# Edge not separating faces

So I’m new to sketchup and am trying to create a city. I am trying to make the natural features like the river and shoreline first. However I keep running into the same problem which is driving me crazy. Whenever I try to create and edge to add on or separate an existing part into separate faces, it doesn’t do so and it acts as if the line isn’t there. I try tracing over it but there is no success. Sometimes for no reason it does work but I cannot make a determination as to what I did differently. Does somebody have a solution to this?

Generally that happens if there’s a gap in the edges or if at least one edge is not actually on the plane. Zooming in and looking at end points can help you identify gaps. Orbiting to look at the drawing from the edge can show edges that aren’t on plane.

Another thing to look for is that you have no overruns of edge segments.

Maybe if you share the model file?

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edges.skp (146.7 KB)

Here’s a systematic way to track down issues with the borders of a face that won’t form:

Draw a new edge between vertices on the opposite sides of the issue face, somewhere around the middle of the face (no need to be exact). There are three things that may result:

1. The face fills in on both sides of the new edge. This means the issue was that for some reason SketchUp failed to realize that the vertices around the perimeter of the face are actually coplanar and should have formed a face. Depending on how you drew the outline, this happens sometimes. If you delete the new edge, the full face should remain.

2. A face fills on one side of the new edge but not the other. This means the vertices to the side that filled are coplanar and gap-free, but the ones to the other side are not. Repeat the process starting with the middle of the side that didn’t fill.

3. No face formed on either side of the new edge. There are defects on both sides of the new edge. Pick either direction from the new edge and repeat the process from the middle of that side.

Eventually you will either have divided the entire face into triangles, at which point there will be a face for every triangle because triangles are necessarily planar. But reaching this point means the edges making up the original outline were not coplanar. Or else you will find a gap or stray internal edge that you can fix.

An alternative is to use the text tool to label the vertices. As you can see in this image, your outline is not planar - the z values vary.

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