Why Precut Stair Stringers Are a Trap (And How to Calculate Your Own)
If you walk into a big box store or lumber yard, you will inevitably see stacks of precut stair stringers. They look like a gift from the carpentry gods—just buy them, slap them up, and you’re walking to the second floor, right?
Wrong. Unless you enjoy failing inspections and creating literal tripwires for your clients, it’s time to step away from the pre-fab aisle and learn how to cut a real stringer.
The Short Version:
- They assume perfect conditions: They are cut to a standard 7” rise and 11” tread depth, which only works if your floor-to-floor distance is perfectly divisible by exactly seven inches.
- They are mostly for exteriors: You can sometimes get away with them on a deck by adjusting the dirt grade at the bottom landing.
- They fail interior codes: On a standard interior build, using a precut stringer will almost guarantee your top or bottom step violates the building code for riser variance.
The “One Size Fits All” Delusion
Let’s look at the math of a standard interior staircase going to a second level. Assuming your floors are perfectly level (a hilarious joke, but let’s pretend), a standard framing setup looks like this:
- 92-5/8” studs
- Single bottom plate
- Double top plate
- 2×10 floor joists
- ¾” subfloor
Add that all up, and your total height between floors is 8’ 11-1/8” (or 107-1/8” for those of us who prefer to just read the tape).
If you use an “easy” precut stringer with a 7” rise, 15 steps will only get you to 8’9”. That leaves your top step exactly 2-1/8” taller than the rest of the flight. Congratulations, you’ve just built a catapult for anyone carrying a laundry basket.
The Inspector Will Laugh at You
Besides the obvious fact that an uneven top step is a massive safety hazard, building codes are completely unforgiving here.
The Code Reality (2018 IRC Section 311.7.5.2): The International Residential Code dictates that the greatest riser height within any flight of stairs shall not exceed the smallest by more than 3/8 of an inch.
That 2-1/8″ difference from your precut stringer? Grab your pry bar, because the inspector is making you tear it all down.
The 3rd-Grade Math Solution
Put down the pre-fab garbage, grab a pencil, and use a math technique they literally teach in elementary school. It takes two minutes to calculate your exact stair layout.
1. Find the total number of stairs Take your total rise (from the top of the subfloor/finished decking to your landing spot) and divide it by 7. Example using our 107-1/8” wall: 107.125 ÷ 7 = 15.3. Since you can’t build 0.3 of a stair, round to the nearest whole number to get 15 risers.
2. Calculate your exact riser height Now, take that total height and divide it by the whole number you just found. Example: 107.125 ÷ 15 = 7.14 (which translates to 7-1/8” on your tape measure).
3. Determine your tread count Because the top step is usually a riser down from the upper floor or platform, you have one less tread than you have risers. For this staircase, you only need to lay out 14 actual treads on your stringer.
Don’t Forget to Drop the Stringer
Here is the part where rookies really ruin a good layout. When you are marking the stringer with your framing square, you have to account for the material you are putting on it.
You must remove the thickness of your tread from the bottom of the stringer. If you forget to “drop the stringer,” and you slap 1” thick treads on your steps, you push the whole system out of whack. Your bottom step will suddenly become an 8-1/8” calf-burner, and your top step will shrink to a pathetic 6-1/8”. And just like that, you’ve busted that 3/8″ IRC tolerance again.
Test Your Math: Make a Template
Before you take a circular saw to an expensive piece of 2×12 lumber, use a scrap of plywood or cardboard to make a template. Trace your layout onto it, cut it out, and test the fit against your actual framing. It is significantly cheaper to throw away a bad piece of cardboard than it is to ruin a good piece of structural lumber.
One Final Pro-Tip: Trust Nothing
Before you even start making layout marks, check the elevation distance from the exact spot where the stairs will terminate.
Do not just drop a tape measure plumb down from the top step and call it a day. Concrete slopes, dirt lies, and floors sag. The grade where your stairs actually land might be a half-inch higher or lower than the spot directly below your header. If it is, you need to adjust your total rise before you calculate your first cut.
Stay organized, measure twice, and cut your own stringers. Your shins (and your local building inspector) will thank you.