Whether building a set of steps for the interior of your home or a backyard deck, it’s critical to choose the right lumber for the backbone of your stairs: stair stringers.
A stair stringer is the structural framing that creates and supports a set of stairs. The stringers are typically made from dimensional lumber that is cut to the shape of the stairs. They should be stout enough to support the weight of the steps and the people using them. Should a stair stringer be 2×10 or 2×12?
It is acceptable and within building code guidelines to use either 2×10 or 2×12 dimensional lumber to create a set of stairs; however, a stair stringer should not be less than 3.5 inches wide at its narrowest point to provide adequate strength for the load the stairs will be supporting.
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What Is a Stair Stringer?
Think of a stair stringer as the framing for a set of stairs. They are cut to create the shape of the stairs. As such, they determine the height of each step as well as the depth of each tread. This is typically referred to as rise over run. Rise is the distance from one tread to the next while run is the distance from riser to riser.
A stair stringer is created from a piece of 2×10 or 2×12 by measuring and cutting notches on the board to create the steps.
Why is a Stair Stringer Important?
When failures do occur with stairs, the size of the notch cut into the stringer is often the culprit. A notch that is too deep for the board creates a throat (the space between the corner of the notch and the opposing edge of the board) that is too narrow to hold an adequate load.
If the throat is too narrow, the stairs could fail, causing catastrophic injury. Unfortunately, when this type of failure occurs, there is typically a little warning. The stairs collapse without warning. This makes using the proper stringer width critical when constructing stairs.
Stair Stringer 2×10 or 2×12?
While one might expect stair framing to have specific code requirements, the fact is there are few specific codes. Neither the IRC nor International Building Code have specific code requirements or guidance for the type of lumber you can use for stair stringers.
That said, what guidelines the IRC and IBC do narrow the field for what dimensional lumber you can use.
The International Building Code stipulates that a set of stairs must be able to handle a minimum concentrated load of about 40 pounds per square foot live, which refers to the temporary load placed upon the steps when someone is using them, or about 10 pounds per square foot dead, which is the amount of constant weight applied to the stairs.
For the throat to be strong enough to support this load, it should be at least 3.5 inches wide, the same width as a 2×4.
The IRC provides guidance for how high the risers can be and how deep the treads can be. According to the IRC, the maximum riser height is 7.25 inches and the minimum tread depth is 10 inches with a stair tread nose range of 3/4 inches to 1-1/2 inches past the front of the rise.
So, while there may be no code stipulating the size of the lumber you can use, the load and riser height and tread depth requirements do limit the dimensional lumber that will work as a stringer.
With these limitations and the 3.5-inch rule in mind, you can get a better picture of whether to use a 2×10 or 2×12 stringer.
Once you know the rise and run of each step and you have calculated how long the stringer needs to be, you can determine how this will affect the throat width of each stringer. Draw the first step using a square to mark the profile of the stairs.
Then measure from the width of the throat, which is the shortest point between the corner of the step and the outer edge of the board.
Imagine drawing a straight line that would connect the corner of each notch in the stair. This line should be no closer to the outer edge of the stringer than 3.5 inches to ensure proper structural integrity.
Can I Use 2×10 for Stair Stringers?
While using a 2×10 is acceptable, keep in mind that it limits the stair size you can use. By looking at standard stair heights, we can determine what dimensional lumber will work and what won’t work.
For example, the standard rise over run for stairs covering 9 feet is a 7-inch rise and 11-inch run. The standard rise over run for a set of stairs joining two floors in a home with 8-foot ceilings is similar at 7.5 inches and 10 inches.
For a 2×10 stringer, the maximum tread width you can achieve with a 7-inch rise and still maintain a 3.5-inch throat is about 10 inches if you use the maximum stair tread nose allowed by the building code of 1.25 inches (the stringer itself would have a run of about 9 inches).
If you want a deeper tread, you’ll have to cut deeper into the board, creating a larger notch and reducing the size of the throat. A 2×10 board may not be wide enough to handle the larger notch while still maintaining a 3.5-inch throat.
While it’s possible to increase the tread depth by decreasing the riser height to make the notch small enough to maintain a 3.5-inch throat in a 2×10 board, it will take a longer run to reach the second level. A longer set of stairs may not fit in the available space.
Can I Use 2×8 for Stair Stringers?
While you may find some information stating that it is okay to use 2x8s for stair stringers, in most cases, it won’t work. The 3.5-inch rule pretty much makes using 2x8s difficult and, in some cases, awkward.
