The third line is where the beam gives way. The line text states simply that the ridgepole is weak and gives evil, and the commentary on the image adds that no help can be brought to it. In a hexagram named for excess, this is excess arriving at the exact point already carrying the most and possessing the least give. Nothing in the line is doing anything wrong. It is bearing more than its span allows, and it has arranged matters so that no relief can reach it.
By the ordinary rule the stroke is entirely correct, a yang line in an odd place, standing at the top of the lower trigram Xun. That correctness is part of the difficulty. Firmness in a firm place, in the middle of four strong strokes, produces a member with no flexibility at all, and a loaded beam that cannot flex is a beam that snaps rather than bends. The only line answering this one from above is the yielding stroke at the very top of the figure, the slightest thing in it, so the support the position would normally draw on is not there.
Set beside the fourth line, which receives the same ridgepole imagery and the opposite verdict, this line marks the difference between a beam braced from beneath and one merely holding out. The text offers no remedy, and the commentary's remark that no help can be given is the reason. By the time the ridgepole is sagging, the correction had to have been made lower down.