If everything is modeled perfectly accurately and aligned, then those lines should not appear.
The probable culprit is your Wall on the right side on the Ground Story because it has issue on both its side and its top.
Check its position and its joints very closely, zoom in very closely to check it.
Also, display the Story Above as Trace Reference, make all Fill Transparent in the Trace Reference Palette and check that the corner on the ground Floor and the corner on the First Floor are exactly at the same point. zoom again very closely to check it.
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Hold down shift to constrain to the direction you are drawing in (and don't let go at last second to click). Check that the direction is on perfect 90 degree angles. I recommend setting working units to show at least one decimal, to clue you in as you are drawing if you are messing things up.
There is also a label on bimcomponents that shows if walls are at 90 degree angles or not.
Sorry to say: it is not a bug. Best thing to do is to get used to modelling precise and neat.
If you are tracing over 2d drawings to remodel in ArchiCAD, remember that these drawings can also have flaws.
Erwin wrote: Sorry to say: it is not a bug. Best thing to do is to get used to modelling precise and neat.
@ Dontknow Exactly as Erwin said. ARCHICAD did exactly what it is supposed to do. The walls did not align, and so the surfaces would not merge. The bug is on the human side.
Besides being sure that you accurately model by using shift constraints, snap lines, trace reference*, etc ... you can often model intersections more accurately by using commands if you're not sure of a snap point (bold vs light check mark).
In your example, select the two lower level walls and issue the Intersect command. Typically in the standard toolbar, I personally use it so often that I assign the shortcut "I" to it. Forces a clean "L" intersection. For "T" intersections, use the 'adjust' command (cmd-minus / ctrl-minus) to adjust a selected wall (or many walls) to the reference line of another.
*Trace Reference footnote: if what you are tracing is not modeled correctly, then your new elements will be off as well. The wall surfaces in this case would merge, making you think all is OK - but in fact, the walls will not be at a 90 degree angle to one another. So of course, only use Trace Reference if you are modeling accurately.
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Thnx for the info guys.
I found it a bit strange because is was my first project where I encountered the problem.
But I may asume that when I draw from corner to corner with a black pencil on my screen that it snaps to the corner............
Architectural construction designer, draftsman, modeller ArchiCAD 25.