Stairs probably impact occupant safety more than any other construction element that carpenters are responsible for. At some point, all of us have stumbled on stairs that were not laid out correctly. Yet this basic element found in practically every structure causes more head scratching than just about anything else.
The building code (IRC, R311.7 Stairways) [Editor's note: link updated to 2021 version] is strict regarding stair layout and includes the minimum tread depth (10 inches), the maximum riser height (7 3/4 inches in the IRC, but this can vary by state), the maximum variance (typically 3/16 inch) between riser heights in a given stairway, and more. Much has been written about stair construction and codes. In this article, we will share some guidelines and tips for both calculating and laying out open stringers (also known in the industry as horses or carriages) to create a safe set of stairs, gleaned from the scores of stairways we have laid out over the years.
- Determine the rise and run for a set of stairs
- Lay out an open stringer.
- Ability to read a tape measure
- Ability to read measurements on a framing square
- Ability to use a circular saw and handsaw
- Tape measure and a sharp pencil
- Construction Master calculator (optional, but extremely helpful)
- Framing square
- Stair gauges
- Circular saw
- Safety glasses
- Most open stringers are cut from 2x12 stock, either dimension lumber or the LVL equivalent.
- Total Rise: The total vertical distance that a staircase will cover
- Total Run: The total horizontal distance that a staircase will cover
- Stringer (or carriage): The main framing member that runs at a diagonal and supports the stairs. In this case we are laying out and cutting an "open" stringer, which is cut in a stepped pattern of vertical and horizontal cuts.
- Unit Rise: The vertical measurement of each individual step on the stringer
- Unit Run: The horizontal measurement of each individual step
- Finished Floor: The material that will be the final finish underfoot once the house is finished
- Tread: Material attached to the horizontal cuts of a stringer. There may be a structural "subtread," and a "finish tread" (the material you walk on when climbing the stairs). Both must be accounted for when calculating the riser height.
- Riser: Finish material attached to vertical cuts on a stringer.
To layout an open stringer, you fist need to determine the total rise and the total run of the stairs. From these measurements you will derive the unit rise and unit run.
Whether you calculate the total rise first or the total run first is sort of a “chicken or egg” question. There are factors (such as limited space for a landing at the bottom of the stair) that can restrict the run. However, the rise measurement is always restricted by the total vertical distance that the stairs need to span, so it makes sense to start with the rise. For the example in the illustration, we'll assume that there are no on-site conditions that would restrict the run, and we will begin by calculating total rise.
1. Find the total rise of the stair
The first measurement to determine is the distance between the top and bottom of the stairs. But don’t just measure up from below the top landing point; instead, level over approximately to where the bottom of the stair will land (Step 1 in the illustration above). The vertical distance between the bottom and top of the stair opening is Distance X, which in our example is 30 3/4 inches.
That distance is just a beginning measurement from the top of the lower subfloor to the top of the upper subfloor. The total rise of the stair is the measurement from the finished floor at the bottom to the finished floor at the top. In this example, the finished floor at the bottom of the stair is 3/4-inch oak strip flooring (Step 2). Because adding the flooring at the bottom of the stair raises the starting point of the stair, subtract that distance from the initial rise measurement (X minus 3/4 inch).
At the top of the stair in our example, the finished floor is 1/2-inch-thick engineered flooring. That number increases the total rise of the stair, so add that number to X (Step 3). The total rise of this stair is then (X – 3/4 inch + 1/2 inch, or X – 1/4 inch). In our example, X = 30 3/4 inches, so the total rise from finished floor at the bottom to finished floor at the top is 30 3/4 – 1/4 inch, or 30 1/2 inches.
2. Find the unit rise
The rise for a safe and comfortable stair is between 7 and 8 inches, so we figure the number of rises by dividing the total rise (30 1/2 inches) by 7, which gives us about 4. We don’t need an exact number at this point (Step 4). To figure out the exact rise per step or unit rise, now divide the overall rise (30 1/2 inches) by the number of steps (4), which gives us 7 5/8 inches (Step 5). That number lands in the sweet spot between 7 and 8 inches, and should make for a safe and comfortable rise.
3. Find the unit run
The unit run determines the depth of each tread. The minimum tread depth allowed by the IRC is 10 inches. There are several formulas for calculating the ratio between rise and run; the simplest is Rise + Run = 17 to 18 inches. Using a target run of 10 inches, the rise/run ratio is well within the “safe and comfortable” formula for this stair: 7 5/8 + 10 = 17 5/8 (Step 6).
4. Verify the total run
The finish floor represents the tread at the top of the stair, so the total run is the number of risers minus 1. Multiply the number of risers (3, in this example) by the the run length (10 inches, in this example):
3 X 10 inches = 30 inches for the total run.
The code requires stairs to have a minimum space of 36 inches for a landing at the bottom of a straight run of stairs. Add this 36 inches to the total run distance to confirm that you have enough space for the landing. If there is less than 36 inches (measured between all the finish material on the walls and stairs), you may have to reduce the unit run. This will reduce the tread depth, but you can't have a unit run less than 10 inches. This is one of the limiting factors that makes fitting a set of stairs into an existing framed opening tricky. In new construction, you need to account for the total run of the stairs when framing the stair opening.