When the new roof was put on (about 20 years ago) the extension roof was switched from a flat roof to a pitched one coming down from the apex of the main roof. Of course, this left the builders with a gap between the top of the extension wall and the new roof (a half gable). They filled this in with concrete blocks, which was fine where the gap was above the main wall.
For the triangular section over the main roof though, they simply put a piece of wood in as a support and carried on with the blocks, pouring a lump of concrete in at the bottom as additional support. Obviously these bits of wood were not sufficient and had bent and flexed, causing the concrete blocks to move and the render to crack. Also, the concrete lumps were poured up against the old thatched roof and once this was removed we were left with some scary lumps of concrete hovering above the wall, supporting a load of heavy concrete blocks balanced precariously on a bent piece of wood!