Expansion joints are intentional gaps in long structures that allow the building to expand and contract with temperature without cracking everywhere. On roofs, these are often filled and sealed—but they still move a bit.
If you see visible cracks, crumbling filler or open gaps along these lines, it suggests the joint treatment has aged. Water can seep in there and travel along, appearing later as damp patches in top-floor ceilings or walls.
Spotting these early lets the society plan proper repair and resealing, instead of waiting until half the top floor is complaining about leaks.
