Measure how wide each step tread will be and using the four-foot square and the tape measure, work out how tall the stairway will be (the "rise") from the ground to the horizontal level of the porch or deck they join.
Then, calculate how many step treads are needed. Divide the rise by the number of the desired step treads to get the height of each vertical support, or "risers," for the treads. Purchase enough pressure-treated wood to complete the project.
Building codes may specify a maximum height of 10 inches for each riser. Add more step treads to shorten the risers to between seven and eight inches. Each step will also be about 12 inches deep on average.
Step 2 - Prepare the Stairway Supports
These supports, called "stringers," are notched beams to which you attach the step treads and risers. The stringers also connect the stairway to the porch. Set the height of the riser from step one onto the measuring square, and draw the correct number of right-angled step treads and risers onto a 2x12-inch pressure-treated board. Cut out these angles on the miter saw, staying within the drawn lines.
Make one stringer for every 18 inches of stair width. If your steps are 36 inches wide, you will need three stringers, two for the edges and one down the middle of the steps. Cut the end of the first stringer to the correct length with a handsaw, and then make the rest identical to the first. Trim the edges of the stringer notches to match the width of the step treads and thickness of the risers.
Step 3 - Construct a Landing
It doesn't matter what materials you choose for your landing; all that matters is that you have one. Soil shifts and goes through many changes throughout the year due to weather and temperatures. This doesn't provide good support for your wood stairs on the bottom, so you need a solid base for them to sit on instead.
Use your cut stringer as a guideline for how large you want the landing area to be. It should extend inside, under the stairs, as well as a few feet forward. Mark your desired dimension with stakes in the ground and a string level.
Gravel can give you a very simple landing style, or you can choose a more polished look with pavers or poured concrete. Whichever you use, always make sure the surface is level and that it has proper drainage so the landing doesn't shift too much or crack.
Step 4 - Cut the Treads and Risers
Based on your measurements, cut the treads and risers from the pressure-treated wood.
Step 5 - Affix the Stringers to the Porch Frame
Drill holes for the L-shaped brackets, and attach them at the farthest left and right end of the stairway, longest side vertical to the porch frame, with three-inch deck screws. Affix the middle stringer to the deck, from the deck outward with two three-inch deck screws, drilled into the back of the stringer.
Step 6 - Attach the Risers
Using three-inch screws, fit the risers onto the stringers vertically. The step treads will sit on top of these supports horizontally.
Step 7 - Add the Step Treads
Screw the step treads down with one screw into the stringer and one into the riser for each stringer support. Walk on the stairs after you're finished to test for firmness.