Newbie Q: decreasing radius on spiral staircase?


Hiya, folks. I’m trying to build a spiral staircase within a decreasing radius tower (theoretical build, not actual architecture). The stairs are built along the inside wall of the tower with the central shaft of the tower remaining open. I’ve built the perfectly symmetrical stairs, but can’t seem to figure out how to gradually decrease the radius of the stairs toward the top. Thus, the stairs protrude from the tower’s exterior surface. (FYI, the base diameter of the tower is 60’ and the top is 50’, the tower height is 190’.)

Would love any input or suggestions on tutorials, etc., that might explain this in depth.



Tricky to do with the native tools, though possible with care and patience.

One way (haven’t tried this, though) could be:

  • make a tread
  • copy and rotate to make another at the correct angle, then move up by the rise height.
  • scale about opposite point (put a guide point at the centre of the spiral) in red/green plane, by a scale factor that you calculate to get you down to top radius after n steps.
  • repeat until done.

An easier way is to use a plugin. There’s Spirix, or a Helical ramp option in SU parametric 3d shapes from Sketchucation that would help, and give you a starting point to build the treads around. You can build a tapering spiral ramp easily.

Or search for Helix or Spiral in plugins on Sketchucation plugin store, or Extension Warehouse, with and without Staircase in the search term


I did a quick example using an Archimedes spiral from the Curvemaker extension and copied a step up the spiral using PathCopy. Both are available in the extension Warehouse.


I suggest Spirix plugin (Direct Download). Its pattern feature suits for this case.


Maybe this model will be of some use.
spiral steps.skp (559.0 KB)


I used my stairmaker plugin in conjunction with Fredo6 Taper


I created a plugin that will use a component definition and create copies along a conical spiral of sorts. The placement of each step maintains the step-to-step rise/run ratio as the radius decreases. The plugin only works in inches, I’m afraid.


Model: staircase.skp (65.6 KB)



Okay, you all are AWESOME! Thanks so much to everyone. Like I said, I’m a total SU newb, so it will take me some time to go through all your answers, and I’m grateful for the opportunity to do so.

Much appreciated, folks!


This topic was automatically closed 91 days after the last reply. New replies are no longer allowed.