if you already read the tutorial, maybe these pictures will help you to imagine what to do:
first, create some walls that will became your roofpanels.
since we don't have a side view in sh3d, you'll have to take the dimensions for the roof from the dimensions of the (sloping) walls, which is illustrated here:
the result, with textures etc., comes out as:
after creating the panels simply follow the export/reimport procedure as described.
the more complicated your roof is, the more work you got on your neck, i'm afraid.
for simple roofs however, this is a quite easy way to do it.
Good luck!
macfrog