While it is possible to reach the minimum stair depth tread of 10 inches (again using the 1.25-inch maximum tread nose depth) while still maintaining the minimum throat width of 3.5 inches, you would have to go with an awkward stair rise of just 4 inches.
This may work in cases where you’re using just a few stairs for a single room with multiple levels, such as a sunken living room. It’s impractical for stairs connecting multiple floors in a home. The total run and number of stairs would need to be nearly double that of a standard set of stairs to reach the second floor!
How to Cut Stair Stringers With a Framing Square
Once you know the rise and run that you want your stairs to be, you can go ahead and draw out the cut lines on the 2×10 or 2×12.
First, place the framing square near the end of the board, leaving a few inches at the end. Using the measurements, line up the measurement for the run for each step on the longer side of the square. Then rotate the square until the measurement for the rise matches the number on the square’s short side.
Once both measurements are lined up, use a pencil to trace the notch on the outer edge of the square.
Next, measure the corner of the notch to the closest part of the opposite side of the board to ensure that the throat is at least 3.5 inches wide.
Then slide the square down the board and continue to trace the shape of the stairs until you reach the other end of the board.
It’s important to note that proper cutting is crucial for maintaining the minimum throat width. When cutting a notch with a circular saw, the circular shape of the blades does not allow you to make a full cut at the endpoint of the cut.
Do not accommodate for this by cutting beyond your mark. This will cause structural weakness in the stringer. Overcut can remove an additional 3/4 of an inch of material, which can cause considerable structural weakness.
Instead, cut to the line with the circular saw, then use a hand saw to finish the cut. A better option is to drill a 1/4-inch hole at the corner of the notch and cut to the hole.
Other Factors That Impact Stringer Size
Things like the wood type, quality, design, span, number of stringers, and cost are also elements to consider when choosing lumber to use for your stair stringers.
While yellow pine is the most common wood used for stringers, it is possible to use hardwoods, such as oak, maple, walnut, ash, and hickory, all of which offer superior strength compared to standard pine boards.
They are also significantly more expensive, costing five times or more than the price of standard pine. Hardwood stringers only make sense if the stringers themselves will be exposed and you want to take advantage of the aesthetic appeal of hardwood.
Some companies also offer engineered wood that can be used for stringers, which also offer better strength qualities.
Span also makes a difference when it comes to selecting lumber size. A 2×12 stringer with a wider throat will need fewer supports than a 2×10 stringer with a 2.5-inch throat. This could make a difference if you have a functional open area under your stairs.
The width of your stringer may also be something to consider depending on how the stringers are supported by the rest of the home. Some stairs have two interior walls on either side, which means the stringer will have a significant amount of support from the house framing.
A set of stairs with one or two open stringers will not have the same support, requiring more strength. In these cases, it makes sense to use a 2×12 board for the stringers.
In some cases in which there is living space under the stairs, it may make sense to use 1 x10 stringer, which will give you an additional couple of inches of clearance for the headroom.
Quality is also an important factor to consider. Since wood is a natural building material, don’t expect all boards to have the same strength qualities. Look for boards that are in good condition with few knots when selecting lumber for a stair stringer. A knot that ends up in the throat of one of your notches will eventually dry out, potentially making the stairs structurally unsound.
It’s also important to note that there is a nominal cost difference when deciding whether to use 2X10 or 2X12 stringers. A 2×10 12-foot board will cost you about $20 at Home Depot, while a 2×12 12-foot board will cost you about $28. Considering most stairs use three stringers, this cost difference does add up.
What Size Lumber for Deck Stair Stringers
The biggest difference you’ll find between deck stair stringers and standard interior stringers is wood type. Deck stair stringers use pressure-treated wood versus standard wood. It’s important to understand that while this treatment makes the wood more resistant to weather and pests, it does not make the lumber structurally stronger.
Because deck wood is exposed to weather, which can cause wear that impacts the structural integrity of the lumber, most pre-cut pressure-treated deck stair stringers use 2×12 lumber for both long and short spans.
Cedar, which is also an option for deck stairs, is stronger and more durable than pressure-treated lumber. As such, pre-cut cedar stringers often use 2×10 boards.
Conclusion
Given how critical the structural integrity of stair stringers is, it almost always makes sense to err on the side of caution and go with 2×12 lumber. Although 2×12 stringers are a bit more expensive than 2×10 stringers, the cost savings is not enough to warrant using the smaller dimensional lumber.
Use 2x12s for your stair stringers unless doing so creates a problem with the clearance in the space beneath the stairs. In the case of a stairwell that is supported on both sides by walls, 2×20 stringers are adequate as they gain the additional support of the wall studs. For decks, use 2×12 stringers unless you’re building a cedar deck.