interfering with lag bolts on a post's other brace, the second set of four knee braces should be cut either longer or shorter than the first set. Measure, mark, cut, and attach these braces as you did the other set.

Set the Joists

With the pencil and tape measure, measure 12 positions on the inside of the floor frame for the joist hangers. There should be six hangers on one side and six on the opposite side. Hangers should be 16 inches apart. You may have to slightly adjust the position of the middle two hangers to accommodate the tree.

Nail the joist hangers into each position with the 10d nails. Firmly pocket the remaining two-by-tens into the joist hangers. Then attach the boards and the hangers with 10d nails.

Build the Flooring

The eleven remaining 2-by-10-inch pieces of wood will be used as deck boards.

Face-nail onto the joists with the 16d nails. Or for a cleaner look, consider purchasing a hidden fastener deck system. Space boards 1/8 inch to 1/4 inch apart. When reaching the tree, cut these boards to length.

Install the Railing and Ladder

Install the hand railing on the top of the deck following the manufacturer's instructions. Be sure to leave a section about 2 feet wide for access.

For a ladder, consider the many rope ladders are available online that can be attached to the treehouse. Or, tilt an extension ladder into place for a sturdier solution. As an option, secure the ladder with bungee cords or metal brackets to stay in place.