Exterior wood stairs are a project a homeowner can accomplish with a few basic carpentry tools. The process is similar for all lengths and heights, and a variety of styles can be made with a few alterations to the basic structure.
Stand a 2-by-4 against the deck stair opening, with one end of the 2-by-4 on the ground.
Place a level against the side of the 2-by-4, and adjust the 2-by-4 so it is plumb.
Measure the distance from the ground to the top of the stair opening. This measurement is the height required for the stairs.
Divide the height to the top of the stair opening by 7, the typical height of a stair riser. For example, if the height is 62 inches, divide that by 7, and the result is 8.86. Divide 62 by 8.86 to determine the exact height needed for each riser. In this example, the height needed for each riser is exactly 7 inches.
Determine the run, or number of treads, required for the stairs by multiplying the depth of each tread by 7. The tread should be at least 10 inches for a comfortable set of stairs. The total distance from the deck to the bottom of the stairs is 70 inches or 10 inches for the treads multiplied by 7 inches for the number of risers.
Place tape or clip-on blocks at 10 inches on the outside of one side of a framing square and 7 inches on the outside of the other side of framing square.
Create the bottom step first. Place the framing square with the tread side of the framing square closest to the end of a 2-by-12, about 3 inches from the end of the 2-by-12. Trace along the outside edge of the framing square. Measure the distance of one riser from the tread. In our example, each riser is 7-inches. Subtract the width of the tread lumber, for example 1½-inch thick deck boards, to get 5½ inches. Measure 5½-inches from the tread line in two spots along the tread line toward the bottom of the board. Align the side of the framing square with the two marks, and trace the edge with a pencil. Place the framing square on this line, with the other side of the square aligned with the outside edge of the bottom riser. Draw a line against the framing square. If a point is not created, adjust the position of the bottom stair by the distance needed so the squared line and the line drawn 5½ inches from the bottom tread create a point on the bottom of the 2-by-12.
Align the tape on the tread side of the framing square with the edge of the 2-by-12 and the line drawn for the rise from the first step. Align the rise side of the framing square with the edge of the 2-by-12. Trace the outside of the framing square to draw the next step on the 2-by-12. Repeat for each remaining step. After the last tread line, continue the riser line all the way to the other side of the 2-by-12.
Cut out each notch with a circular saw. Do not overcut. Finish the cut with a handsaw. Use the finished stringer as a template for the other two pieces of 2-by-12. If the stairs will be more than 48 inches wide, cut an additional stringer. Cut out the notches as with the first stringer.
Create a cleat to hold the stairs on the deck. Cut a 2-by-6 the width of the opening for the stairs, minus the width of two stringers. Cut a notch in the back of one of the stringers---the area that will be against the deck when in position---the same width as the thickness of a 2-by-6 cleat. Measure the length of the cleat. Divide by 2 and subtract the thickness of the 2-by-12 stringer. Use a square to mark the location with a 90-degree line down the front of the cleat. Align the centre stringer to the line, and secure it to the cleat with at least three exterior, corrosion resistant screws inserted from the back of the cleat.
Align the cleat to the deck, centred to the stair opening. Screw the cleat to the deck facing with exterior corrosion resistant screws. Position the centre stringer so it is square and plumb to the deck.
Attach the side stringers to the sides of the cleat with exterior corrosion resistant screws. Place a level across the first tread to ensure that the stringers are level. Place the level on the sides of each stringer to ensure that the stringers are plumb. Position a square between the side of the deck and the stringers to ensure that the stringers are square to the deck.
Place deck boards cut to length on the bottom tread on the stringer with two exterior corrosion resistant screws on each stringer. Install all of the treads for each step.
Install a handrail and spindles according to local building codes if there are more than three steps.
A 16 foot 2-by-12 can only create a stringer to hold 14 steps. Use a landing between stringers to make the staircase work when taller stairs are needed.