How to Calculate & Cut Stair Stringers


If you're planning to add a set of stairs to your deck, understanding how to calculate and cut stair stringers is essential. Stair stringers are the structural components that support each step in a staircase, providing stability and safety. They are typically made from lumber and are positioned at an angle to support the treads and risers of each step.

In this guide, we'll walk you through the process of cutting stair stringers, from calculating the rise and run of the stairs to finding the appropriate angles and making precise cuts. Whether you're a seasoned DIY enthusiast or a beginner in construction projects, this guide will provide you with the necessary knowledge and confidence to build stair stringers that will enhance both the accessibility and aesthetics of your deck. So let's dive in and learn how to build stair stringers for your deck steps.

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


On average, 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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A deck stair stringer is the structural component of the deck stairway that supports the decking and fascia. 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. Stringers are typically placed on either side of the staircase and are angled to support the treads and risers. They are responsible for bearing the weight and forces exerted on the steps during use. Without stringers, the steps would lack the necessary support and would be prone to sagging, flexing, or collapsing under the weight, making the staircase unsafe and unusable. 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?

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. Additionally, incorrect stair stringer sizing or spacing can lead to uneven steps, making it difficult and potentially dangerous to navigate the staircase. By ensuring that the stair stringers are correctly measured, cut, and installed, you create a safe and sturdy deck staircase that meets building codes and regulations and should last for years to come.

How Many Stringers Are Needed for Stairs?

The number of stringers needed for a staircase depends on the width of the staircase, the materials used, and local building codes. 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. Placing a stringer on each end of the staircase provides a solid foundation and prevents excessive flexing or sagging of the steps. For wider staircases or those with a particularly long span, additional stringers might be necessary to ensure adequate support. Be sure to check with your decking manufacturer to determine how far the deck board can span between stringers. Keeping to this spacing helps ensure strong, stable steps while placing a stringer on each end of the staircase provides a solid foundation and prevents excessive flexing or sagging of the steps. To determine the number of stringers needed for your deck staircase, divide the width of your staircase by the length the decking you’ve chosen can safely span in stair applications. When working with PVC or composite boards, you’ll find this information in the manufacturer’s instructions, which should always be consulted before you begin any project using deck boards. For wood, the same information is included in local building codes. Request documents from the municipality before you start building your staircase. 

Can You Build Stairs Without Stringers?

You can build stairs without stringers, but it’s a more complex construction process or less economical 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, which can be more challenging than traditional stair construction as it requires meticulous planning, precise measurements, and careful installation to ensure visible supports look good. Building stairs as boxes also means each tread provides its own structural support, but the process involves increased labor and materials costs.

If you’ve never used or calculated stair stringers, or even if you have but it’s been a while, you’re probably asking “What is the formula for cutting stair stringers?”. While it’s a task that requires precision and just a little math. Before you calculate stair stringers, make sure you have:

  • A quality measuring tape
  • A pencil
  • A framing square 
  • A circular saw
  • A handsaw
  • Safety glasses 
  • Gloves

Step 1: Measure Your Stairs’ Total Rise

The first step is to calculate the total rise of your stairs is the distance from the ground to the top of your deck boards. To accurately work out the total rise, hold a tape measure to the top of your deck boards, measuring vertically to the ground. If you haven’t added boards to your deck’s frame yet, adjust for the height of the board you’re using. Take note of the measurement, which will be used in subsequent calculations to determine the number of steps and rise per step. Keep in mind, the ground can slope away from the deck's edge so measuring the height at the point where the stairs land is important to designate. To ensure this is accurate, use a level. 

Step 2: Decide How Many Steps You Need

Before you calculate stringer size you’ll need to decide how many steps you need. In most cases, the steps on deck staircases are from 7–7 1/2 inches tall. Be mindful that taller steps can be tough for those with accessibility issues and little ones. Also, the International Residential Code (IRC) states that 7 3/4" is the max rise for a stair rise, but it is always a best practice to check with your local building code for the most recent rules and regulations in your area. 

Step 3: Lock in the Exact Rise Per Step

Now for the math. Take the total rise of your stairs, and divide it by the desired height for each step you’ve just decided on. This will allow you to calculate the number of steps. Remember to round the result to a whole number and adjust if necessary to ensure a consistent and comfortable rise for each step. This consistency also provides safety by reducing the likelihood of the users to trip or fall on the stairway. Ensuring this comfort and safety is one of the reasons local building code dictates the minimum and maximum height allowed per rise. Always check with the municipality to check their rules. 

When considering how to calculate stair stringers bear in mind that the total number of stringers is based on their spacing. For example, if your deck staircase will have 10 steps, you’ll need 9 treads. This is because the top step doesn’t require a step or tread as it leads directly to your deck surface.

Step 4: Determine the Run of Each Step

The run of a step is its width. A very narrow run can make staircases dangerous, as people using them won’t be able to properly place their feet. A very wide run can make it a stretch to get from step to step. Additionally, the run of each step affects the run of the stairs, and therefore how much space they will take up in your yard. Ultimately, however, allowable run is dictated by code. Check with your local municipality to verify what’s acceptable where you’re building. 

An easy option for the run of each step in a deck staircase is 10 inches. Standard decking boards are usually 5 1/2 inches in width. Two boards plus the 3/16" spacing required between them gives 11 3/16" total. A 10-inch run for each step leaves a 3/4 inch overhang, which is aesthetically pleasing and means any wear and tear to step edges tends to be cosmetic rather than structural. As close as possible to an 11-inch tread for steps is also recommended by most local municipalities. If your deck boards are a different width you can still use this trick, just adjust for your numbers. 

Step 5: Calculate the Total Run of the Stairs

The total run is the horizontal distance covered by the staircase. Measure the horizontal distance from the face of the first riser to the face of the last riser. This measurement will determine the length of the staircase. Accurate calculation of the total run ensures that the staircase fits properly within the available space and provides a comfortable and balanced walking surface. It’s difficult to say exactly what the normal rise and run is for stairs, but a comfortable angle for a staircase is usually between 30 and 45 degrees, and 12 is the common maximum for stairs in a home or yard. 

Step 6: Sketch Out Stair Stringers

Once you have determined the total rise, number of steps, rise per step, and total run, it's time to sketch out the stair stringers. To make a stringer template, you can use a piece of scrap plywood or cardboard. Start by marking the top and bottom of the stringer, indicating the total rise and total run measurements. Then, use this template to mark the positions of each step, taking into account the rise per step and tread width. Connect these marks to create the outline of the stringer. Repeat this process for each stringer required for your staircase.

Once you have determined the stair dimensions, including the total rise, total run, tread depth, and riser height for your stair stringer, it’s time to cut. Use a framing square or your DIY template to determine the angle and length of the stringer. Mark the steps on the stringer, accounting for the thickness of treads and risers. Cut along the marked lines using a circular saw or handsaw. Don't over cut where the lines intersect. Use the handsaw to finish the cut. Test-fit the stringers and make any necessary adjustments for a secure fit. For more detailed information on how to cut stair stringers, check out Chapter 2 of Trex Academy’s How to Build Stairs.


Circular Saw

Framing Square

Measuring Tape


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