How to Build Treehouse Stairs

Lean two 2- by 6-inch boards against the front opening of your tree house so the bottom end touches the floor, and the slope is suitable for your stairs. Use your circular saw to cut the bottom edges of the boards so they stand flush against the ground at the angle they will be installed.

Mark the cutting angle at the top end of your 2-by-6 boards with your straightedge and a pencil. With the two boards extending past the floor of the tree house, place a straightedge against the front edge of the platform so it extends upward. Use your pencil to mark the line of the straightedge onto the boards. Use your circular saw to cut along the pencil mark.

Mark the position of each step in the staircase with your tape measure. Stand the stringers in position against the tree house as they will be installed. Mark a spot on the back edge of each board 8 inches down from the very top of the stringer. Place your level at this mark and then extend a plumb pencil line toward the front of the stringer. This mark is the guideline for your top step. Extend the tape measure downward from the first step marker line, and repeat the process until all the steps positioned have been marked.

Measure and cut pieces of 1- by 4-inch lumber to fit each tread marker on the stringers. These pieces will serve as tread supports. Do not extend the supports to the edges of the stringers or the ends will have to be mitered to fit. Place the tread supports along the bottom edge of each pencil line. Use your screw gun and 2-inch decking screws to fasten them in place.

Cut stair treads from 2- by 6-inch lumber with a circular saw. Cut each step to fit the width of the staircase as needed. Do not exceed 16 inches or the wood may be too weak to withstand the load. Place the treads between your stringers onto the tread supports. Use your power drill to make pilot holes through the stringers and into the stair treads. Next, use your screw gun and 3-inch decking screws to fasten each tread in position. Drive the screws through the stringers and into the treads.

Move to the underside of the tree house and fasten the staircase to the rim joist. Use your power drill to make pilot holes through the rim joist and about 1 inch into the stringers behind it. Use your screw gun to drive 3-inch decking screws into the holes, two for each stringer.