Josh Grant just answered a similar issue here
For quick reference:
From my experience, this is the best flow for making custom meshes that support player builds:
- Create a Blueprint and select the Building Prop option.
- In the Blueprint editor, select your mesh
- Enable ‘Is Player Buildable’
- Enable ‘Do Not Block Building’
- Enable ‘Structurally Support Overlapping Actors’
- Compile your Blueprint
- Drag your Blueprint in
- Build on it!