How to Build Stairs - Complete Deck & Interior Guide
Master stair construction with detailed building codes, rise and run calculations, cutting stringers, and professional framing techniques for deck and interior stairs.
Dr. Snezana Lawrence
Mathematical Historian
15+ years experience
PhD from Yale University. Published mathematical historian ensuring precision in all calculations.
Education
PhD in Mathematical History - Yale University
Bruce Zawalsky
Survival & DIY Expert
28+ years experience
28+ years wilderness survival and hands-on construction experience. Author of "Canadian Wilderness Survival."
Education
Canadian Armed Forces Wilderness Survival Training
Quick Answer
Building stairs requires precise calculations following building codes (7-7¾" rise, 10-11" run), cutting stringers accurately with a framing square, proper framing with minimum 3 stringers, and secure attachment at top and bottom. Mathematical precision is critical - small errors multiply across all steps, creating unsafe, uncomfortable stairs.
Project Overview
Difficulty Level
Advanced skill level required. Mathematical precision is critical - 1/8" errors multiply across all steps. Best for experienced DIYers comfortable with framing squares and calculations.
Time Required
Simple deck stairs (3-4 steps): Planning 2-3 hours, cutting stringers 2-3 hours, installation 3-4 hours, treads/risers 2-3 hours, handrails 3-4 hours.
Cost Estimate
Deck stairs (4 steps, 36" wide): 2×12 stringers $75-120, 2×6 treads $60-90, handrail kit $150-300, hardware $30-50, concrete footings $40-60.
Understanding Stair Terminology
Before you start building, you must understand these terms. Stair construction uses specific vocabulary that's critical for calculations and communication.
Total Rise
Vertical distance from bottom to top (floor to floor)
Total Run
Horizontal distance the stairs travel (calculated, not measured)
Unit Rise
Height of each individual step (must be 7"-7¾" by code)
Unit Run (Tread Depth)
Horizontal depth of each step (minimum 10")
Stringer
Diagonal support beam cut in saw-tooth pattern (typically 2×12)
Tread
Horizontal step surface you walk on
Riser
Vertical board between steps (optional on exterior)
Nosing
Front edge overhang of tread (typically ¾"-1¼")
Headroom
Vertical clearance above stairs (minimum 6'8" by code)
Building Codes & Regulations (IRC)
Critical: Always check local building codes! Requirements vary by jurisdiction. Many areas require permits and inspections for stairs. Failing to meet code can result in forced demolition and liability issues.
| Requirement | Specification | Notes |
|---|---|---|
| Rise Requirements | Maximum 7¾", minimum 4" | Max ⅜" between any two steps |
| Run Requirements | Minimum 10" (11" recommended) | All runs must be equal |
| Headroom | Minimum 6'8" (80") | Measured perpendicular from nosing |
| Stair Width | Minimum 36" clear width | Handrails can project 4½" each side |
| Handrails | Required if >4 risers, 34-38" height | Must be graspable (1¼"-2" diameter) |
| Guards/Railings | Required if drop >30" | Max 4" gaps between balusters |
Rise & Run Calculations
This is the most critical part of stair building. Get the math right and everything else follows. Get it wrong and you'll have unsafe, uncomfortable stairs.
Step-by-Step Calculation
- 1Measure total rise: Floor to floor height (finished surfaces)
- 2Calculate steps: Total rise ÷ 7" = number of steps (round to whole number)
- 3Exact unit rise: Total rise ÷ number of steps (must be 7"-7¾")
- 4Choose unit run: 10"-11" (11" is more comfortable)
- 5Total run: Unit run × (steps - 1) = horizontal distance
- 6Verify comfort: (2 × Rise) + Run = 24"-25"
Example Calculation
Given: Deck is 35" above ground
1. Total rise: 35"
2. Number of steps: 35 ÷ 7 = 5 steps
3. Exact unit rise: 35 ÷ 5 = 7" ✓
4. Unit run: 11" (chosen)
5. Total run: 11 × 4 = 44"
6. Comfort check: (2 × 7) + 11 = 25" ✓
Result: 5 steps, each 7" rise × 11" run
💡 Pro Tip: Use our Stair Calculator to automatically calculate rise, run, and stringer dimensions for your project.
Tools & Materials
Tools
Essential Tools
- •Framing square with stair gauges (essential!)
