Building the stairs you need for a new deck or old porch can be accomplished following the procedure used by carpenters and home builders everywhere. With a little planning and some basic geometry, you can build outdoor wooden stairs in an afternoon that will provide lasting service for years to come.
Extend a straight board horizontally from the top surface of the deck or porch platform and over the surface where the base of the steps will land. Use a level positioned on top of the board to make certain the extended board is horizontally level. Take the vertical measurement with a measuring tape. This measurement is referred to as the stair's "rise."
Divide the rise determined in Step 1 by 7 inches, which is the standard average for a comfortable step height. It is unlikely the rise will divide equally, so round the result up or down to the nearest whole number. Divide that number into the rise measurement. For example, a rise of 45 inches divided by 7 equals 6.4 steps. Round 6.4 down to 6, the nearest whole number, then divide the rise (45 inches) by 6 . The result is 6 step risers at 7 1/2 inches each.
Determine the tread depth desired for the steps. For this procedure, the treads will be two two-by-six boards, side by side, with a 1/2-inch space between the two boards and a 1/2-inch nosing overhang, for a total tread depth of 12 inches. Treads also can be a single board but are typically between 11 and 12 inches deep.
Rest a length of 2-inch by 12-inch pressure-treated lumber on sawhorses to lay out the first stair stringer, which will support the risers and treads.
Mark the first tread riser notch by laying a framing square flat on the stringer board face. Align the riser dimension on the short arm of the framing square and the tread depth dimension on the longer arm of the square, aligned with the edge of the stringer board. Scribe a pencil line along both edges of the square to mark the notch to be cut out of the stringer.
Repeat Step 2 for the number of treads and risers determined in Section 1. Align each tread line with the previous riser line marked.
Measure the bottom riser line to the riser dimension, minus the thickness of your tread material. In this case, it's 1 1/2 inches for two-by-six treads. Extend a line from this measurement square to the riser line and parallel to the first tread cut line marked in Step 2. This marks the base cut for the stair stringer.
Lay out the top cut of the stringer. Measure the top tread line from the outside edge of the stringer board to the tread depth measurement, minus the thickness of the riser boards. In this case, the riser boards are 1 inch by 8 inches, so 3/4 inch is subtracted from the tread depth. Square this line down from the top tread line and parallel to the previous riser line.
Use a circular saw to cut the tread and riser lines. Cut only to the inside corner of the notches, then finish the cuts with a handsaw or jigsaw. Do not cut beyond these lines.
Use the completed cut stringer as a template to mark the other stringers needed for the stairway. Generally, there are a minimum of three stringers for steps 36 inches wide, and four or more -- spaced a maximum of 18 inches apart -- for wider stairs.
Measure down vertically from the deck or porch surface to a point that equals the riser height used for the stringer layout, plus the thickness of the tread material, and make a pencil mark. Use a level to extend this line horizontally across the width of the stairs.
Position one of the outside stringers with the top cut against the rim joist of the deck or porch, and the base cut resting flat on the landing surface. Hold the top surface of the top tread cut at the horizontal line marked in Step 1. Attach the stringer to the rim joist with a 1 1/2-inch decking screw to hold it in place. Use a framing square to verify the stringer is square and perpendicular to the line of the deck or porch. Use a level to make certain the stringer is plumb.
Repeat Step 1 for the centre stringer and the outside stringer on the opposite side of the stairs. Verify that the top surface of the top tread to the deck or porch surface equals the riser measurement, plus the thickness of the tread material. This ensures the top riser is correct when the treads are installed.
Cut two-by-six blocks to fit tightly between the stringers and against the face of the rim joist. Attach these blocks to the deck or porch rim joist with 2 1/2-inch plated decking screws, then attach the tops of the stringer boards to the blocks. Use a builder's level across each of the tread surfaces to ensure the steps are level with each other.
Cut a length of riser material to the width of the steps. Attach it to the first riser face of the stringers, using at least two vertically spaced 1 1/2-inch decking screws at each stringer.
Measure the width of the stairs. Cut two lengths of two-by-six tread material to this dimension, plus 1 inch. The added inch allows for a 1/2-inch overhang on each side to match the 1/2-inch overhang of the tread nosing that was included in the tread depth calculation. Attach the tread material at the first tread position, using at least two 2 1/2-inch decking screws in line with each stringer on each board. Keep the tread's back edge tight to the riser board you installed in Step 5 and centred on the stringers to overhang the outside edges. Maintain a 1/2-inch space between the two boards.
Repeat Steps 5 and 6 for each of the treads and risers to the top of the stairs to complete the assembly.
For outdoor stairs, a wood that is less vulnerable to the elements, such as cedar or redwood, is recommended, even if the stairs are protected with paint or stain. For stability and safety, use or create a concrete, stone or similar level, permanent surface to support the base of the completed stair assembly. Framing square guides are small brass or steel clips that attach to the square to make marking repetitive cut lines easier.