Stair with First 4 Treads Compensated, Then Normal

[Heftor]

Hello everyone,

I'm trying to model a stair where the first four treads are "compensated" (meaning the nosing is adjusted to fit a specific landing or winder condition), and from the 5th tread onwards, they are standard, equal treads, like the floor plan below.

I'm working in Archicad 28 and using the Stair Tool. I can successfully create the compensated treads and risers by dragging the nodes in the floor plan, but the problem is that the stair's underlying structure remains unchanged, which causes issues for the rest of the flight.

Is the best approach to model this as two separate stair elements? Or is there a way to define a "transition node" within a single stair object where the tread compensation stops and the structure recalculates for standard treads?

Does anyone has any experience in this?

Thanks in advance.

Hector

Operating system used: Windows

[Heftor]

Hi @Lingwisyer , @Barry Kelly, and all,

Thank you both for your replies and for linking the video. I appreciate the help.

I believe I’ve hit a more specific constraint. The techniques shown (and in the linked video) work perfectly for designing a new stair freehand, where you can adjust the boundary lines freely until it looks right. Like this one I just made

My situation is different: I am redrawing an existing stair from precise field measurements. I have fixed dimensions for:

• Total rise and run.

• The exact geometry of the compensated treads.

• The exact point where the stair transitions from this curved start to a straight run.

When I follow the standard method—adjusting the boundary lines while keeping the stair path (blue line) curved—the structure still fails to match the tread and the rise finish.

My Question: Is there a way to force the stair's structural geometry to strictly follow user-defined node placements on the boundaries, even if it creates a more complex or less "ideal" transition? Or, when documenting an existing irregular stair with precise dimensions, is the accepted workflow to:

1. Model the treads/risers with the Stair Tool for scheduling.

2. Then manually model the actual, as-built structure/soffit using the Morph tool?

I’ve attached a portion of my blueprint with the key dimensions highlighted. Any guidance on the best practice for accurate documentation (not just design) would be incredibly valuable.

Thanks again for your time.

Hector

[Barry Kelly]

---

@Heftor wrote:

My situation is different: I am redrawing an existing stair from precise field measurements. I have fixed dimensions for:

• Total rise and run.

• The exact geometry of the compensated treads.

• The exact point where the stair transitions from this curved start to a straight run.

When I follow the standard method—adjusting the boundary lines while keeping the stair path (blue line) curved—the structure still fails to match the tread and the rise finish.

---

Looking closer at your images, the dimension on the right side of the treads all appear to be the same ... 170mm.

If these are being generated from a curved stair structure where the treads emanate from a single centre point, this will not be possible.

As you can see in your last image.

So, this method only works if your treads emanate from a single centre point.

The equal treads in your example will all have separate center points (or pints of origin).

Rather than a circular or spiral beginning to your stair case, you can have winders with varying origin points.

Something like this, but I just don't know how to incorporate a winder into your stair.

I don't model a lot of stairs and when I do they are not usually curved.

At the moment I can only suggest modelling those angled treads from separate slabs.

Barry.

[Heftor · Accepted solution]

Hello again everyone, especially @Barry Kelly ,

I'm writing to close this thread with the solution and to thank everyone—especially Barry—for the crucial insight that solved my problem.

It was possible to model the stair correctly from a single center point, once I understood how to define it.

In my earlier attempts, I was adjusting the stair's boundary lines and nodes based on my 2D plan, but I was doing so without correctly setting the stair’s actual center of rotation. As a result, the geometry was inconsistent, and the 3D structure would not update properly—it remained as if the stair was still purely tangent, even when the treads looked right in plan.

Barry’s last comment made me realize I wasn't defining the problem correctly for the tool. I went back and properly set the stair’s center point in the stair settings—aligning it with the real, physical center of the existing curved flight. Once I did that:

• The treads automatically compensated correctly, I have to aproximate some measures.

• The thread finish updated perfectly.

• The transition from the curved start to the straight run was cleaner.

The key takeaway for others:
If you’re modeling an existing curved or partially curved stair and the 3D structure won’t follow your 2D adjustments, double-check the stair’s center point definition. The tool needs that reference to calculate the tread compensation and the structural soffit correctly. The boundary line edits alone aren’t enough—they have to work in concert with a properly placed center.

Thanks to @Yves for the video: https://youtu.be/8I4PLZBbuUc to correct the curve stair.

I’ve attached a screenshot of the final, correct stair, both in plan and 3D. The structure now matches the existing conditions perfectly.

Thank you again to everyone who chimed in. Sometimes the answer isn’t in fighting the tool, but in stepping back and making sure you’re giving it the right foundational information.

hopefully this helps someone else avoid the same head-scratching.

Best regards,

Hector

[Barry Kelly]

So glad you got it working.

I kept having trouble getting the 5th riser to be horizontal and the curved goings (1,2,3,4) to all be equal.

I guess it is a case of knowing exactly where all the critical points are.

Then I just remembered you can model a stair from top to bottom.

You know the top start point.

You know where riser 5 is.

You know where the centre of the curved treads is.

That is all you need to know, the last point of the stair is automatic.

So in 4 clicks (5 if you include changing the pet palette), you can place the stair.

Barry.

[Barry Kelly]

But as I look at this again, the riser between 4 & 5 is not exactly horizontal.

Maybe I still did not pick the exact points to get it all lined up properly.

It is close though.

Barry.