Mathematical Derivation & Field Specifications: 12-Step Basement (90" Rise)
For a finished floor-to-floor vertical elevation of 90" (7.50 ft / 2286 mm) using a Standard Sub-Riser Drop Ledger Mount and 10" tread depth, dividing the total rise by the ergonomic target riser height of 7.35" yields 12.24 raw steps. Locking the riser integer to 12 vertical risers establishes an exact unit rise of 7.500" (7-1/2" / 191 mm) per step.
Because this layout specifies a drop top connection with 1" finished tread stock, the stringer carriage requires 11 horizontal tread cuts spanning a total horizontal floor projection (Total Run) of 110.0" (9.17 ft) at a stair pitch angle of 36.9°. The bottom plumb cut must be shortened (dropped) by the 1" tread thickness so the finished first step finishes at exactly 7-1/2" above the lower landing: bottom notch rise = 6-1/2" (6.500"). To maintain the mandatory 80" minimum vertical headroom clearance through a 10.5" upper floor assembly, frame the stairwell rough opening to at least 121" (10.08 ft) and purchase 13 ft 2×12 Douglas Fir or Southern Yellow Pine stringer stock (true hypotenuse length: 154.1" across a 36" stair width).
Riser Count Sensitivity Matrix for 90" Total Vertical Rise
Comparing adjacent riser counts around the optimal 12-riser configuration demonstrates why adding or removing steps shifts the unit rise toward the 7.75" building code ceiling or expands floor run footprint:
| Riser Count | Unit Rise (Frac / Dec) | Metric Rise | Total Run (10" Tread) | Slope Pitch | Code Verdict |
|---|---|---|---|---|---|
| 10 Risers | 9" (9.000") | 229 mm | 90.0" (7.50 ft) | 42.0° | EXCEEDS 7.75" MAX |
| 11 Risers | 8-3/16" (8.182") | 208 mm | 100.0" (8.33 ft) | 39.3° | EXCEEDS 7.75" MAX |
| 12 Risers (Selected) | 7-1/2" (7.500") | 191 mm | 110.0" (9.17 ft) | 36.9° | IRC R311.7 PASS |
| 13 Risers | 6-15/16" (6.923") | 176 mm | 120.0" (10.00 ft) | 34.7° | IRC R311.7 PASS |
| 14 Risers | 6-7/16" (6.429") | 163 mm | 130.0" (10.83 ft) | 32.7° | IRC R311.7 PASS |
Cumulative Framing Square & Tape Measure Layout Schedule (12 Steps)
To prevent cumulative rounding drift of up to 0.75" across 12 steps, hook your tape measure at the bottom stringer heel and mark the cumulative hypotenuse and elevation points below:
| Step # | Notch Rise Cut | Cumulative Elevation | Metric Elev | Cumulative Run | Stringer Edge Hypotenuse |
|---|---|---|---|---|---|
| Step #1 (Bottom Drop) | 6-1/2" | 7-1/2" (7.50") | 191 mm | 10.00" | 12.50" |
| Step #2 | 7-1/2" | 15" (15.00") | 381 mm | 20.00" | 25.00" |
| Step #3 | 7-1/2" | 22-1/2" (22.50") | 572 mm | 30.00" | 37.50" |
| Step #4 | 7-1/2" | 30" (30.00") | 762 mm | 40.00" | 50.00" |
| Step #5 | 7-1/2" | 37-1/2" (37.50") | 953 mm | 50.00" | 62.50" |
| Step #6 | 7-1/2" | 45" (45.00") | 1143 mm | 60.00" | 75.00" |
| Step #7 | 7-1/2" | 52-1/2" (52.50") | 1334 mm | 70.00" | 87.50" |
| Step #8 | 7-1/2" | 60" (60.00") | 1524 mm | 80.00" | 100.00" |
| Step #9 | 7-1/2" | 67-1/2" (67.50") | 1715 mm | 90.00" | 112.50" |
| Step #10 | 7-1/2" | 75" (75.00") | 1905 mm | 100.00" | 125.00" |
| Step #11 | 7-1/2" | 82-1/2" (82.50") | 2096 mm | 110.00" | 137.50" |
| Step #12 | 7-1/2" | 90" (90.00") | 2286 mm | 110.00" | 150.00" |
Official Building Code & Industry Standard Citations
- ICC International Residential Code (IRC R311.7) Governs maximum 7.75" riser height, minimum 10" tread depth, 3/8" riser variance, and 80" minimum stair headroom. Reference: ICC Digital Codes (codes.iccsafe.org).
