Dear Zita,

yue can go two methods:
1) I assume you have to use a little trick to get what you want:
The roof beam has to changed to 3D ( use "Convert Elements" , pic1) and than 3D to archtectural Elements.
But if you do so you'd better make a backup of all "frame construction" in case they will be needed as "frame construction" later again.

2) If you don't want to lose the attributes you should consequently use layers for the frame construction" Within the selrctional view all elements maintain the layers from the original 3D frame construction. (Of course you must not assign an layer to the clipped components in the settings for sectional view of clipping then).
After making the hidden line image you can select the lines with Filter of the corresponding Layer of the frame construction and adjust the "Pen thickness" in palette "properties".

regards
Undine