When Designing stairs with a platform or landing in the middle, a common mistake is to first build a landing at an arbitrary level, then design and build the upper and lower staircases. Ninety-five percent of the time this results in the upper and lower staircases having noticeably different geometry. By analyzing your entire staircase first, then building your platform at the correct level, you will end up with matching flights.
First of all, let me say that stair-building is one of the most complicated aspects of carpentry (or ironwork), so DON’T RUSH. Rushing usually results in poor results and wasted lumber.
In summary, here are the steps (no pun intended) …
DESIGN A PHANTOM SET OF STAIRS FOR THE ENTIRE RISE (IGNORING THE PLATFORM)
CALCULATE A Rise Per Step THAT MEETS LOCAL CODE (e.g. 7 1/8)
BUILD A PLATFORM AT ONE OF THE STEP LEVELS (e.g. 21 3/8)
DESIGN THE UPPER FLIGHT, USING THE SAME Rise Per Step AS THE PHANTOM STAIRCASE
DESIGN THE LOWER FLIGHT, USING THE SAME Rise Per Step AND Run Per Step AS THE UPPER STAIRCASE
NOW YOU HAVE TWO FLIGHTS WITH MATCHING GEOMETRY, MAKING A COMPOUND STAIRCASE
In greater detail …
Most importantly, you want the Rise Per Step for both the top and bottom flights to be the same. Your local building code probably requires this, and even regardless of code, the stairs will look and feel better if Rise Per Step, Run Per Step and all the other figures are the same for both flights.
In order to ensure that you have equal Rise Per Step on both flights, first design a phantom set of stairs using your total Overall Rise like you’re making one long set of stairs instead of breaking it in two. (You don’t really need to worry about the Overall Run at this point.) Take your Overall Rise and divide it by your local building code maximum Rise Per Step (7-1/2 inches is a common value.) This tells you the number of steps you will need. Since you can’t have a fraction of a step, round this number up to get an integer, then divided your Overall Rise by this new number to get your calculated Rise Per Step.
Here’s an example:
84.5 Overall Rise
7.5 Building Code Maximum Rise Per Step
divide 84.5 by 7.5 = 11.27
11.27 is the ideal number of steps
round up to 12 full steps
now divide 84.5 by 12
7.04 this is your calculated Rise Per Step
You can now build a platform or landing for your compound staircase at a multiple of 7.04 inches, and both the upper and lower flights will have the same Rise Per Step. For example, if you build the platform at 21.12 inches (3 x 7.04), it would be three steps up from the bottom. If you build it at 35.20 inches, it will be five steps from the bottom.
Now, since the upper flight usually has space constraints, and the lower flight usually doesn’t, design your upper flight first. Use the same process for the Upper Overall Rise and you should end up with the same Rise Per Step (7.04). Calculate your Run Per Step and make sure to include some overhang for the treads. (I’ve developed a staircase calculator for this at: http://www.Shalla.Net .) Now use these same Rise Per Step, Run Per Step, Tread Size, and Tread Overhang to design your lower flight. Your upper and lower flights will now have matching geometry.
The key point here is that you have to build your platform at the right level in order to have matching upper and lower flights.