How to make stairs with a flat bottom

Question

I'm trying to make a procedural staircase using an array and solidify modifier. I created two planes (a rise and a run) like this:

Then apply the array with a relative offset of 1 in the Y and Z directions and solidify everything into a staircase that looks like this:

I would like to be able to generate a watertight mesh after applying the modifiers with the underside of the staircase being one single face (instead of the zigzag I've found in every other tutorial). I may be going about this entirely wrong as I think like a Sketchup user but here is what I'm trying to achieve:

Is there a way to do this non-destructively where the rise and run, stair count, and flat bottom can all be controlled parametrically and independent of each other?

I had to apply the modifiers and still couldn't come up with a clean mesh for the example image above. I still couldn't come up with a clean topology, and it broke my mirrored staircase on the other side:

Answer 1

Here you are:

result:

node setup:

if you add a realize instance node before the output and apply it, you won't get one single face on the downside (sorry for that), but a pretty clean topology:

The step node just creates this:

a box and the two vertices will be just pushed on the x axis.

the mesh line instances the steps in z direction. With the indentation value i push the vertices and at the same time move the instances on the x axis so that it fits.

maybe you are asking: is this all it can do?

well...no...you can add e.g. a simple deform modifier, and you get this:

if you change the step node to this:

you can build stairs for your basement:

so with negative height you now get downstairs :D

Answer 2

It seems I'm a bit late, but here's my Geometry Nodes entry with all your criteria satisfied, including optional start/end steps and perfectly manifold geometry:

To get a manifold geometry I had to forgo using steps as instances and tried drawing the side section of the whole staircase edge by edge, and extruded the finished profile in one go instead:

Answer 3

Here's a non-node approach.
Create a subdivided plane as shown below

First join, with J, the verts cirlced then select, with B to make it faster, faces as shown

Remove the selected faces and Y-extrude