Building a safe set of stairs for a deck (or any structure) requires some carpentry skills, math, and accuracy. Sure, the first set of steps may be a ladder, but it’s not the easiest or safest method for accessing a deck, especially with an armload of grilling supplies, beverages, furniture, or children. If you’re wondering how to lay out a deck stair stringer, we’re here to help!
A stringer is often a 2×12 that connects the deck to the ground or another surface. It’s usually notched to accommodate steps at a uniform 7” to 7.75” step rise and 10” to 11.75” step run. There are often two matching stringers used to make a set of steps, but there may be more depending on stair width and load requirements.
In this guide, we’ll explain what a deck stringer is, its typical dimensions, and what the International Residential Building Code (IRC) requirements are for deck stairs. We’ll explore stair stringer spacing and how many stringers are needed, and how to measure them for a deck. Plus, we’ll discuss stair stringer calculators, the best lumber sizes for stringers, and explain how to lay out a stringer. Our aim is to provide you with the information you need to build deck stairs.
- What Is a Stringer on a Deck
- Typical Dimensions of Stair Stringers
- Deck Stairs Building Code Requirements
- Stair Stringer Spacing
- How Many Stringers Do I Need for Deck Stairs?
- How to Measure Stair Stringers for a Deck
- Deck Stairs Stringer Calculator
- What Size Lumber Is Best for Stair Stringers? 2×10 or 2×12?
- Stair Stringer Layout [Diagram]
- How to Lay Out Stair Stringers
What Is a Stringer on a Deck
A stringer or carriage is a structural component used to create an inclined plane or stepped ramp connecting one elevation level with another. It may be saw-tooth or zig-zag in profile or smooth but typically has a fixed slope range for ease of movement. A stringer is used to support stair treads uniformly spaced to ease the step up or down between ground and deck, deck and deck, or structure and deck.
Stringers are commonly made from pressure treated 2×12 or two 2x6s fastened together along their length. This ensures that there is approximately 5.5” of solid wood perpendicular between the stringer’s back and the narrowest point where the rise and run meet.
There are usually two or more stringers depending on the width and shape of the stairs. Typically, there’s one at each end of the step with others equally spaced in between if necessary.
The stringers support horizontal steps or treads made of pressure treated 2-by lumber or other materials. The treads may rest on and fasten to the horizontal saw-tooth profile of the stringer, or be supported on a bracket or board fastened to the inside face of smooth stringers. The stringers often fasten to the rim or band board and sit on pavers, blocks, the ground, or other material at the other end.
Deck stringers may be standard and flush mount too. A standard mount is fastened so that the last step is formed by the deck itself. A flush mount is where the top of the last tread is level with the top of the deck boards. There are advantages to both, but the flush mount is often easier to brace against and fasten to the rim or band board of the deck. Plus, it forms a sort of ‘landing’ if there is a gate to be opened.
The angle of the stringer with the deck often depends on the distance and desired slope. A common comfortable rise is 7 inches, while the run is often 10 to 12 inches, but can be more if desired. It is important, though, that the rise be consistent from top to bottom.
The distance between the deck surface and the ground or other surfaces, in inches, is divided by the desired rise to identify the number of steps or treads required. The run then determines the slope – the shorter the run, the steeper the slope, and vice versa.
Typical Dimensions of Stair Stringers
Building stairs begins with an understanding of basic terminology and some reasonably simple math. There are some specific measurements that are required before the stringers can be laid out and cut. Since the stringer supports the steps or treads, those measurements are also necessary for laying out the stringers.
It’s also important to decide on the type of staircase that best suits your requirements. The easiest is a straight staircase, but an ‘L’ shaped, ‘U’ shaped, curved, or spiral may work better. Since the straight staircase is the most common and easiest, we’ll focus on that design. Image 2 below may assist with the terminology too.
The stair riser is used to set the distance you must lift your foot up to advance to the next step. For safety and comfort, it is usually the same for all steps in a staircase. The riser is commonly between 7” and 7.5” but shouldn’t be greater than 7.75”. However, it can be less than 7” provided it is uniform for the entire staircase. The height of the stair riser is used to calculate the number of steps based on the distance of the stringer run.
The stringer run or length is the actual board length required to bridge the distance between the deck surface and the ground based on the stair angle selected. The greater the angle, the shorter and steeper the stringer run, and vice versa.
The stringer run is also dependent on its total rise from the ground to the deck surface, and the total run from the deck face to where the stair will meet the ground. It should be noted, though, that stringers that are standard mount are often longer than flush mount stringers of the same configuration for fastening purposes.
The total rise is the distance from the ground, deck, or landing pad where the stringer will land measured vertically upward to the deck surface or building’s floor. So, it isn’t necessarily the same as the vertical distance directly below the deck face from the deck surface.
It is often measured using a long level, board and level, or string level outward from the top of where the stairs will begin. The distance down measured at 90° from the level line to the ground or surface where the stairs will land, is the total rise. The total rise shouldn’t exceed 151”.
