Building a safe and sturdy deck starts with properly designed deck stairs. The key to perfect stairs is calculating your stringer correctly — the diagonal board that supports the steps.
This Deck Stringer Calculator helps you quickly determine the number of steps, total run, stringer length, and stair angle with just a few inputs: total deck height, tread depth, and riser height.
Let’s see how to calculate each step manually before using the calculator.
1. Formula to Calculate Number of Steps
The number of steps depends on the total deck height and riser height.
Number of Steps=Riser HeightTotal Deck Height
2. Formula to Calculate Total Run
Once you know the number of steps, multiply it by the tread depth to find the total horizontal distance covered by the stairs.
Total Run=Number of Steps×Tread Depth
3. Formula to Calculate Stringer Length
The stringer length is the diagonal length between the top and bottom of the stairs.
It’s calculated using the Pythagorean theorem:
Stringer Length=(Total Run)2+(Total Deck Height)2
4. Formula to Calculate Stair Angle
To find the angle of inclination (stair angle), use the tangent formula:
Angle=tan−1(Total RunTotal Deck Height)
Example Calculation
Let’s say you’re building a deck with these dimensions:
- Total Deck Height = 36 in
- Riser Height = 7.5 in
- Tread Depth = 10 in
Step 1: Calculate Number of Steps
Number of Steps=7.536=4.8≈5
Step 2: Calculate Total Run
Total Run=5×10=50 in
Step 3: Calculate Stringer Length
Stringer Length=502+362=61.6 in
Step 4: Calculate Stair Angle
Angle=tan−1(5036)=35.8∘
Why Use a Deck Stringer Calculator?
- Saves time and ensures accuracy
- Prevents miscutting lumber
- Helps design stairs that meet building code requirements
- Gives you quick results for planning and cost estimation
FAQs
1. What is a deck stringer?
A deck stringer is the sloped support board that holds the steps of a staircase. It’s usually cut from a 2×12 piece of lumber and serves as the backbone of the stairs, supporting each tread and riser securely.
2. How do I calculate the number of steps for a deck stringer?
To calculate the number of steps, divide the total deck height by the riser height:
Number of Steps=Riser HeightTotal Deck Height
Round the result to the nearest whole number to get the correct number of steps.
3. What angle should deck stairs be?
Most deck stairs have an angle between 30° and 37° for comfortable and safe use. A lower angle gives longer, easier steps, while a steeper angle makes stairs more compact but harder to climb.
4. What size lumber should I use for deck stringers?
The most common lumber size for deck stringers is 2×12, as it provides enough depth for both treads and risers while maintaining strength. For small stairs or light-duty applications, 2×10 may also work.
5. How many stringers do I need for deck stairs?
For standard stairs, you typically need one stringer every 16 inches of stair width.
For example, a 48-inch wide staircase should have 3 stringers — one on each side and one in the center for proper support.
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