When space is tight and you need access to a loft, attic, or storage area, a complete guide to staircase design usually recommends conventional stairs with standard rise and run dimensions. But many homes have constrained floor plans where a full staircase simply will not fit. Alternating-tread ladder stairs offer a practical compromise, combining the steep angle of a ladder with the walking convenience of steps. Unlike spiral stairs or ship ladders, alternating-tread designs allow the user to descend facing forward, making them safer for regular use. This article covers the design math, layout techniques, construction methods, and safety considerations for building your own alternating-tread ladder stairs.
Understanding Alternating-Tread Ladder Stair Design
Alternating-tread stairs get their name from the distinctive pattern of the treads. Each step is wider on one side than the other, and consecutive steps alternate which side is wide. This creates a staggered climbing surface where the user places each foot on a full-width tread while the opposite foot swings past the narrow side of the same tread.
How the Alternating Pattern Works
When climbing an alternating-tread stair, the user faces the stairs and steps up with the right foot onto a tread that is wide on the right side. The next step uses the left foot onto a tread that is wide on the left side. The narrow portion of each tread provides just enough room for the opposite foot to pass without obstruction. This alternating sequence creates a natural climbing rhythm that feels more stable than a standard ladder.
Key Design Parameters
The critical dimensions for alternating-tread stairs fall within specific ranges:
- Stair angle: Between 50 and 70 degrees from horizontal. Angles above 70 degrees feel too much like a ladder. Below 50 degrees the stair becomes too long for tight spaces, defeating the purpose.
- Unit rise: Between 6.5 and 9 inches per step. The difference in height between the right and left treads is the unit rise.
- Tread depth (wide side): 7 to 10 inches of usable walking surface.
- Tread depth (narrow side): 3 to 5 inches, sufficient for foot passage.
- Overall tread length: 18 to 30 inches depending on the stair width needed.
- Number of carriages (stringers): Three for most residential installations.
Odd Number Rule
A critical design requirement for alternating-tread stairs is that the total number of unit rises must be an odd number. This is because on each side of the stair, every other tread is omitted. To have the same number of treads on both the right and left sides, one side must begin with an omitted tread and the other side must end with an omitted tread. This geometry forces the total number of rises to be odd. For example, a total rise of 79.75 inches divided by 11 gives a unit rise of 7.25 inches, which falls within the acceptable range.
Layout and Math for Alternating-Tread Stringers
Proper layout requires the same fundamental approach used for conventional stairs: accurate rise and run calculations, followed by careful story pole setup and stringer marking.
Calculating Total Rise and Number of Treads
Start by measuring the total rise from finished floor to finished floor. Divide this number by a target unit rise between 6.5 and 9 inches, then round to the nearest odd whole number. Divide the total rise by this odd number to get the actual unit rise.
Example Calculation
- Total rise: 79.75 inches
- Target rise: 7 to 8 inches
- 79.75 / 7.25 = 11 (odd number, perfect fit)
- Actual unit rise: 7.25 inches
- Total treads per side: 5 treads (one side gets 5, the other gets 5, with alternating omission)
Setting Up the Story Pole
Once the unit rise is determined, set up a story pole marked with the total rise in unit-rise increments. Use two different colors of ink or marker to distinguish right-side tread heights from left-side tread heights. One color indicates tread heights for the right side carriage, and the other color indicates heights for the left side carriage. The center carriage gets marks from both sides.
Marking the Carriages
Alternating-tread stairs use three carriages (stringers). The process follows these steps:
- Set the right-hand carriage and the center carriage in place temporarily.
- Transfer the right tread heights from the story pole onto the right carriage and the right-hand side of the center carriage.
- Set the left carriage in place and transfer the left tread heights from the story pole onto the left carriage and the left-hand side of the center carriage.
- Keep all carriages intentionally long during layout and cut the bottom edge last.
- Use the first completed carriage as a template to cut the bottom angle on the other two carriages.
Construction Methods for Alternating-Tread Stairs
Building alternating-tread ladder stairs requires precise joinery between treads and carriages. The most professional method is to mortise the treads into the carriages, similar to housed stringer construction.