The total run is the horizontal distance the stringer must traverse. It is often calculated based on the depth of the treads and the rise between treads. So, if the rise is 7” per step, divide the total rise by 7 to determine the number of steps. If each step has a run of 10”, multiply that by the number of steps to identify the total run required.
A tread’s width is typically the long measurement of the rectangle, and the shorter side is the length, run, or depth. So, the total width the stingers must support is commonly measured from one end of the tread or step to the other along its long face. The stair width usually doesn’t include the handrail, while the total width may.
Deck Stairs Building Code Requirements
Building Codes often set minimal safety requirements for different aspects of construction and building. Many national, state, and local codes are based on standards set out in the International Residential Building Code (IRC) which is updated every three years. The most recent is the 2021 edition, however, many State and local codes aren’t updated as frequently, so they may be using an earlier edition. So, always check with your local building department.
The codes for deck stairs are similar to those for any residential stairs as they are considered a means of egress. Section 311 of the 2021 IRC addresses most aspects of stair construction requirements that affect stringer construction.
The landing on either end of the staircase should be the width of the door they serve or greater, and extend without obstruction at least 36” in the direction of travel (R311.3). Landings, decks, and stairs must be self-supporting or anchored to resist vertical and lateral forces (R311.5).
The vertical rise of a flight of stairs shouldn’t be greater than 12’-7” (R311.7.3). Risers shouldn’t exceed 7.75”, and the maximum deviation between risers within a flight can’t be greater than 3/8”. Additionally, open risers more than 30” vertically above the surface below can’t permit the passage of a 4”-diameter ball (R3126.96.36.199).
The tread depth shouldn’t be less than 10” and the maximum deviation between all treads also can’t be greater than 3/8” (R3188.8.131.52). The code also identifies where and how measurements should be done, thus negating confusion. The code even addresses nosings (R3184.108.40.206), the curvature at the tread’s leading edge, and how far they can protrude.
The width of a stairway is addressed in Sub-section R311.7.1, and identifies that the width needs to be a minimum of 36” and no narrower than 31.5” due to one handrail or 27” if a handrail is on both sides. This typically sets the spacing between stringers at 36” or less, although in some situations it could be as much as 40”.
Exterior deck construction, which includes exterior stairs, is addressed in Section R507. It identifies the type of material requirements and fasteners required for manufacturing all components of a deck, including those used for stringers and treads. Section R507.2.2 is especially helpful as it deals with the use of composite materials used for treads and other purposes.
Stair Stringer Spacing
Stair stringer spacing depends on several factors such as tread thickness and material, the width of the door they service, deck design and stair layout, and local building code requirements. The maximum spacing between stringers, according to Sub-section R311.7.1 of the 2021 IRC is 36”, although 40” is permitted if the tread thickness is nominally 4”. Most deck stringers and treads are manufactured from #2 grade or better pressure treated 2x12s or 2x6s, but metal, concrete, and other materials can be used.
Identifying the maximum spacing at 36” is for safety reasons based on wood strengths and potential loads the stairs could be exposed to. One toddler or adult going up or down the stairs at a time is much different from a group of adults sitting, standing, traversing, or dancing on the stairs for a party or picture.
Many builders often add an extra stringer or two to provide additional support and reduce bounce. Thus, making the stringer spacing 12” to 16.75” on center.
If using composite material for the treads or 5/4” decking, the spacing between stringers is usually 12” to 12.75” on center. This provides greater support since the thinner material is more flexible than 2-by lumber. It’s important to note, though, that most composite manufacturers provide detailed information online if using their products for stair treads and the appropriate stringer spacing.
The overall stringer spacing may also depend on the design. Curved stairs may require narrower spacing, and the stringers may sandwich the tread or support and be covered by the tread ends. Thus, making the spacing slightly different. Additionally, how railings or guards are attached can also affect spacing too. Always check the local building codes and manufacturers’ websites when designing stairs.
How Many Stringers Do I Need for Deck Stairs?
The number of stringers depends upon the width of the staircase and the type of material being used for the treads. The maximum spacing between stringers is 36” according to Sub-section R311.7.1 of the 2021 IRC. So, in most situations, only two stringers are required if the treads and stringer are 2-by material.
Stringers spaced 36” apart can be used for treads up to 48” long, which means the treads will overhang the outer stringer face by 5.5”. However, due to bounce, many builders use three or four stringers equally spaced apart for 36” wide staircases. This is especially true when using composite material or 5/4” lumber for treads.
How to Measure Stair Stringers for a Deck
There are several ways to measure stair stringers, however, all require the measurement of the total rise between the deck surface and wherever they begin or end. Whether the stringer will be flush mounted or standard/drop mounted will also affect the length of the stringer, so that needs to be decided in the design stage.
You’ll need to know the total rise of the stairs and the depth of the step run for most calculations. The first must be measured, the second is a design factor.
One simple method to identify the total rise is to measure the distance vertically between where the stringer begins and ends using a level of some sort and a tape measure. Most steps begin either level with the deck surface or one step rise lower.
Since the ground usually slopes away from the house, then the landing will often be lower than the ground directly below the face of the deck. Identify approximately where the stair landing will be and measure vertically up from there to a line level with where the stairs start. That is the total rise.
