offset surface isn’t working…
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i know nurbs dosen’t support triangular faces,
is there a way to approach shapes like this in rhino?
There are many ways to do this. What is the result supposed to be like?
i just want to give the surface a thickness, this is the roof of a building
i generated the surfaces by doing curve boolean, so they are all trimmed surfaces, i am seeing that manually generating 3 points surfaces generates a better result, but still not perfect
One quick way to create the ‘pyramid’ is by using _ExtrudeCrvToPoint with the solid option
i tried that way but it dosen’t produce a clean outcome
Then _Shell for the thickness…
Make sure the bottom perimeter of the roof is a closed polyline.
Post your file if it doesn’t work
shell only works on a closed polysurface, i don’t show it in the example i gave before, but i need the bottom perimeter to be free, so i can’t create a solid out of it
but shell does create a clean outcome in the simplified situation
I don’t understand what you mean by free.
Again, post your file if it does not work.
i don’t have the problematic file with me at the moment,
but with free i mean that the bottom should be able to not be coplanar
Ok, understood.
So after _ExtrudeCrvToPoint you can create an offset surface and the result is closed.
The edge requires some clean up
this is what worries me, the project where i need to do this has a way more complex polysurface so it will be a mess to create this 
Does the thickness of the roof need to be exact?
Is the result going to be 3D printed? Maybe a mesh based approach could be used.
no i need to generate sections for an architectural drawing
i was just experimenting with meshes, and it does work, but it dosen’t create a 90° angle at the edge, in “reality” this would be made out of solid rectangular panels, with this workflow it generates prisms
i’m doubting that this is an achievable shape at all
Note that a solid offset of uniform thickness (i.e. like built from sheet material) that results in the panel edges meeting perfectly is not geometrically possible in most cases - the different dihedral angles will result in different width mating surfaces. This is a common problem oft discussed in architectural circles.
Many cases can be solved by allowing non-planar mating surfaces, but this requires 5-axis machining of the panel joints.
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To expand on the post by @Helvetosaur with a uniform thickness offset distance the resulting offset surface will be more complex than the original surface and the individual surfaces will not meet at a single point. The bottom edges will not have simple intersections. That is how the basic geometry works. It is not a bug or problem with Rhino.
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