- •Circular saw
- •Jigsaw or handsaw
- •Level (4' and torpedo)
- •Measuring tape (25')
- •Speed square
- •Pencil
- •Clamps
- •Drill/driver and impact driver
Specialty Tools
- •Stair gauges (clips for framing square)
- •Construction calculator with stair function
- •String line
- •Post level
Materials
| Material | Use |
|---|---|
| 2×12 pressure-treated | Stringers Minimum 3 for 36" wide stairs |
| 2×6 or 5/4×6 decking | Treads Per tread width needed |
| 4×4 posts | Handrail posts 2 minimum (top and bottom) |
| Handrail kit | Rail and balusters 1 per side |
| Joist hangers | Top attachment 1 per stringer |
| Concrete mix | Footings 2-3 bags for pad |
| 3" deck screws | Treads to stringers 2 per tread-stringer connection |
| ½" × 4" carriage bolts | Stringer attachment 2 per stringer |
Step-by-Step Construction Guide
Measure Total Rise
The total rise is the vertical distance from the finished bottom surface to the finished top surface. This measurement is critical - all other calculations depend on it.
Key Tips:
- ✓Measure from finished floor to finished floor
- ✓Include deck board thickness at top
- ✓Include any flooring materials
- ✓Measure in multiple spots if ground slopes
- ✓Use the average for sloped ground
Measuring to framing instead of finished floor is the #1 mistake. Always measure to finished surfaces!
Calculate Number of Steps
Divide the total rise by the ideal step height (7") to get the number of steps. Round to the nearest whole number, then recalculate the exact rise per step.
Example: 35" total rise ÷ 7" = 5 steps. 35" ÷ 5 = 7" exact unit rise. Total run: 11" × 4 = 44"
Key Tips:
- ✓Total Rise ÷ 7" = approximate number of steps
- ✓Round to nearest whole number
- ✓Total Rise ÷ Number of Steps = exact unit rise
- ✓Unit rise must be between 7" and 7¾"
- ✓Use Rule of Thumb: (2 × Rise) + Run = 24-25" for comfort
All rises MUST be equal. Code allows only ⅜" maximum variation between any two steps.
Mark the Stringers
Using a framing square with stair gauges, mark the rise and run for each step on your 2×12 lumber.
Key Tips:
- ✓Attach stair gauges to framing square at rise and run measurements
- ✓Start at top of 2×12
- ✓Align both gauges on board edge
- ✓Mark rise (vertical) and run (horizontal) lines
- ✓Repeat for each step
- ✓Subtract tread thickness (1½") from first rise ONLY
The first rise adjustment is critical! If you don't subtract the tread thickness from the first rise, your first step will be too tall.
Cut the Stringers
Cut along the marked lines carefully. Use your first stringer as a template for the others.
Key Tips:
- ✓Cut with circular saw for most of each line
- ✓Stop just before corners (don't overcut)
- ✓Finish corners with jigsaw or handsaw
- ✓Overcutting weakens the stringer!
- ✓Remaining throat should be minimum 5"
- ✓Test fit first stringer before cutting others
- ✓Trace first stringer onto remaining boards
Overcutting corners significantly weakens stringers. Stop the circular saw at the corner and finish with a jigsaw.
Install the Stringers
Secure stringers at top to the deck/floor and at bottom to concrete footings or a pad.
Key Tips:
- ✓Space stringers maximum 18" apart
- ✓Outside stringers at edges, center stringer(s) equally spaced
- ✓Use joist hangers or stringer hangers at top
- ✓Pour concrete pad or set post anchors at bottom
- ✓Check level and plumb before final fastening
- ✓Use carriage bolts for secure top attachment
Minimum 3 stringers for 36" stairs. Using only 2 stringers results in bouncy, weak stairs.
Attach Treads and Risers
Install treads starting from the bottom step, working up. Add risers if required (interior stairs).
Key Tips:
- ✓Pre-drill to prevent splitting
- ✓Two 3" screws per stringer connection
- ✓Overhang tread ¾"-1¼" for nosing
- ✓Check level side-to-side on each tread
- ✓For double-board treads, leave ¼" gap for drainage
- ✓Risers optional on exterior, required on interior
Countersink screws slightly. Protruding screw heads are trip hazards and uncomfortable underfoot.