Since the individual step rise shouldn’t be more than 7.75” and is often 7”, dividing the total rise by 7” will provide the number of step-rise increments. Don’t round up or down as you want the step rise to be consistent. The result is the number of treads or steps required to comfortably ascend or descend the total rise. You may need to adjust the rise by a fraction to get a whole number.
If the depth of the treads is 10”, then multiplying by the number of step-rise increments will identify the total run length. If the tread depth is 11.25”, the actual width of a 2×12, then the total run length will be longer and the angle of the slope shallower.
To determine the stringer length or run, use the Pythagorean formula a² + b² = c², or (total run)² + (total rise)² = (stringer length)². Plugging in the total rise and total run, squaring each, and adding them together will give the stringer length squared. Finding the square root of the sum will provide the stringer length. If you don’t feel up to doing the math, there are several good online stringer calculators available.
Deck Stairs Stringer Calculator
Calculating the stringer length using a stringer calculator is quicker and easier and often makes multiple calculations simpler. Some calculators provide multiple variables, while others are more simplistic and easier to use. My calculator of preference provides an automatic mode and a manual mode. Both require the total rise to be inputted, but the automatic mode allows for the selection of the step rise, tread thickness, and depth to be selected from drop-down menus.
The manual requires you to input all four values. Using the calculator makes it easy to alter step rise for easier results too.
What Size Lumber Is Best for Stair Stringers? 2×10 or 2×12?
Both 2×10 and 2×12 lumber can be used for stair stringers. Many pros prefer #1 or better grade pressure treated 2×12 lumber for strength and look. Additionally, it is easier to maintain a minimum of 5.5” at the narrowest point between where the step rise and run meet and the bottom or back of the stringer. A 2×10 typically allows for 3.5” at the narrowest point, making it a weaker choice.
Stair Stringer Layout [Diagram]
Laying out stair stringers begins with two or more 2×12 planks to use as stringers. Stairs may have an open riser if the total rise doesn’t exceed 30”. If it does, then the risers should be solid or closed, or a partial riser used to ensure the opening won’t permit a 4” diameter sphere from slipping through.
A solid riser won’t alter the run calculation since the thickness added on one run is taken up by the riser on the next run. A partial riser also doesn’t interfere as it doesn’t lessen the usable tread depth. So, while the addition of a riser doesn’t change the calculated rise and run values, it’s important to double-check all calculations and measurements. It’s also helpful to draw the stringer plan to scale on paper and include all measurements.
If the rise is 7”, then the bottom rise needs to be 5.5” so it will be 7” when the 1.5” thick 2×12 tread is added on top. The rest of the unit rises will be 7” to accommodate the tread thickness. All the unit runs should be the same, typically 10” to 11”, which allows for a nosing depth of 1.25” to 0.25” depending on the cut depth, as seen in Image 1 and 2. The third image shows the finished product.
How to Lay Out Stair Stringers
Stair stringer layout is usually done after the total rise and the number of risers and tread length have been determined, and the stringer length or run calculated. Before laying out the cut lines, consider how the stringers will attach to the deck as it may require the stringers to extend beyond the top step. A flush mount doesn’t require extra material, but a standard/drop mount may.
Drawing the stringer plan to scale on paper with all measurements and how it will attach to the deck will help ensure all measurements are accurate and identified. With the plan in hand, begin laying out the stringer on your 2×12 plank. We recommend not cutting the board to length or making any cuts until the stinger plan is fully drawn on the wood.
A framing square is the tool of choice when laying out a stringer. The framing square is used to lay out each individual step rise and run measurements, as in image 1 above.
If the rise is 7” and the run or tread is 10”, mark those values with tape or another method on the inside face of the opposing arms of the square. There are stair gauges that can be attached to the arms of the square for easier adjustment and alignment.
The first step to lay out can be either the top or bottom step, however, make sure to leave enough space for the bottom rise or top tread. It’s a good practice to measure and mark a starting point to ensure enough board is available at either end.
Place the square on the plank at your starting end mark – if beginning at the top step the 7” marked arm goes at the starting mark, if at the bottom step, the tread or 10” marked arm goes to the starting mark.
Make sure the 7” and 10” marks are perfectly aligned with the plank edge so the lines will form a perfect 90° angle when cut out. Hold the square tight with one hand once it’s in place and draw the lines so they go all the way to the plank edge from the inside corner.
If you can’t hold the square so it won’t shift, have a helper do so or use clamps – even a small shift can throw the measurement off. Once the two lines are drawn, check their distance apart at the plank edge and compare it with the distance between the two marks on the square, they should be the same.
Slide or move the square along the plank in the necessary direction and align the mark on the appropriate arm with the line you just drew on the board. If going to the top, the mark on the tread arm aligns with the riser or shorter line, if working toward the bottom, the riser arm mark aligns with the tread or longer line.
Make sure the mark on the square arm intersects exactly with the end of the line at the edge of the plank. Hold the square firmly and draw the next set of lines. Repeat this process until you have the number of steps required.