The single most expensive mistake with a roof leak is repairing the wrong spot. Water enters at a defect, runs along a rafter, a felt lap or a purlin, and drops through the ceiling somewhere entirely different, often a metre or more away and sometimes on the other side of the room.
Start inside, in the loft
- Go up during or just after heavy rain if you safely can, with a torch rather than the loft light.
- Look for the highest point of wet timber, not the wettest point. Water travels down, so the top of the damp trail is closest to the entry.
- Check the underside of the felt or membrane for staining, tide marks and drips at the laps.
- Look for daylight at the ridge, around the chimney and at valley boards.
- Mark wet timbers with chalk and check again after the next downpour to see whether the pattern is repeating or historic.
The usual entry points, in rough order of likelihood
| Entry point | Typical sign inside | How obvious from the ground |
|---|---|---|
| Slipped or missing slate or tile | Localised damp after wind driven rain | Sometimes visible, often not |
| Cracked or lifted chimney flashing | Staining down one side of the chimney breast | Almost never visible |
| Failed valley between two roof slopes | Long damp streak along a rafter | Not visible |
| Perished mortar at ridge or hip | Damp near the top of the roof space | Not visible |
| Blocked or split gutter overflowing | Damp at the top of an external wall | Sometimes |
| Cracked render or pointing above a flashing | Damp that appears in wind driven rain only | No |
Why the ground gives you almost nothing
From street level you are looking at the roof at a shallow angle, usually from one side, and the parts that leak most often are precisely the parts you cannot see: the ridge line, the valleys, the back pitch, the flashing detail tucked behind a chimney stack. Binoculars help a little. They do not show a hairline crack in lead or a nail sickness pattern across a slate field.
Leaks that are not leaks
Not every ceiling stain comes from the roof. Condensation in a poorly ventilated loft produces widespread, even damp with no clear high point, often worst in cold weather rather than after rain. Plumbing leaks are steady regardless of the weather. Testing the pattern against the weather is the quickest way to separate them.
When to stop guessing
If you have had two repairs and the leak has come back, the problem is diagnosis rather than workmanship. At that point a full photographic survey of the whole roof is cheaper than a third attempt, because it shows every defect at once rather than the one a roofer happened to notice from the ladder.