Plenty of roofs behave perfectly through weeks of drizzle and then let water in during one bad night. That pattern is useful information. A leak that only appears in certain conditions is usually a defect that needs a particular combination of volume, wind direction or overflow before water can get past it.
Heavy rain only: a capacity problem
If volume is the trigger, the likely culprits are drainage rather than the covering itself. Gutters that cope with light rain overflow in a downpour, sending water down the wall or back under the lowest course of tiles. Valleys with debris behind them do the same. Undersized or partly blocked downpipes back up and push water sideways at the joints.
Wind driven rain only: a detail problem
If the leak follows a particular wind direction, the defect is almost always in a vertical or junction detail on that elevation. Lifted flashing at a chimney, open mortar joints on a gable, a cracked verge, or slates missing their nibs on the exposed pitch. In normal rain the water falls vertically and never reaches the gap. In a gale it is driven horizontally straight into it.
| Pattern you notice | Most likely cause | Where to look |
|---|---|---|
| Only in prolonged downpours | Gutter, valley or downpipe overflow | Eaves, valleys, rainwater goods |
| Only when wind comes from one side | Flashing, verge or exposed slate detail | That elevation, chimney and gable |
| Worse in cold weather, not rain | Condensation, not a leak | Loft ventilation |
| After snow rather than rain | Ice damming or snow driven under tiles | Eaves and shallow pitches |
| Started after a storm | Displaced covering | Whole roof, ideally photographed |
Why intermittent leaks get repaired twice
An intermittent defect is usually invisible in dry weather, which is exactly when the roofer visits. Nothing looks obviously wrong, so the nearest suspicious slate gets replaced and everyone waits for the next storm to find out whether it worked. Photographing the full roof in high resolution short circuits that cycle, because the actual defect is on record whether or not it happens to be raining.
What to record before you call anyone
- The date and rough wind direction each time the leak appears.
- Whether it starts within minutes or hours of the rain beginning.
- Photographs of the internal stain at its worst, with dates.
- Whether the gutters were seen overflowing at the same time.
That short record, combined with a photographic survey of the roof, usually narrows an intermittent leak to a single defect in one visit.