If you have spent any time staring at the blueprints for our Alexis A-frame or the Frances cottage, you have likely run into the same headache every tiny house builder faces: the vertical trade-off. You want the loft—that cozy, separate sleeping space is crucial for keeping your sanity in 300 square feet—but you hate what it costs to get there.
A standard staircase is a space hog. To meet traditional code, a staircase needs a horizontal run that can easily eat up 10 to 12 feet of your floor plan. In a 20-foot house, that is half your living room gone just so you can walk to bed. The alternative? A ladder. But let’s be honest—climbing a vertical ladder at 2 a.m. to use the bathroom isn’t “rustic charm”, it’s a hazard. And good luck carrying a laundry basket up one.
This is where the alternating tread stair (ATS) comes in. It is the architectural sweet spot between a ladder and a staircase. It’s steep, it’s weird-looking, and it might just be the best decision you make for your build.
First, Let’s Kill the “Witch” Myth
You have probably seen these on social media called “witches’ stairs.” The story goes that early American colonists built them because they believed witches couldn’t walk up staggered steps due to their unnatural anatomy.
It’s a great story to tell guests, but it’s total nonsense. There is zero historical evidence for it. The truth is much more practical and frankly, more “Jeffersonian”. Thomas Jefferson was obsessed with efficiency and hated wasted space; he famously hid narrow stairs at Monticello to maximize his living areas. The alternating tread design is a direct descendant of that philosophy: pure utility.
The modern version was actually perfected for factories and oil rigs by a guy named J.M. Lapeyre in the 80s. He needed a way for workers to carry tools down steep descents without falling face-first. That’s the pedigree we care about—not folklore, but industrial safety.
The Biomechanics (Or: Why You Won’t Shin Yourself)
When you look at an ATS, your brain might glitch for a second. It looks like a staircase with missing teeth. But that negative space is the whole point.
On a normal steep stair or ship ladder, the tread above you is constantly in the way of your shin. To avoid banging your leg, you have to use shallow treads, which means you are climbing on your tiptoes. That feels precarious because it is.
The ATS removes the unused half of the tread—the side your foot isn’t using on that specific step. This creates a pocket of air for your shin to travel through. The result? You can place your entire foot firmly on a tread, even at a 60-degree angle. It allows you to walk upstairs essentially the same way you walk up a hill, rather than hauling yourself up rungs.
The Math: Planning the Cut
This is where the rubber meets the road. Designing an ATS isn’t like framing a wall; it’s a trigonometry problem.
First, you need to see if it’s even worth it for your space. Grab a tape measure and find your total rise (floor to finished loft floor). Let’s say you are working with a 108-inch rise.
If you plug that into a standard stair calculator, you will see that a standard comfortable stair requires a run of over 10 feet. That’s likely a dealbreaker. But an ATS operates comfortably at around 56 to 65 degrees. At that angle, you can slash that horizontal run down to about 5 or 6 feet. That is four feet of floor space reclaimed for your kitchen or a wood stove.
Once you commit, you have to do the layout. You take your total rise and divide it by your desired riser height (usually between 7.5 and 9 inches for these types of stairs).
Here is the trap: the math rarely works out to a clean number. You might end up needing a riser height of 8.379 inches. Trying to mark “.379” on a dusty 2×12 with a carpenter’s pencil is a recipe for a wobbly staircase. It is highly recommended to use a rounding calculator to convert those messy decimals into the nearest 1/16th of an inch. Consistency is everything here; if one step is off by a quarter-inch, you will trip on it every single time.
The Material: Keep It Heavy
At Pin-Up Houses, we talk a lot about Hygge—that feeling of cozy, sturdy contentment. A flimsy staircase is the opposite of Hygge.
Do not use 2x10s for your stringers if you can avoid it. Go for 2×12 kiln-dried Douglas fir or Southern yellow pine. You want meat on those bones because you are going to be cutting deep notches or routing out heavy channels.
For the treads (the paddles), skip the construction lumber. Softwood treads will dent, scratch, and eventually cup. Spend the extra money on 5/4 or 8/4 hardwood—oak, ash, or maple. Not only does it look beautiful against a painted stringer, but it holds screws better and feels solid underfoot.
The Code: Will the Inspector Laugh at Me?
Ten years ago? Maybe. Today? Probably not.
The building code has finally caught up to the tiny house movement. The 2021 International Residential Code (IRC) specifically addresses alternating tread devices in section R311.7.11.
Even better, if your state has adopted Appendix Q (the tiny house appendix), you are in the clear for lofts under 200 square feet. The code acknowledges that a 300-square-foot house cannot support a mansion-sized staircase.
To stay compliant (and safe), remember these three rules:
- Handrails on BOTH sides: You cannot just have a rail on the wall. You need rails on both sides to create a “chute” that guides you up and down.
- The 20-inch rule: You generally need at least 20 inches of clear width between those handrails.
- Open risers: Don’t box them in. You need that open space for your toes and shins to pass through.
The Fabrication: Dado vs. Cleat
You have two main ways to build this.
The easy way (cleats): You screw 2×2 wooden blocks onto the inside of your stringers and rest the treads on top. It’s fast, but it relies entirely on the shear strength of your screws. If you go this route, use structural screws (like GRK or Simpson), not drywall screws.
The right way (dados): This is for the woodworkers. You use a router to carve a 1/2-inch deep channel into the stringers for the treads to slot into. This locks the tread into the wood, providing immense structural support. Glue it, screw it, and it will never squeak or budge. It takes twice as long, but it lasts forever.
Living With It
We won’t lie to you—the first time you walk down an ATS, it feels weird. Your brain screams “ladder”, but your feet find a comfortable stair.
Because the steps are staggered, you have to start with the correct foot. If your first step is on the right, you lead with the right. If you lead with the left, you’ll end up doing a weird hop-skip. But here is the amazing thing: it takes about ten trips to develop muscle memory. By the third day, you won’t even look at your feet.
You’ll be walking down face-forward, carrying your morning coffee, looking out the window at your view, appreciating that you saved 15 square feet of living space without sacrificing your neck.
That is the essence of building small. It’s not about suffering through less; it’s about being smarter with what you have.
