Try placing a marquee around just a small portion of you model in plan and then view that in 3D.
If that works try different areas with the marquee.
I am wondering if the roof maker has created a very long element.
I am sure I have seen a post here (somewhere) about an extremely long element (due to tapering or trimming) causing a problem in 3D.
Barry.
One of the forum moderators.
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