If your treehouse is too high up for a single set of stairs, you can add platforms between smaller stair sets and have them wrap around your tree.
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Calculate the rise and run of your stairs. Lay a board flat on the floor of the treehouse. Measure the vertical distance from the ground to the bottom of the board to find your run or how tall your stairs need to be. Take the run number and divide it by 7, which is the typical rise of an individual stair. This will tell you how many stairs you need in your staircase. If your answer isn't an even number, round down and divide your run by the new figure to get the correct rise for each stair.
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Cut four 2-by-12 boards into stringers for your staircase using a circular saw. Use a framing square with stair markers to measure where each of the stairs should be. If you're unsure of the square's measurements, measure each of the rises and the treads with a simple tape measure. Treads are typically at least 10 inches wide. Mark your measurements with a pencil, and create cut lines. Use the circular saw to cut out the stair steps in each stringer. Cut away any excess length from the board, once you have finished.
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Cut two 2-by-8 joists approximately 8 inches longer than the planned width of your steps. Attach these boards to the bottom framing of the treehouse where you plan to attach the stairs. This will add strength and support to your stairs. Space the stringers evenly apart with the top of each stringer against the attached joists. Screw or nail the stringers to the joists. Leave approximately 4 inches of joist free on each side.
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Block the first step using the additional pieces of 2-by-8 wood. Measure the width, depth and length of the top step of your staircase. Cut a board for each side of the first step and along the rise. Attach the boards inside the first stair to reinforce the staircase. Screw or nail one board to the inside of each outside riser. Attach the final longer board across the rise to construct the first step.
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Measure the width of each step rise. Cut a 1-by-8 into board pieces to create risers. Each step needs one riser. Attach the boards to the rise of each step using screws or nails.
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Measure the width of each stair tread. Cut 2-by-6 decking boards to width. Place and attach two to three decking boards across each tread space, depending on the depth of the stairs. Leave approximately one-eighth inch between each board and the next as you lay them to allow for any board swelling resulting from moisture.