Sometimes walls disappear when trimming elements to roof, this might be due to the position of the trimming bodies in relation to the position of the walls to be trimmed and because trimming bodies are not displayed in default settings of ArchiCAD most users are not aware of their importance and their existence.
1. To start, go to the 3D window where you have walls and roof ready for trimming, by default trimming bodies are not displayed in ArchiCAD, to display them, go to View-On-screen View Options- Trimming Bodies as illustrated on the screen shot (i) below. Alternatively we may display them by going to Design-Connection-Trimming Bodies as per video at the bottom.
2. Trimming Bodies will be displayed with a "reddish" translucent colour and from the roof settings we may set them to project from the "Pivot Lines Down" ( Angle Point of the Roof) or from the "Contours Down" ( Edges of the Roof), refer to the screen shot (ii) below. For Walls to trim correctly they must be within the margin of the trimming bodies.
3. Alternatively, we may trim the walls to roof with an operation called SEO (Solid Element Operations), quite an accurate tool and does not require trimming bodies to trim correctly, to activate it go to Design-Solid Element Operations as per screen shot (iii) below. Then follow the Video for illustration of all the above mentioned steps.