I am struggling to add a face here as when I try to do it my SubD completely breaks…
I tried it to do with a mesh and it works fine.
mesh and subd.3dm (68.5 KB)
mesh and subd.3dm (68.5 KB)
Search in the web for “Non-manifold geometry”.
You can see your face or surfaces (brep or subd) as a “geometrical space limit” that divide “the inside” from “the outside”.
If you have an edge shared with 3 different faces, you’ll see that detecting “the inside” from “the outside” is no longer possible.
The fact you can do it with meshes is not good. It would be best if you avoid doing so even with meshes.
If an edge is shared between 3 faces, how the “subdivision”/smoothing of the SubD would occur? … impossible…
Okay… thank you for the insight Ricardo.
Strange case of not being able to do something virtually that is possible in reality 
How would you solve/model this? 3 faces coinciding is perfectly common in architecture.
I am using the subd due to ease of modelling plus smoothness and then convert to a mesh and apply a noise to imitate a 3D printed surface.
Not the best edge condition but at least I was able to close the hole.
Note, I was able to solve it this way for this specific case but I really need to solve the problem with subd geometry as it is part of a larger model.
Just to make sure, if you were using Symmetry, it has been removed?
Haven’t used it, so yes. I think it is what Ricardo said about no being able to create 3 edged geometry.
Yes, except you will find nowhere in architecture objects that are only 2D. Each, and every objects is 3D.
If you model this starting with a box and extrude sides you will be able to make this easily. Just make sure you ensure your faces have actual substance will help here.
I mean I can put 3 pieces of paper together in reality, sure.
Of course, but paper still has a 3rd dimension…
(very thin admittedly, but it is there).
Yes, technically, but it’s irrelevant. In real life, there’s no need to perform any mathematical subdivisions. You could stick three pieces of a 2D object together with glue or use any method to combine them. There are different ways to connect things in reality.
Here I am stuck with stitching points and the software does not let me do it.
paper is 0.1mm thick
a wall is 200 mm thick
to be fully fair dry walls basically are paper, filled with plaster though but still paper.
2D is a mathematical abstract concept that mainly helps to understand or calculate processes that are abstract and incomplete. though an area for instance is 2d and useful as such it still provides data for real 3D matter such as people or furniture or anything else. but in reality you can not glue 2d objects together which are fundamentally not real. one would be able to glue them together as a concept creating in your example something like an abstract 3dimensional 2d geometry like a non manifold merge in Rhino but what would that bring you for further processes, are you using it for display purposes or is it going to be manufactured? in the latter these abstract surfaces would have to become real, meaning you would apply some thickness to them.
subd is currently not made for such a concept like non manifold merge though it might be possible to redefine it it would probably defy the logic of it. surfaces that are adhered in subd are meant to converge into a smooth adaptation of 2 tangencies, if you would have 3 surfaces like in your case there would be no conversion possible since they would equal each other in both directions making a hard crease only. meshes and nurbs dont have those problem there such a merge is conceptionally more useful.
what you probably could do in your case is to open the seams where you want to add the face and duplicate it then stitch is back together like a loop but i have a hard time understanding what that would be good for to be honest.
here is what i meant:
flat display of the looped subd
smooth display
creasing applied
