A roof that stays dry for nine months can pour water in April because long-rains weather is different: more volume, longer duration and, crucially, wind that pushes water uphill along the sheet.
The usual suspects, in order
- Valleys choked with leaves and silt — water ponds above the blockage and laps over the sides
- Short side laps — fine for vertical rain, no match for wind-driven water
- Ridge caps fixed without closures or sealant — rain blows straight under the ridge
- Failed abutment flashings where the roof meets a wall, especially on parapets
- Worn screw washers that have hardened and cracked with UV exposure
Where the stain lies is a clue
Water follows purlins. A stain at the exterior wall often begins at a ridge several metres away. Trace the member uphill from the damp patch before sealing anything.
Before the rains
Clear valleys and gutters, check the ridge by hand for movement, and look up inside the ceiling after the first heavy downpour. Finding a drip in March costs a fraction of finding it through a ruined ceiling in May.