How to plan a porch stair.
A porch stair is an outdoor entry stair — a few open-stringer steps leading from grade or path to a deck, terrace, garden door, mobile platform or container. Compact, easy to build, and (with the side-mount variant) self-standing so it works against a trailer or a removable deck without an anchor in the upper floor.
When is a porch stair the right choice?
Use it whenever you need a short outdoor flight (typically 2–8 risers, total rise ≈ 300–1500 mm) — terrace and patio stairs, deck access, garden steps, technical platforms, container ramps, market-stand stairs. The calculator outputs an open-stringer (saw-tooth) carcass with optional risers, optional railing, and full multi-stringer support for wide spans. For indoor primary stairs use the Straight or L-shaped calculator instead — those generate closed-stringer joinery with proper dado pockets.
Four numbers to get right before you cut wood
- Slope 25–35°. Outdoor stairs need to feel safe in wet conditions. Indoor codes allow up to 38°; outdoor comfort starts dropping above 35°. The calculator accepts up to 45° for tight spaces.
- Rise per step ≈ 150–180 mm. Slightly lower than indoor — a porch user is often carrying something (groceries, plants, kids). Above 200 mm becomes a chore.
- Tread depth ≥ 280 mm. Deeper than indoor for two reasons: wet shoes need more landing, and a multi-plank tread with a drainage gap eats into the effective surface. The default 280 mm gives a generous foot.
- Stringer span ≤ 800 mm. Tread planks sag if the span between stringers gets too wide. Add an intermediate stringer at every ~80 cm of width — a 120 cm porch uses 2 stringers, a 190 cm porch needs 3, a 400 cm deck stair needs 6.
One stringer type — open (saw-tooth)
Porch stairs always use open stringers: the saw-tooth profile is cut directly into the top edge of each beam and the treads sit on top, exposed from the side. There's no dado pocket to trap rainwater or decaying leaves. The carcass dries fast, runs air underneath, and is much cheaper to mill than a closed-stringer with dadoes.
Floor mount vs. side mount
Floor mount is the classic setup: the top newel post stands on the upper floor / deck past the stair end, anchored to the slab. Foot post stands on the lower ground.
Side mount bolts both newel posts to the BROAD SIDE of the outer stringers — the top post extends all the way to the ground (no slab anchor needed). The stair becomes self-standing and can be placed against a trailer, container, market-stand, or a removable platform that has no upper floor to anchor into. Pair side-mount with a 4-leg geometry and you have a free-standing entry step that travels.
How to read the live preview
The 3D view shows the porch in context with all stringers, treads, optional risers and the configured railing variant. The 2D Top view is what you hand the carpenter or send to the CNC. The X-Ray view reveals the stringer profile so you can verify the saw-tooth cuts before milling. All three update live from the same geometry model.
What you'll get when you export
Five formats, all derived from the same geometry as the preview above:
- PDF · multi-sheet A3 construction drawing. Cover with project summary, plan view, side elevation, scaled workshop drawings for every stringer + every tread (+ riser / handrail / baluster pages when those are enabled), and a bill of materials. Print at A3 for the workshop; do not use as a 1:1 cutting template.
- DXF · 1:1 CAD/CNC zip. One file per category (plan, elevation, each part) at true 1:1 scale in the chosen unit. Named layers separate stringers, treads, risers, posts, handrail. Loads straight into AutoCAD, BricsCAD, LibreCAD, and any CNC post-processor.
- SVG · 1:1 vector zip. Same drawings as the DXF, also at true 1:1 scale, in plain SVG — for browsers, Illustrator, Inkscape, Affinity, or SVG-input laser cutters.
- STL · 3D printable mesh. A single solid model for slicing and 3D printing a tabletop reference, or for importing into Blender / Twinmotion / Lumion for renders.
- STEP · parametric CAD exchange. Each part as a separate B-Rep body. Opens natively in SolidWorks, Inventor, Fusion 360, FreeCAD, SolidEdge.
What the files do not include
The downloads are a construction foundation, not a turnkey outdoor build. Specifically:
- Foot anchoring is not modelled. Outdoor stairs need a proper foundation — typically a concrete pad with post brackets (H- or U-shaped post bracket), a poured pier footing below the local frost line, or adjustable steel deck feet on a gravel pad. The PDF gives you the stringer foot position; the anchor hardware and footing are supplied on site.
- Top connection is not modelled. Floor-mount: bolt the top newel into the deck joist or sill plate with through-bolts and washers. Side-mount: lag the top post into the stringer with M10 carriage bolts (two minimum, one near the foot, one near the rail entry). The drawings show the geometry — the hardware list is your call.
- Weather treatment is not modelled. Use a wood species in durability class 1–3 (oak, larch, robinia, pressure-treated pine, or WPC). Seal end-grain, route a drip kerf under each tread nosing, and brush down after rain. Without weather treatment any outdoor stair rots in 3–5 years.
Always verify every dimension on site before cutting wood, and check your local building code for handrail / fall-protection requirements (in Germany, state building codes such as BauO NRW §35 typically require a handrail above 3 risers or 1 m drop height).