- NFPA 70 / National Electrical Code (NEC Article 690) Governs photovoltaic maximum system voltage temperature correction (690.7) and 156% continuous OCPD sizing (690.8). Reference: National Fire Protection Association (nfpa.org).
- TCNA Handbook & ANSI A108.02 Tile Standards Establishes symmetrical perimeter cut layout rules, 33% max offset for 24"+ rectified porcelain, and mortar coverage. Reference: Tile Council of North America (tcnatile.com).
- APA Engineered Wood & AWC NDS Span Standards Governs structural panel thickness, kerf tolerance, and Southern Pine / Douglas Fir stringer throat bending capacity. Reference: APA Engineered Wood Association (apawood.org).
Material Buying Guide, Where to Buy, Warranty Protection & Installation Protocol for 12-Step Basement (90" Rise)
1. Should You Buy Pre-Cut Stringers or Raw 2×12 Lumber for a 90" (12-Step) Staircase?
For a 90-inch total vertical rise (12 risers at 7-1/2" each), you should never buy off-the-shelf pre-cut 12-step stringers from a big-box store. Retail pre-cut stringers are milled to a fixed 7.50" × 10.00" step without a bottom heel drop for your 1" tread stock—which would throw your finished 90" flight out of level and fail the IRC R311.7.5.1 3/8-inch riser variance inspection. Instead, purchase 4 boards of 13-foot #1 Prime KDAT (Kiln-Dried After Treatment) 2×12 Southern Yellow Pine or Select Structural Douglas Fir (true hypotenuse length: 154.1" across your 36" stair width).
2. Where to Buy, Exact Hardware SKUs & Manufacturer Warranty Protection
Source your 13 ft 2×12 stringer blanks from a local contractor lumberyard rather than picking through wet #2 grade boards at a retail center. Check the butt-end plastic tag to confirm UC4A/UC4B Ground Contact retention. To preserve the lumber treater's Lifetime Limited Rot Warranty (AWPA M4), you must brush two coats of 2% copper naphthenate end-cut preservative onto all 11 saw-cut notches before fastening treads. Anchor the top of all 4 stringers using Simpson Strong-Tie LSCZ (ZMAX G185) Adjustable Stringer Connectors fastened exclusively with hot-dip galvanized or `#9 × 1-1/2" SD9112` structural connector screws (using drywall or deck screws in structural hangers voids Simpson Strong-Tie's load rating and warranty).
3. Step-by-Step Jobsite Installation Checklist for This 12-Step Layout
- Step 1 (Crown & Layout): Sight down each 13 ft 2×12, place the crown (hump) facing UP, and lock brass stair gauges at 7-1/2" (rise) and 10" (run).
- Step 2 (Bottom Heel Drop): Shorten the bottom plumb riser cut by 1" so the bottom notch measures 6-1/2"; once the 1" tread is installed, Step #1 returns to 7-1/2".
- Step 3 (Apex Over-Cut Protection): Stop your 7-1/4" circular saw 1/4" before the inside corner of all 11 notches and finish the cut with a handsaw to preserve the full 5.25" structural throat spine.
- Step 4 (Ledger & Landing Bearing): Mount via sub-riser drop ledger framing and set the bottom heel on a pressure-treated sleeper plate or concrete pad at exactly 110.0" (9.17 ft) horizontal projection.