Stair design looks simple—until your permit reviewer starts asking about width, landings, risers, slope, and fire protection. Here’s a concise breakdown of what the 2025 California Residential Code (CRC) and California Building Code (CBC) require for single- and two-family dwellings (R-3, Type VB).
🔹 Minimum Stair Width
- 36 inches minimum clear width
- Measured above the handrail and below headroom
- Applies to all interior and exterior stairways serving a dwelling
Quick tip: Don’t dimension to finish—dimension to clear width.
🔹 Maximum Rise per Flight
- A single flight of stairs may rise no more than 12 feet without a landing.
- Landings required at top and bottom of each stairway.
- Interior garage door and exterior door exceptions exist, but only in narrow scenarios.
🔹 Handrails
- Required on at least one side when there are four or more risers.
- Can be Type I or Type II graspability profiles.
- Must be continuous with code-compliant returns (no open ends).
If you want a graspability post, we already have one—easy to link directly from here.
🔹 Stair & Landing Slope
- Max slope: 1:48 (2%) for interior walking surfaces.
- Landings that must drain water may slope up to 1:20 (5%).
- This applies to both treads and landings.
🔹 Guarding & Openings on Open Stairs
- Stair walking surfaces with openings cannot permit passage of a ½-inch spherical object.
- Applies to open treads, decorative panels, grating, etc.
This rule often catches modern “open tread” designs—check your gaps carefully.
🔹 Space Under the Stairs
Interior stairs:
- Usable enclosed space may be protected with ½-inch gypsum board on the enclosed side.
- Applies only when serving a single R-3 dwelling unit.
Exterior stairs:
- Cannot have enclosed usable space below unless the space is constructed as 1-hour fire-resistant.
- Open space below exterior exit stairs must remain unused.
🔹 Exterior Stairway Fire Separation
For R-3 dwellings, exterior exit stairways must be at least:
- 5 feet from property lines or adjacent structures (exception for R-3).
Other occupancies must maintain 10 feet, so this is a useful residential concession.
🔹 Structural Attachment
Exterior stairs, decks, landings, and balconies must:
- Resist vertical and lateral forces,
- Not rely solely on nails, and
- Use connectors, tension ties, hangers, or engineered hardware.
Always specify mechanical connectors—inspectors will flag “nail-only” details immediately.
🔹 Key Measurements Recap
- Width: 36″ min
- Vertical rise per flight: ≤ 12′
- Slope: 1:48 max (1:20 allowed only where draining)
- Handrails: Required with ≥ 4 risers
- Under-stair protection: ½″ gypsum (interior); 1-hour (exterior if enclosed)
- Fire separation (exterior stairs): 5′ min for R-3
🧭 Bottom Line
Stair requirements in the 2025 CRC/CBC are straightforward once you know the key constraints:
- Keep widths compliant,
- Control slope and rise,
- Use proper handrails,
- Maintain fire separation for exterior stairs,
- Protect under-stair areas appropriately.
