I believe to reduce the number of errors you got for unsolvable unions, the new CSG system offset parts by a fraction of the amount you can manually in studio. This can cause some visual offsets as you have pointed out, you could try going to File > Settings > Physics and enable DisableCSGv2. You may find that the union is unsolvable with the legacy CSG solver, but this is the only way of fixing it.
I’m not sure if CSG is appropriate for this case. Check Settings → Physics → ShowDecompositionGeometry. For a something like a staircase, which is large, somewhat complex, and demands fairly precise collision geometry, I think you’ll find that the collision geometry is not what you expect, and might not be sufficient for stairs. Why do you want to union the stairs in this case?
To answer your question, in general you can often avoid this problem by building your union subtractively from one large part, rather than additively from multiple parts. For a shape like this, it would be easy set up multiple negative parts to cut away stairs.
2 Likes
The real question is. Why does Roblox’s CSG system goof up so much. >.>
4 Likes
It’s purely for visuals. I replace the actual stair with a slope.
Negative parts might be the solution in this case, but that takes time.
Making a union of these stair parts to generate a slope-like collision is a horribly inefficient solution to this problem. This impacts performance so much more than if you were to just make the parts non-cancollide and add an invisible cancollide wedge over it. You really shouldn’t union parts together for that kind of reason (even better: don’t use unions at all if you can).
2 Likes
I hope this helps you 
You’re saying those misalignments are caused by CSGv2?
That’s exactly what I said. Add an invisible slope.
What’s the reason for making a union out of this in that case?
Yes, that’s it.
Then I wouldn’t agree with that at all. CSGv1 also had those issues.
I can say it’s caused only by Floating Point, by the looks of it.
Yeah, ahaha. I’ve had problems with it in the past, now I just ignore them. 
#MeshesForLife CSG is only good for bricks lol.
Yeah dude! I’ve been making lots of new meshes in Blender recently, although not top tier quality. 
1 Like
Keep going the more you learn the better and faster you’ll become!
1 Like
Thanks Veov
1 Like
No problem!
I believe that’s just one example. If you’d make more tests like these, you’d probably see that the version doesn’t affect it very much. Not sure though, maybe there is something with v2 that changes the shape or position slightly when unioning.
But I built a lot with v1, and I can tell from experience such misalignments happen mostly (if not only) because of number inaccuracies caused by Floating Point.
To get rid of the unnecessary triangles that are not visible to the player.