Problem with ceiling using elevators

I'm making a cinematic that plays in an elvish library, and I'm using the elvators as a ceiling, but for some reason it won't work ingame : when a unit comes under the roof, it (the unit) suddenly becomes invisible. Could someone help me.
N.B.: I didn't use the normal elevator doodad but a custom one on which I made very minor changes :
- no doodad shadow (but changing it won't make the units visible)
- changed path texture (of normal and dead doodad) to "mushrom..."
- changed name to plafond (French for ceiling)
maybe I should try to change doodad classification from bridge to environment ...?

Try what you said.
Also, makesure"Is Walkable" is false. Then set the pathing to NONE.

OK, I changed walkable to False, I'm gonna try it tomorrow (too late now), hope it will work. Anyway thanx for help.

Yeah, what Zurg said ought to work. The unit isn't actually becoming invisible, its walking up onto the roof.