Curved Surfaces Intersection Issues


I am making a model of a room where two curved ceiling surfaces intersect each other. I am struggling with some strange things happening with the faces of these surfaces.

I made the model by first making the two key wall faces, and then pushing them the appropriate length through each other. Then I selected the two created ceilings and did “Intersect Faces” “With Selection”, which creates the edges between the two desired faces. However, once I do this, I see that there is one edge that is not drawn in, shown below.

Then, I delete all the unnecessary faces and geometry created from that, trimming it down to only the faces that should be the ceilings. However, once I do that, I am left with one face for the entire ceiling, which is not what I would like. The ceiling appears correctly, but ideally it would appear as two different faces, one for each curved surface.

Not sure what I’m doing wrong. If anyone could help me out, that’d be great!

Here is the sketchup file:
Curved Ceilings1.skp (201.8 KB)

If you look at your model with “Hidden Geometry” showing you can see how the ceilings are meeting --or not. If you just want an image with a complete valley, you only have to draw the edge that his softened / smooth. You will see however that somewhere in the process you have created asymetrical faces and in general the shapes are not meeting cleanly. To do an intersection that is “perfect” like this the two ceilings need to be identical in height width and radii, AND have the same number of segments. --essentially the crossing ceiling is a copy of the main one.
However it is possible to have two ceilings intersecting that are not the same size. The meeting line will not be perfect but using forms that fully cross over each other faces, and using “Interesect Faces”" they will intersect in a common set of edges. There will be “roughness” in the joint according to how fine or rough (how many segments) your arcs are–because the edges and faces cannot align.

Dave mentioned drawing the side arc “by hand”. I am not sure how, but I think the idea is you start each edge of the side ceiling at an edge of the main ceiling. I just did a model of a large wine cave and few of the intersections were equal. It works. You just have to be aware of the limitations of working without true arcs and building a model of faces, with the level of detail required for your project.

Here’s a way to draw the roof of the side extension so the vertices of the curves line up. First, I identified where the center of the extension’s roof meets the center of the main roof and I also identified where the sides intersect. I drew a rectangle between those lines. Note, since the width of the addition is different from the main length of the building, these rectangle is not at 45°. If the widths were the same, there’d be no problem with your intersection.

I intersected that rectangle with the main run of the building.

Then I extended lines out from where that rectangle crossed the edges on the main roof and connected them at the outboard end to get the faces.

Copy the half of the roof, flip it in the green direction and move it into place. Complete the outline of the rest of the extension and soften the edges.


Thanks Dave. That’s a cool!

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Dave you really are the best. Thank you so much. This was giving me a headache.

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