A roof is a system for moving water: shed it fast, overlap every path it might take, and seal the places where shedding isn't enough. Leaks happen where that system breaks down — and it breaks down in predictable places, because some parts of a roof work far harder than others.
The junctions fail first
The open field of a roof — plain tiles overlapping down a slope — is remarkably reliable. The hard-working parts are the junctions: flashings where the roof meets chimneys and walls, valleys where two slopes drain into one channel, and the mortar-bedded ridges and verges along the edges. These concentrate water, movement and weather at single points — which is why the majority of leaks we trace end at a junction, not a tile.
Then the materials age
Tiles and slates crack, spall in frost, or slip when their fixings corrode (tile replacement covers this). Lead fatigues along its fold lines after decades of thermal movement. Mortar — the shortest-lived material on the roof — cracks and sheds. And beneath everything, bitumen felt embrittles, turning near-misses that the felt once caught into ceiling stains (see how the underlay works).
And sometimes it isn't the roof
Two impostors round out the list. Blocked rainwater paths — gutters and valleys overflowing against walls — produce "roof leaks" that a clearing fixes (damp after heavy rain). And condensation produces wet lofts and stained ceilings with no hole anywhere (the full explanation). Between them they account for a striking share of leak call-outs — and neither is fixed by new tiles.
Using this when something leaks
Start with the symptom, note when it happens, and work the list junction-first — which is precisely what our leak diagnosis guide walks through. Or skip ahead: photographs to the contact page and we'll take it from there.