Thanks guys for all the suggestions!,
I already tried making it a seperate object in scene mode, without any luck.. =(
The faces are the right way around too. So I started remodeling the corrugated part, first one 'wave', that gave a shadow (yay!), then i copied it and welded it to the first one, still worked with shadows.. after a while i had like 6 waves and added another 6, thats when it stopped giving shadows. So what I did is just have a few seperate pieces of 6 waves each and just place them next to each other, without actually joining the solids.
That gave almost a good result, still some artifacts, so I changed the lighting from a local to an infinite light source, and raised the corrugated part a bit from the underlying beam.. that solved the problem completely for my case.
I still don't know why it won't just work when i join the solids to one big piece..
Thanks for all the help though guys, a million more roof tiles to come so I hope anim8or can get rid of the shadow bugs sometimes