Flat trapezoid being automatically split into triangles

I’m a complete beginner to SketchUp and am having issues with shapes being split into triangles. I’ve worked my way through some tutorials and am aware of the need to group items and of Hidden Geometry view option. Knowing these things is not helping. I’ve also searched on this forum, and whilst I can see other people are having similar issues, I haven’t found a solution to the problem I am experiencing.

What I am trying to create are two halves of a form that could them be used to make a mould that fit together. The below image should give you an idea of what I am trying to do. The bottom half is made of 5 shapes, the top should only be 6. As you can see the shapes on top have extra triangles that I don’t want:

Open Flint

I’m starting by building my shape as a flat form in the plan template. This is the flat form for the top half:

Flat form

Using the move tool, I’m then moving each edge up the blue (z) axis. Regardless of whether I select to move on the edge or midpoint SketchUp is automatically splitting the trapezoid shapes into two triangles:

Split shape angle 1

Split shape angle 2

If I select Hidden Geometry view it shows me nothing, and group at this point won’t help as it will prevent me moving the shapes up like this (as far as I can tell). Is coplanar something I need to be thinking about? I’ve had a quick look but don’t understand it very much and thought that if the shape came up with the grey colouring it was coplanar?

Any help would be greatly appreciated. Thanks.

Can you share your .skp file? That would be a big help.

In your last screen shot you show an edge being moved upward and the resulting triangles. Since the edge you moved isn’t parallel to the opposite edge, the polygon has to be divided into triangles.

Instead of moving the outer edges, you should be rotating the faces using the edge between the inner polygon and the tab as the center of rotation.

It looks to me as if you need to make sure all of the outer edges are parallel to the inner ones and they are the same distance out.

Use Move tool + ‘Alt’ key

Thanks both for your speedy replies, the info has been really helpful. I’ve got round it in a slightly different way but the key to it does indeed seem to be parallel lines.

This time I drew the outer shape first to get a form I liked, then duplicated it, made it smaller, then placed the smaller shape within the original larger shape. This means I can still have something that looks fairly irregular but maintains parallel lines and its very simple to add in connecting lines between the shapes. I’ve then used @mihai.s way of dragging the centre of the shape up.


You might want to look at using the Offset tool. It would do what you did in one step.

Thanks Dave, tried that but the offset tool doesn’t allow me to make the internal shape small enough without loosing edges and this way I have more control over where the internal shape goes.


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