Mortising Treads into Carriages
Unlike conventional stairs where treads sit on horizontal cuts, alternating-tread stringers have angled support surfaces. Each tread must be cut with a precise compound angle to sit flush against the carriage. Use a router with a spiral bit and a custom jig to cut consistent mortises at the correct angle for each tread position. The wide side of each tread must be clearly labeled during cutting to avoid confusion during assembly.
Cutting Treads
Each tread in an alternating-tread system has a unique shape. Cut all treads from the chosen material before starting assembly. Label each tread for its position in the stair. The tread material should be solid hardwood or engineered lumber rated for stair applications. Plywood is not recommended due to edge wear over time.
Assembly Sequence
- Dry-fit all treads into the carriages to verify fit before applying adhesive.
- Apply construction adhesive to each mortise.
- Insert treads and fasten with screws driven through the back of the carriage into each tread.
- Counterbore screw heads and plug with wood plugs for a finished appearance.
- Allow assembly to cure for 24 hours before moving the completed stair into position.
Comparison of Tread Attachment Methods
| Method | Strength | Appearance | Skill Level | Tools Needed |
|---|---|---|---|---|
| Mortise and tenon | Excellent | Clean, no visible fasteners | Advanced | Router, mortising jig |
| Dado and glue | Very good | Good, slight gap at joint | Intermediate | Circular saw, chisel |
| Metal brackets | Good | Industrial look | Beginner | Drill, wrench |
| Screws and adhesive | Good | Fair, screw heads visible | Beginner | Drill, countersink bit |
Safety Features and Code Compliance
Alternating-tread stairs serve best as access to unoccupied areas such as lofts, attics, and storage mezzanines. Most building codes do not require approved stairs for these areas, which is why a ladder-type stair is acceptable. However, safety features remain essential.
Handrail Requirements
Install a handrail on at least one side of the alternating-tread stair. For stairs wider than 30 inches, provide handrails on both sides. The handrail should be continuous from the top landing to the bottom landing, with a graspable profile that fits comfortably in the user’s hand. Mount the handrail at a height of 34 to 38 inches measured vertically from the tread nosing.
Landing Area
Provide adequate landing space at both the top and bottom of the stair:
- Top landing: At least as wide as the stair and a minimum of 24 inches deep.
- Bottom landing: At least 24 inches beyond the last tread in the direction of travel.
- Clearance: Maintain at least 80 inches of headroom at the landing areas.
Tripping Prevention
The bottom of the carriages presents a tripping hazard because they extend below the first tread at an angle. Plumb-cut the bottom front edge of each carriage below the first tread to minimize this hazard. Paint or stain the bottom few inches of the carriages a contrasting color to improve visibility. Designing safe and functional staircases requires attention to these small but important details that prevent accidents over the life of the stair.
Structural Considerations
The carriages must support a live load of at least 40 pounds per square foot for residential use. Use 2×10 or 2×12 lumber for carriages, selecting straight-grained stock free of knots in critical areas. Preventing stair stringer shrinkage is particularly important in alternating-tread designs because the angled mortise joints cannot accommodate much movement. Use kiln-dried lumber and acclimate it to the installation environment for at least two weeks before cutting.
For the site-built stair construction approach, all connections must resist both upward and downward forces. The alternating geometry creates torsion forces on the center carriage that are not present in conventional stairs. Use through-bolts or lag screws at critical connections, not just nails or deck screws.
When to Use Alternating-Tread vs Other Space-Saving Options
Alternating-tread stairs are not the only space-saving solution. Consider these alternatives and when each is appropriate:
- Ship ladders (50 to 70 degrees): Safer than alternating-tread but requires more horizontal run. Best for regular daily access.
- Spiral stairs: Elegant and space-efficient but difficult to move furniture through. Best for secondary access.
- Alternating-tread stairs (55 to 65 degrees): Best balance of space savings and usability for loft and attic access.
- Standard attic ladders: Folding ladders for occasional use only. Not suitable for daily traffic.
Alternating-tread ladder stairs provide a practical, buildable solution for accessing elevated spaces in tight floor plans. With careful layout, precise construction, and proper safety features, they serve reliably for years. Always check with your local building department before starting construction, and consider having your design reviewed by a structural engineer for installations over 12 feet of total rise.