thanks for your help - here is what i got from it:
a) solid element operations tend to get messed up - especially when you already have a slanted wall and you want to make another slant within it
b) trim to roof works the best almost no errors
c) that slanted small wall method does work

but only on straight walls (this is what i wanted) - doesn't work on already slanted ones
all in all trim to roof seem to be the best way to do it ... but this archicad has so much stuff to learn

especially if you want to work with non-straight walls
thanks again will be back with more questions