How to create a flat surface inside a curved polysurface


I’m using Rhino 7 for Windows, and I am not sure how to create a flat floor and solid surface on my polysurface geometry. I’ve tried creating a box and Boolean Difference, but it just erases the actual geometry. I can’t upload my actual rhino file to this topic as it keeps giving me an error saying try again :thinking:
I appreciate your help in advance!

I want to create the flat solid on top of this cut of my geometry:


If you still have the uncut geometry, you can draw a cutting plane on the height that you want to introduce the floor slab. This plane can then be used to intersect your base geometry, and will leave you with perimeter curves. Since, you seem to have a wall thickness, you’ll get an inner and outer curve. It’s up to you to decide which one you want to become your floor boundary. Delete the other one!
Then connect the open ends with straight lines thus closing the perimeter curve.
You can now extrude this closed, planar curve as a solid or first get a planar boundary surface and later extrude that one.

Thank you so much, it worked and I just trimmed the rectangular extruded surface to fit the curved geometry. I appreciate your help a lot! Thank you :pray:

You’re welcome. Glad it helped!