How to Build and Cut Deck Stair Stringers
Stair stringers are the angled boards that support your deck stairs. Cutting them requires careful calculation and precise layout—every step must be identical for safe, comfortable stairs. This guide walks you through the math and cutting process for perfect stringers every time.
Quick Summary
2-4 hours
Moderate to Challenging
$50-150 (materials)
Understanding Stair Terminology
Before diving into calculations, understand the key terms:
- Rise: The vertical height of each step. Typical range is 7-7.75 inches.
- Run (Tread depth): The horizontal depth of each step. Typical range is 10-11 inches.
- Total Rise: The total vertical distance from ground to deck surface.
- Total Run: The horizontal distance the stairs cover from bottom to top.
- Stringer: The notched diagonal board that supports the treads.
Tools and Materials
Materials
- 2x12 pressure-treated lumber – one per stringer (minimum 3 for stairs up to 36" wide)
- Stringer hangers – metal brackets to attach stringers to deck frame
- Structural screws – for stringer hangers (or lag bolts)
- Concrete landing pad – if stairs land on soil (prevents rot and settling)
Essential Tools
- Framing square – essential for layout (the large L-shaped carpenter's square)
- Stair gauges – small clamps that attach to framing square ($5-10)
- Circular saw – for straight cuts along step notches
- Jigsaw or handsaw – for finishing inside corners of notches
- Tape measure – for measuring total rise
- Calculator – for rise/run calculations
Step 1: Measure Total Rise
The most critical measurement. Measure from the deck surface (not the rim joist) straight down to where the stairs will land:
- Use a long straight board or level extending from the deck
- Measure from bottom of board to ground
- Account for any planned landing pad thickness
- Measure in multiple spots if ground slopes
Critical: An error here affects every step. If your total rise is wrong by 1/2 inch and you have 5 steps, each step will be off by 1/10 inch—enough to feel awkward and potentially fail inspection.
Step 2: Calculate Number of Steps
Divide total rise by your target rise (typically 7.5 inches) and round:
Example Calculation
Total rise: 42 inches
Target rise per step: 7.5 inches
- 1. 42 / 7.5 = 5.6 steps
- 2. Round to 6 steps
- 3. Actual rise per step: 42 / 6 = 7 inches
If rounding up gives a rise under 7 inches or rounding down gives a rise over 7.75 inches, adjust your landing height slightly if possible.
Step 3: Determine Run
The run (tread depth) affects stair comfort. Use the 2R + T formula to find a comfortable combination:
- R = Rise (7 inches in our example)
- T = Tread/Run (what we're solving for)
- 2R + T should equal approximately 25 inches
For 7-inch rise: 2(7) + T = 25, so T = 11 inches
Common combinations that feel natural: 7"/11", 7.25"/10.5", 7.5"/10"
Step 4: Lay Out the First Stringer
Set Up Stair Gauges
Clamp stair gauges to your framing square:
- 1.Clamp one gauge on the tongue (shorter leg) at your rise measurement
- 2.Clamp other gauge on the blade (longer leg) at your run measurement
- 3.These gauges let you repeat the exact same angle for every step
Mark the Steps
Position the framing square on the 2x12 and trace each step:
- 1.Place gauges against the board edge
- 2.Trace along both legs of the square to mark the first step
- 3.Slide square along board until gauge aligns with previous mark
- 4.Trace again for next step
- 5.Repeat for all steps
Mark Top and Bottom Cuts
The stringer needs specific angles at each end:
- Top cut: Follows the rise angle, flush against rim joist
- Bottom cut: Follows the run angle, sits flat on landing
Account for Tread Thickness
Critical step often forgotten: The bottom of the stringer must be shortened by the thickness of one tread, or your first step will be too tall.
- If using 2x6 treads (1.5" actual), remove 1.5" from bottom of stringer
- Mark a line parallel to the bottom cut, 1.5" higher
- This is your actual bottom cut line
Step 5: Cut the Stringer
Cutting requires two different tools for clean, accurate notches:
- 1.Set circular saw depth to just under the board thickness
- 2.Cut along each marked line but stop about 1 inch before the corner
- 3.Finish inside corners with jigsaw or handsaw—don't overcut with circular saw
- 4.Cut top and bottom angles with circular saw
Avoid This Mistake: Never overcut the corners with your circular saw. Overcutting creates stress points that weaken the stringer and can lead to cracks over time. Take the extra minute to finish corners with a handsaw.
Step 6: Test and Copy
Before cutting more stringers, verify your first one is correct:
- Hold stringer in position against deck and landing
- Measure from deck surface to first tread position—should equal one rise
- Check that stringer sits flat on landing and flush against rim joist
- Verify at least 3.5 inches of solid wood remains at narrowest point
Once verified, use your first stringer as a template to trace and cut the remaining stringers.
Common Mistakes to Avoid
Forgetting to Subtract Tread Thickness
The most common stair-building error. If you don't shorten the stringer bottom by one tread thickness, your first step will be too tall and last step too short—a trip hazard that fails inspection.
Using 2x10 Instead of 2x12
After cutting notches, a 2x10 doesn't leave enough solid wood (3.5 inches minimum required). Always use 2x12 for stair stringers.
Overcutting Corners with Circular Saw
The kerf extends beyond the visible cut, weakening the stringer at each step. Stop short and finish with a handsaw for maximum strength.
Only Using Two Stringers
Even for narrow stairs, code requires three stringers. The center stringer prevents treads from flexing and breaking under load. Add extra stringers for wide stairs (one every 16 inches or less).
FAQ
What is the ideal rise and run for deck stairs?
Building codes typically require a maximum rise of 7.75 inches and minimum run of 10 inches. The ideal comfortable stair has a 7 to 7.5 inch rise and 10 to 11 inch run. Use the formula 2R + T = 25 inches for comfort.
How many stringers do I need for deck stairs?
For stairs up to 36 inches wide, you need a minimum of 3 stringers. Wider stairs need a stringer every 16 inches or less. Two stringers alone are never sufficient.
What size lumber should I use for stair stringers?
Use 2x12 pressure-treated lumber. After cutting notches, you must have at least 3.5 inches of solid wood remaining. A 2x10 doesn't leave enough material.
Do all stair risers need to be the same height?
Yes, building codes require that all risers be within 3/8 inch of each other. Inconsistent rise heights are a major trip hazard. This is why careful calculation before cutting is critical.