Stair Stringer Layout: Complete Code Reference
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Stair Stringer Layout: Complete Code Reference
Stairs are the one part of a framing job where the building inspector has zero sense of humor. The IRC requirements for residential stairs are specific, measurable, and enforced — and for good reason. An inconsistent riser height is a trip hazard that someone uses every day. Get one step wrong by 3/8" and you've built a lawsuit waiting to happen.
This reference gives you every code requirement, calculation, and table you need to lay out stair stringers right the first time. I keep this bookmarked on my phone for every deck and porch job. The math isn't hard, but it needs to be exact.
IRC Stair Code Requirements (2021)
These are the numbers you cannot fudge. They apply to residential stairs per the International Residential Code. Your local code may be stricter — always check — but it won't be more lenient.
Dimensional Requirements
| Dimension | Requirement | Code Section |
|---|---|---|
| Maximum riser height | 7-3/4" | R311.7.5.1 |
| Minimum riser height | 4" | R311.7.5.1 |
| Minimum tread depth | 10" | R311.7.5.2 |
| Minimum tread nosing | 3/4" | R311.7.5.3 |
| Maximum tread nosing | 1-1/4" | R311.7.5.3 |
| Riser variation (max) | 3/8" | R311.7.5.1 |
| Tread variation (max) | 3/8" | R311.7.5.2 |
| Minimum width | 36" | R311.7.1 |
| Minimum headroom | 6'-8" | R311.7.2 |
The 3/8" Rule
Per IRC R311.7.5.1, the largest riser cannot exceed the smallest riser by more than 3/8 inch. Same for treads. This is the most commonly cited violation during inspections.
What this means practically: All your rises must be within 3/8" of each other. If your target rise is 7-1/4", every riser must be between 7-1/16" and 7-7/16".
Rise and Run Quick Reference
Ideal Combinations (Rise + Run = 17" to 18")
| Unit Rise | Unit Run | Comfort Total | Angle |
|---|---|---|---|
| 6-1/2" | 11" | 17.5" | 30.6° |
| 6-3/4" | 10-3/4" | 17.5" | 32.1° |
| 7" | 10-1/2" | 17.5" | 33.7° |
| 7" | 11" | 18" | 32.5° |
| 7-1/4" | 10-1/4" | 17.5" | 35.3° |
| 7-1/4" | 10-3/4" | 18" | 34.0° |
| 7-1/2" | 10" | 17.5" | 36.9° |
| 7-3/4" | 10" | 17.75" | 37.8° |
The comfort formula: 2×Rise + Run should equal 24" to 25" for optimal comfort.
Why Comfort Matters
- Rise too steep + run too short = awkward climbing, dangerous descent
- Rise too shallow + run too long = shuffling gait, wasted space
- Consistent rhythm = safer stairs, natural stride
Calculating Number of Risers
This is where people overthink it. The process is simple — divide, round, recalculate. The key is measuring your total rise accurately, because every riser is calculated from that one number.
The Process
- Measure total rise (floor to floor or landing to landing)
- Divide by target rise (start with 7")
- Round to nearest whole number
- Recalculate exact rise
- Verify against code maximums
Common Total Rise Solutions
| Total Rise | # Risers | Unit Rise | Notes |
|---|---|---|---|
| 24" | 4 | 6" | Low step-up |
| 28" | 4 | 7" | Standard 2-step |
| 32" | 5 | 6.4" | Comfortable |
| 35" | 5 | 7" | Good balance |
| 36" | 5 | 7.2" | Typical deck |
| 40" | 6 | 6.67" | Easy climb |
| 42" | 6 | 7" | Very common |
| 48" | 7 | 6.86" | Good for porches |
| 49" | 7 | 7" | Typical porch |
| 56" | 8 | 7" | High deck |
| 63" | 9 | 7" | Tall deck |
| 84" | 12 | 7" | Interior floor |
| 96" | 14 | 6.86" | 8' ceiling |
| 108" | 15 | 7.2" | 9' ceiling (use 16 @ 6.75") |
Formula for Any Total Rise
Number of risers = Total Rise ÷ Target Rise (round to nearest whole) Exact rise = Total Rise ÷ Number of Risers
Example:
- Total rise: 47 inches
- 47 ÷ 7 = 6.71 → round to 7 risers
- 47 ÷ 7 = 6.714" = 6-11/16" per riser ✓ (under 7-3/4" max)
Stringer Sizing Requirements
Minimum Remaining Wood After Cuts
Per engineering practice, you need at least 3-1/2" of solid wood remaining at the narrowest point (the throat) after cutting the stringer notches.
Stringer Size by Rise
| Unit Rise | 2×10 | 2×12 | Notes |
|---|---|---|---|
| 6" | 3.25" throat | 5.25" throat | Both OK |
| 6-1/2" | 2.75" throat | 4.75" throat | 2×12 preferred |
| 7" | 2.25" throat | 4.25" throat | 2×12 required |
| 7-1/4" | 2" throat | 4" throat | 2×12 required |
| 7-1/2" | 1.75" throat | 3.75" throat | 2×12 required |
| 7-3/4" | 1.5" throat | 3.5" throat | 2×12 minimum |
Throat calculation: Board depth − (rise² + run²)^0.5 × (rise/run)
Rule of thumb: Always use 2×12 for stair stringers. The small cost difference isn't worth the structural compromise.