Add Handrails
Install posts and handrails according to code requirements. Rails are required if there are more than 4 risers.
Key Tips:
- ✓Posts must extend to ground or through deck framing
- ✓Bolt posts through stringers securely
- ✓Rail height: 34"-38" from tread nosing
- ✓Use string line to ensure consistent rail height
- ✓Balusters maximum 4" apart
- ✓Rail must be graspable (1¼"-2" diameter)
Handrails are required by code if more than 4 risers. Skipping this step means failing inspection.
Common Mistakes to Avoid
| Mistake | What Happens | How to Fix |
|---|---|---|
| First step different height | Trip hazard, code violation | Subtract tread thickness (1½") from first rise |
| Not using stair gauges | Inconsistent steps | Invest in stair gauges for your framing square |
| Overcut stringer corners | Weak stringers, potential failure | Stop cut at corner, finish with jigsaw |
| Measuring to framing | All steps wrong height | Always measure to finished floor surfaces |
| Only 2 stringers | Bouncy, weak stairs | Minimum 3 stringers for 36" width, 4 for wider |
| Skipping permit | Failed inspection, liability | Check local codes, get permit before starting |
| Unequal rises | Uncomfortable, dangerous stairs | Calculate precisely, verify each cut |
| No handrail | Code violation, unsafe | Required if more than 4 risers |
When to Hire a Professional
✓ Good DIY Projects
- • Simple deck stairs (3-5 steps)
- • Straight run, no turns
- • Comfortable with framing square
- • Understand the math
- • Outdoor stairs (more forgiving)
✗ Hire a Pro For
- • Interior stairs: Visible, must be perfect. Higher standards for finish.
- • L-shaped or U-shaped stairs: Complex calculations for landings and turns.
- • Winder steps (pie-shaped): Difficult geometry, code requirements for minimum tread width.
- • Curved or spiral stairs: Specialized skills, custom fabrication required.
- • Stairs with landings: Additional framing, code requirements, more complex.
- • Building inspector will be strict: Some jurisdictions require professional installation.
💵 Cost Comparison
Simple Deck Stairs (4 steps)
DIY: $300-600
Pro: $800-1,500
Interior Stairs (13 steps)
DIY: Not recommended
Pro: $2,000-5,000
Frequently Asked Questions
What is the formula for building stairs?
(2 × Rise) + Run = 24-25 inches for comfortable stairs. Unit rise should be 7-7¾", unit run minimum 10" (11" preferred). All rises must be equal within ⅜". Use our Stair Calculator to get exact measurements for your project.
How many stringers do I need for stairs?
Minimum 3 stringers for stairs up to 36" wide. Add a 4th stringer for stairs 48" wide or more. Maximum spacing between stringers is 18". Never use only 2 stringers - stairs will be bouncy and dangerous.
Why is my first step a different height?
The most common mistake is forgetting to subtract the tread thickness (1½") from the first rise. If your unit rise is 7", the first rise should be cut at 5½" so when the tread is added, it equals 7" like all the other steps.
Do I need a permit to build stairs?
Usually yes for new stairs or replacing existing stairs. Deck stairs, interior stairs, and any stairs with more than 3-4 steps typically require permits. Check with your local building department - requirements vary by location.
What size lumber do I need for stair stringers?
Use 2×12 pressure-treated lumber for most residential stairs. After cutting the stair notches, you need minimum 5" of material remaining at the narrowest point (throat). Deeper stairs or longer spans may require 2×14.
Related Resources
This guide has been reviewed by multiple experts to ensure both technical accuracy and practical application.
Dr. Snezana Lawrence
Mathematical Historian | PhD from Yale
Dr. Lawrence is a published mathematical historian with a PhD from Yale University. She ensures mathematical precision and accuracy in all our calculations, conversions, and academic score calculators. Her expertise spans computational mathematics and educational assessment.
LinkedIn ProfileBruce Zawalsky
Survival & DIY Expert | 28+ Years Experience
Bruce is a wilderness survival expert with 28+ years of experience and author of "Canadian Wilderness Survival." His extensive hands-on experience in building wilderness shelters, construction projects, and practical DIY work, combined with his military training, ensures all our construction guides and survival content are field-tested and practical.
LinkedIn Profile