Stairs are the most vulnerable component of a porch. Exposed to the elements (intense sunlight, rain, snow, frost heave), and often limited by poor choice of materials, bad craftsmanship, and lack of maintenance, it’s no wonder they usually fail long before the rest of the porch. But new stairs—whether simply utilitarian or attractively dressed with trim—can be constructed to last many decades. The key is to use quality materials and a design that accounts for weathering.
First, purchase and assemble your materials. For longevity and reliability, construct the 2x12 stringers and 2x4 frame with double kiln-dried, pressure-treated lumber rated for 40 PCF ground contact. (Visit a top-notch lumber supplier, not a home store, to find these materials.) For the treads, go with 5/4 vertical-grain Douglas fir, and clear cedar or a cellular PVC product like AZEK for the trim, including the risers and cove mold. You’ll also need hot-dipped galvanized nails and a tube of construction adhesive. For tools, you’ll need a circular saw, handsaw, framing square, tape measure, and miter saw. A waist-level worktable will make the project easier on your back.
Measure the rise, run, and width of the existing stairs. Appropriate stringer layout must take into account the thickness of tread stock and porch decking. The run for porch steps is usually 11", which is the width of fir tread stock available at most lumberyards; a 9" run is code minimum. A 1" lip overhanging the riser is typical, but if you’re using a cove molding and need more overhang, you can go as far as 1¼" (the limit mandated by building codes). Treads can overhang the end stringers as much as you prefer, but matching the lip overhang provides nice balance. Double-check your measurements—it’s very important that the rise-and-run is correct, both to prevent a tripping hazard and to meet code requirements.
Now it’s time to cut stringers and construct the frame. The number of stringers needed depends on the stair width and thickness of the treads—for 48"-wide 5/4 Douglas fir treads, you’ll need 24" centers, or three stringers. (Note: If using 3/4 treads—not recommended for durable stairs—you’ll need four stringers on 16" centers for the same span.) Lay out the stringer rise and run cuts using the guide on your framing square, allowing for a 1⁄8" to ¼" pitch to the front for water drainage. If cutting with a circular saw, finish the cut with a handsaw so you don’t over-cut your guide lines. Cut the 2x4 frame parts precisely with a miter saw.
Assemble the frame with hot-dipped galvanized nails and construction adhesive, making sure the unit remains square with each operation. Construct a triangle under each stringer to provide support, and use 2x4s to hold the stringers together. Add diagonal 2x4s between the stringers to keep the frame rigid from side to side. The final frame assembly should include any “feet” or shims necessary to create level placement on an often unlevel pad or surface. To provide ventilation between the stringers and trim, apply ¼" pressure-treated shims to the stringer adjacent to each riser. </td>
With the frame ready, fasten the treads, risers, and side trim. If you’re using wood trim, back-prime all surfaces, including cut joints, before you attach it. (You can skip this step if using AZEK or pre-primed finger-jointed cedar trim.) For the most weather-resistant finish, paint the new stairs with two coats of floor and deck enamel, using a thinned coat of the paint for primer. (Clear finishes and stains are difficult to maintain.) For safety in areas prone to icing, add non-skid aggregate to the last coat of paint applied to the treads.