Building a spiral staircase around a tree brings a fantastic Swiss Family Robinson aura to any outdoor area. This can easily be accomplished with large enough trees, using long bolts and t-nuts in the steps. While these steps can be installed quite easily in the tree, they cannot hold very large loads, but are appropriate for lighter use or for low heights. These steps must be made of treated wood and galvanized t-nuts in order to resist the weather of the outdoors.
Drill a 6-inch hole in the tree at the location of the first step using a 3/8-inch drill bit. Slide a 12-inch long, 1/2-inch diameter bolt into the hole and tighten it securely using the pliers if necessary until the bolt is six inches into the hole.
Drill a 1/2-inch diameter hole in the center of one end of a treated 1-by-8-by-24-inch stair board. Hammer a 1/2-inch T-nut into the hole. Screw the stair onto the bolt until the stair comes in contact into the tree.
Drill a 4-inch galvanized screw into the tree just above the stair board, toward its rear, to prevent the board from rotating upward when it is stepped upon.
Repeat this process with each stair board, installing them in the tree 8 inches higher than the previous step with an eight inch gap horizontally between the steps. Continue to install the steps around the tree, going no higher than 8 feet off the ground.