There is no problem in SketchUp, but when I take .fbx and upload it to a system, why doesn't the bottom part appear?

Top and bottom are shown in SketchHup.

When I load it in the application, the top part appears.

But the bottom part is not visible in the application.

Share your .skp file so we can see what you’ve got. From just your screenshots it would appear that the application you are importing into won’t display back faces but that’s only a guess based on almost no information.

BTW, please correct your profile. There is no 2022 Make version.

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9.skp (277.9 KB)

Can you write my mistake?

Since I can only guess at what the other application is and needs, I don’t know for sure. Try the attached file. I gave the road some thickness so there are no exposed backfaces. This should at least confirm that it’s an issue with the application not reading back faces.
9.skp (302.2 KB)

Again, there is no 2022 Make version. Please correct your profile as that information helps us help you. Also put in the right operating system.

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Some systems don’t fill in the back of faces. In the FBX export options you can export two sided faces. That may help.

Dave’s trick of adding thickness should also solve the problem, even without exporting double sided. That way does involve more geometry, but you can choose carefully only the areas that need thickness, instead of double sided, which is a bit like adding thickness everywhere.

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Yes, your solution is better, thank you.

Its intentional by design of the 3d software. Many/most 3d applications use the ‘front-face=visible’, ‘back-face = invisible’ as a way to speed up the rendering of the viewport. Rendering the bottom part of a road or the inner part of a wall would just take extra calculation time for the gfx chip. Its a ‘trick’ from the beginning of 3d graphics era.

A workaround in many programs is that you can assign double sided materials. These sometimes mess up the lighting of the object.

DaveR’s solution by giving the object some thickness creates new front-faces on the bottom and sides so the object is visible from any viewing direction but it will cost a bit extra performance. For AAA games that’s a no-go but for your use case it probably won’t be a problem.

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