HOW-TO GUIDES

How to Calculate & Cut Stair Stringers

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If you're planning to add a set of stairs to your deck, understanding how to calculate and cut stair stringers is essential for stability and safety. Without properly cut stair stringers, your deck's stairs run the risk of falling through and leading to unwanted accidents.

Stair stringers are the structural components that support each step in a staircase. They are typically made from lumber and positioned at an angle to support the treads and risers of each step.

In this guide, we'll walk you through the process of how to cut stair stringers, from calculating the rise and run of the stairs to finding the appropriate angles and making precise cuts. No matter your experience level, this guide offers the tips you need to be confident building stair stringers that will enhance both the accessibility and aesthetics of your deck.

Disclaimer: Instructions provided by Trex® are based on the build specifications that Trex® demonstrated in the Trex® Academy video series and are not universal to all homeowners’ projects. A homeowner’s build may vary from Trex’s instructions based on the homeowner’s local municipality requirements, individual design preference and project specifications. For questions or assistance needed during your project, please reach out to howto@trex.com

On average, it will take 1.5 hours to cut two stair stringers with a team of 2 people.
*Times may vary
Step-by-Step Instructions
Chapter 01: Deck Stair Stringers - What Are They and Why Do They Matter for My Deck Build?
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Building stairs

A deck stair stringer is the structural component of the deck stairway that supports the decking and fascia. They’re typically placed on either side of the staircase, are angled to support the treads and risers, and are responsible for bearing the weight and force exerted on the steps during use.

Without stringers, the steps would lack the necessary support and be prone to sagging, flexing, or collapsing under the weight of those using them, making the staircase unsafe and unusable. 

The stringer will be cut out of a 2”x12” piece of pressure-treated lumber with the appropriate rise and run determined by the finished height of the deck. Your deck stairway will need multiple stringers with a span of 9-12 inches each, with thinner deck boards requiring shortened stringer spans.

Why Is It Important to Get Stair Stringers Correct?

As a core element of your stair's construction, there are a few key reasons why knowing how to calculate stringers for deck stairs is essential to your project's success:

  • Support: Stair stringers provide structural support and stability for the entire staircase. If they are not accurately calculated and cut, the steps can become weak and unstable, posing a significant safety risk for anyone using the stairs. 
  • Safety: Incorrect stair stringer sizing or spacing can lead to uneven steps, making it difficult and potentially dangerous to navigate the staircase.
  • Durability and regulation: By correctly measuring, cutting and installing your stair stringers, you can create a safe and sturdy deck staircase that meets building codes and regulations and lasts for years to come.

How Many Stringers Do Stairs Need?

The number of stringers needed for a staircase depends on a number of factors:

  • The width of the staircase: For wider staircases or those with a particularly long span, additional stringers might be necessary to provide adequate support. Placing a stringer on each end of the staircase provides a solid foundation and prevents excessive flexing or sagging of the steps.
  • Manufacturing guidelines: Be sure to check with your decking manufacturer to determine how far the deck board can span between stringers. Different boards may have different requirements, which can affect your calculations.
  • The materials used: When you’re using composite deck boards, thicker full-profile boards can hold up to a 12-inch span between stringers. Scalloped boards or thinner boards need shorter spans between stringers for proper support, therefore they require more stair stringers.
  • Local building codes: Your local building codes may require a certain number of stringers, depending on your stair size. Check beforehand to avoid any unwanted re-dos or citations.

Can You Build Stairs Without Stringers?

You can build stairs without stringers, but it can be both more complex and less economical to construct than the stair stringer method. Two common alternatives to stair stringers are an open riser design and constructing stairs as boxes:

  • Open riser staircases use supports for each tread instead of stair stringers, offering an open gap between each stair. These stairs are more design-friendly and provide better airflow. However, they can be more challenging to build than traditional stair construction as it requires meticulous planning, precise measurements, and careful installation to ensure visible supports look good.
  • Building stairs as boxes means each tread provides its own structural support, but instead of being open, these stairs are essentially different sized boxes placed on top of each other. These stairs are good for wide and short decks that don't require more than three stairs, but the process involves increased labor and materials costs.

Tools

Circular Saw

Framing Square

Measuring Tape

Pencil

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