How to Calculate Stair Stringers for a Deck?

The stair stringers are crucial for the staircase’s stability and safety. To figure out how many deck stair stringers you need, first find the Total Rise (the height of the deck to the landing). Then, divide that by a preferred riser height (around 7–7.75″) to get the number of steps, rounding up to the nearest whole number. Next, multiply the number of steps by the tread depth you want (around 10–11 inches) to get the Total Run. Then, use a framing square to mark and cut the notches in your stringer board (typically a 2×12) using the rise and run measurements you picked.

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What is a Stair Stringer?

Stair stringers form the skeletal framework of a stair system. They serve as the primary load-bearing components supporting the entire staircase’s weight. Visually, stringers resemble long wooden planks cut into sawtooth patterns. The stepped contours created by these teeth precisely support each stair tread and riser.

In deck stair construction, stringers are typically fabricated from 2×12 pressure-treated lumber. This dimension provides sufficient structural strength and cutting allowance. It can withstand user weight and environmental stresses.

Why Are Stair Stringers So Important?

Stringers bear the critical responsibility of transferring all stair loads to the floor or lower structure. Proper stringer design ensures:

• Even load distribution to prevent localized stress concentration.
• A stable tread surface to avoid step deflection or deformation.
• Resistance to structural fatigue caused by long-term use and environmental changes.

Incorrect stringer calculations and installation can lead to:

• Inconsistent step heights.
• Insufficient tread support, creating bending or fracture risks.
• Overall structural instability, potentially causing collapse accidents.

How to Calculate Deck Stair Stringers?

Most stairs will have four to five stringers, but this depends on the stair width and tread material. Therefore, you need to measure and calculate following these steps.

Prepare Measuring Tools

You may need the following tools for calculation and measurement.

• Measuring Tools: 25-foot tape measure, 4-foot level, laser distance meter, chalk line.
• Marking Tools: Carpenter’s pencil, marking knife, framing square.
• Safety Gear: Safety glasses, dust mask, work gloves.

Key Terminology Explained

Before beginning calculations, the following critical terms must be understood:

• Total Height: The vertical distance from the finished deck surface to the stair tread’s bottom contact point. This serves as the foundational dimension for calculations.
• Total Length: The horizontal projection distance from the deck edge to the front edge of the lowest step.
• Riser Height: The vertical height of a single step. The ideal residential stair riser height ranges between 7 and 7.5 inches.
• Tread Depth: The horizontal depth of a single step, measured from the leading edge to the trailing edge. The ideal tread depth is typically 10 to 11 inches.
• “Rule of 27”: Riser Height + Tread Depth ≈ 27 inches. This formula, based on ergonomic research, ensures stair comfort and safety.
• Headroom: The least amount of space that has to be above the stairs, according to the code, is at least 80 inches.

Accurate Measurement of Total Height

If the frame is built but the deck boards haven’t been put on yet, put a real deck board at the place where the measurement is supposed to be to get an idea of how high it will be when it’s done. If the stairs terminate on solid ground, measure to that ground level. If terminating on soil, account for the final finished surface height.

Measure the height at least three times in the stair placement area. If the differences in height are more than an inch, you might need to change the plan or get ready to level things out.

Determining the Number of Steps and Riser Height

Select a riser height between 7 and 7.5 inches based on specifications and ergonomics. Number of steps = Total height ÷ Riser height. Round the result up to the nearest whole number to obtain the total number of columns.

Tread Depth

The “Rule of 27” can be applied to determine tread depth. Ideal tread depth = 27 – riser height. In real life, the tread depth of deck stairs is usually based on the width of normal deck boards. Using two boards installed side-by-side yields a tread depth of approximately 11 inches.

Calculating Total Stair Length and Stringers

For stairs with n cut steps: Total horizontal length = Tread depth × Number of cut steps. The actual stringer length is longer and requires calculating the diagonal dimension.

Minimum joist count = Stair width ÷ Maximum allowable span. Most building codes require: – At least 3 joists for stair width ≤ 36 inches. – At least 4 joists for widths between 36 and 48 inches. – For widths > 48 inches, add 1 joist for every additional 12 inches.

Drawing the Stair Stringers

Finally, use all the dimensions calculated earlier to draw the stair stringers. It is best to mark the total height and total span, as well as the height and run of each step on the stringers.

Conclusion

Building a safe and durable deck staircase requires patience, precision, and attention to detail. The joists, serving as the staircase’s framework, are crucial to the entire structure. So, when you’re planning, be sure to carefully measure each part and use good materials and tools. This approach will enable you to construct a staircase that is both aesthetically pleasing and safe, enhancing the functionality of your deck.

If you have further questions, feel free to contact us. COOWIN provides high-quality composite decks and is here to address your inquiries.

FAQ

What if the total stair height isn’t an exact multiple of the riser height?

This is common. Solutions include:

• Slightly adjust the height of the first or last step.
• Modify the deck or ground level.
• Accept minor tolerances. Best practice is to distribute discrepancies across multiple steps rather than a single one.

How to Handle Sloped Ground?

You can cut the bottom of joists into steps to match the terrain. Alternatively, adjust the final step height to accommodate the slope. Some homeowners also build a small level landing at the bottom of the stairs.

Are There Special Requirements for Joist Calculations with Composite Decking Materials?

Composite materials have distinct properties. Their thermal expansion rate differs from that of wood, potentially requiring appropriate expansion gaps. Additionally, their strength and connection methods vary, possibly necessitating closer joist spacing. Refer to the manufacturer’s installation guidelines for specifics.

How Do You Calculate Joists for Curved Staircases?

Curved staircases typically require professional design and calculations. However, fundamental principles include:

• Inner joists are shorter than outer joists.
• Treads fan out in a curved pattern.
• More frequent joist support is necessary.
• Using specialized stair design software or consulting an engineer is recommended.