Number of Stringers Required
| Stair Width | Minimum Stringers | Recommended |
|---|---|---|
| Up to 30" | 2 | 3 |
| 30" to 36" | 3 | 3 |
| 36" to 48" | 3 | 4 |
| Over 48" | 4+ | Per engineer |
Why more is better: Three stringers on a standard 36" stair eliminates bounce and provides redundancy if one cracks.
Layout Step-by-Step
This is the hands-on part. If you've ever watched an experienced carpenter lay out a stringer, it looks effortless — square walks down the board, pencil follows, done in three minutes. Getting to that point takes practice, but the process itself is mechanical.
Step 1: Set Stair Gauges
On a framing square:
- Rise on the tongue (narrow arm)
- Run on the blade (wide arm)
Example: 7" on tongue, 10-1/2" on blade
Step 2: Mark First Rise and Run
- Position square at one end of 2×12
- Gauges against the top edge of board
- Mark along both the tongue (rise) and blade (run)
- This creates one "step" in your layout
Step 3: Step Off Remaining Layout
- Slide square so previous run line meets new rise position
- Mark next rise and run
- Repeat for total number of risers
- Keep gauges tight against edge
Step 4: Mark Top Cut
The top cut connects the stringer to the deck or floor:
For ledger attachment:
- Extend final rise line to top
- Mark horizontal cut equal to ledger height
For flush attachment:
- Mark plumb cut only
- Will use metal hanger
Step 5: Drop the Stringer (Bottom Cut)
Critical step: The first riser must equal all other risers AFTER tread installation.
Drop amount = Tread thickness
| Tread Material | Thickness | Drop |
|---|---|---|
| 2× lumber | 1-1/2" | 1-1/2" |
| 5/4 decking | 1" | 1" |
| 2× with 5/4 nosing | 1" | 1" |
| Composite | Per product | Per product |
Process:
- At bottom run line, measure tread thickness DOWN
- Draw new parallel line
- This becomes the actual bottom cut
Step 6: Mark Both Sides
Why this matters: Saw blade follows visible line. Misaligned marks create angled cuts that won't seat properly.
With traditional square: Flip board, realign marks, transfer each point With Rapid Rafter: Marks both faces and edge in one motion—guaranteed alignment
Cutting the Stringer
Recommended Tools
- Circular saw for primary cuts (stop before inside corners)
- Jigsaw or reciprocating saw for corner finish
- Handsaw for precision cleanup
The 1/4" Rule
Never overcut corners. Circular saw blade extends beyond the visible cut line on the opposite face. Overcuts weaken the stringer and are visible.
Process:
- Cut rise lines from outside, stop 1/4" before corner
- Cut run lines from outside, stop 1/4" before corner
- Finish corners with jigsaw or handsaw
Test Fit Before Production
After cutting first stringer:
- Set in position
- Check level across multiple treads
- Measure each rise with tape
- Verify rise + run at each step
- Confirm top and bottom connections
Only after verification: Use first stringer as pattern for remaining stringers.
Mark Stringers Faster — The Rapid Rafter marks both faces at once, ensuring your cuts align perfectly through the board. Get yours →
Connection Details
Top Connection Options
| Method | Hardware | When to Use |
|---|---|---|
| Ledger | 2× ledger + screws | Most common deck attachment |
| Stringer hanger | Simpson LSC/LSCZ | Code-required in many areas |
| Direct mount | Bolts through rim | Commercial applications |
| Post support | 4×4 posts at sides | Freestanding stairs |
Bottom Connection Requirements
Stringers need positive connection to landing to prevent kick-out.
| Landing Type | Connection | Hardware |
|---|---|---|
| Concrete | Angle bracket | Simpson A34, L50 |
| Concrete | Direct bolt | 1/2" concrete anchor |
| Wood kicker | Toenail + clip | Simpson A34 + 16d nails |
| Gravel (temporary) | Stakes | Not code-compliant |
Common Calculation Mistakes
I've seen every one of these on real jobs. The tread drop and the total rise measurement are the two that catch people most often.
| Mistake | Result | Prevention |
|---|---|---|
| Forgetting tread drop | First riser too tall | Always drop by tread thickness |
| Measuring total rise wrong | All risers wrong | Measure twice, plumb bob best |
| Rounding incorrectly | Risers vary too much | Recalculate, verify 3/8" rule |
| Running short treads | Last tread too shallow | Count treads = risers − 1 |
| Overcut corners | Cracked stringers | Stop 1/4" short, finish by hand |
Quick Formulas
Stringer length (approximate): Stringer Length = √(Total Rise² + Total Run²) + 12"
Total run: Total Run = (Number of Risers − 1) × Unit Run
Number of treads: Treads = Risers − 1 (deck surface is final tread)
Angle: Angle = arctan(Unit Rise ÷ Unit Run)
Material Checklist
| Item | Specification | Quantity Formula |
|---|---|---|
| Stringers | 2×12 PT | Width ÷ 16", minimum 2 |
| Treads | 2×12 or 5/4×12 PT | Risers − 1 |
| Ledger | 2×8 or 2×10 PT | Stair width |
| Stringer hangers | Simpson LSC/LSCZ | 1 per stringer |
| Angle brackets | Simpson A34 | 2 per stringer at base |
| Tread screws | #10 × 3" deck | 4 per tread per stringer |