I just checked using roof accessories, roof surfacer, corrugated roof,
in AC 10 and had no problem making a zero pitch roof with a zero
pitch corrugated roof. I initially drew the roof at 45°, enabled the
roof surfacer, then changed the pitch of the roof to 0° and the
surfacer object followed the new pitch. No problem.
I checked the pitch of the object and it showed as 0°.
I do not know what is causing the problem you are having.
Peter Devlin