Help on extruding a non-planar curve onto another surface

Hi All,

As a new user of Rhino, I’m trying to create a staircase with half-of-circle return in the middle of the flight and its balustrades, shown as the image:

The question I have now after creating the balustrade is that there are gaps in-between some of the steps of the stairs and the balustrade itself. I’ve also created a interpolate curve at the underside outer points of these steps. What I intend to do is to create a surface from the interpolate curve to the underside of the balustrade and ultimately give this new surface a thickness to match the balustrade.

I’ve tried to loft and create new planarsrf and join all together but the result isn’t really what I wanted, shown here:

Thanks in advance.

Hi Rhyno - hard to say without the file - can you export the bits you are asking about and post the file here or to



Hi Pascal thanks for the reply.


Please have a look at the file that I attached here.
Stair exported.3dm (704.5 KB)


Hi Rhyno - I think what you need is to extend the vertical walss down a bit (ExtractSrf, ExtendSrf, maybe .3 or so) then trim the extensions back with the stairs.

Stair exported_PG.3dm (619.7 KB)


Hi pascal - thanks for the insight. Yes it worked as I’ve tried your method in my original model. Cheers!

Another question while I was doing the trimming - how do you select the upper side of the steps or the upper side of the zigzag line as the cutting object to trim the extended glass? I’ve extended down but found it only trimming the underside parts of the glass.

Thanks in advance again.

Hi Rhyno- for the outer edge, the easiest is probably to use the “CRV” selection filter to trim - start Trim with no selection and type ‘crv’ and . Then, click on the edges of the stairs - you’ll see only the edge curves light up as selected. Enter when you have them selected and then click to trim off the lower part of the surface.
For the inner wall, the stairs can be selected as is to trim with - this might even work for the outer one, I did not try, I used the crv filter to just use the edges.


Hi pascal - Great! Another thing learned.

Thank you so much!

Hi pascal - sorry forgot to ask. What do you think about why it won’t work as intended when I extend the curve to the bottom of the glass?

Hi Rhyno - I’m not sure what you tried exactly - adding a new surface to the bottom rather than extending, correct? From your images, it looks like the curves were too dissimilar that you tried to hook up make the added part. I guess the take-away here is to think of surface modeling as surfaces and not as volumes that can be arbitrarily extended - there are some important constraints imposed by surface modeling - the overall definition of volumes is by enclosing the volume with surfaces - it’s usually up to you to sort out how to best arrange the surfaces and to make sure curves are compatible. or remove extra internal faces if needed and so on.

You could have added surfaces to the bottom - not the ones you made, from the look of it, but made some other way - one difference compared to what I made is that there would have been a seam between the new surfaces and the original , with all of the potential for complication in terms of joining cleanly, having good continuity, etc. that come along with that. Making the surface over-sized but simple & trimming back to the complex edge just seems to be a cleaner solution.


Hi pascal - Thank you very much again for the advice & sharing your knowledge on rhino modelling in general. Really appreciate it!