To install prefab outdoor stairs, mark the correct position of the stairs, construct a temporary support, fasten the staircase in place, and finish it with waterproof treatment. Tools needed for the installation include a tape measure, a level, a saw, a T-bevel, a hammer and finishing nails. A two-by-four-inch piece of lumber and helper are required as well. Depending on the location of the staircase installation, a stud finder may also be helpful.
Place the prefabricated staircase in the proper position so that it lines up correctly with the top and bottom landing locations. Use a level across the stairs to ensure that the position is level and square with the landings. Mark the top and bottom locations of the stairs on each landing. Use the T-bevel to measure the angle of the staircase rise, and write it down. Measure to the center of the stairs, and mark its location, then measure from the mark to the ground, and note the distance.
Measure and cut a piece of the two-by-four to the height of the staircase center, and cut one end of it to match the angle of the stairs to construct a temporary support. Position the stairs in place with the marks, make sure it is level, and hammer a couple of finishing nails through the top framing element and into the upper landing face. Nail the temporary support in place under the staircase with a single nail. Finish nailing the top and bottom of the stairs to their respective landings, and install any support brackets included in the kit.
If the stairs run along an exterior wall, locate the studs in the wall, and nail the staircase to them securely. Cover the stairs with the desired waterproof stain or coating.