my 1 year old cathedral ceiling has cracks running down the seems at the top.i have a flat level area at the top of the ceiling made with 2x6(the flat area is 4 feet wide).i have noticed this problem on alot of ceilings like mine.i have R30 insulation,2x10 rafters and vent boards runnng down every bay.what causes this and how do i fix it(? stronger mud or tape etc...)