Flashing is the metalwork that seals a roof where it meets something else: a chimney, a wall, an abutment, a valley between two slopes. It is a small fraction of a roof's area and the source of a large fraction of its leaks, because every flashing is a junction somebody formed by hand rather than a component that arrived in a box.
The kinds, and where each one sits
Apron flashing runs across the front of a chimney or the head of a lean-to. Step flashing climbs the side of a chimney in pieces that follow the courses of the brickwork. Back gutter and soakers deal with the uphill side of a chimney and the sides of a slated abutment. Valley flashing lines the trough where two slopes meet, in lead, glass fibre or a preformed metal liner. Lean to roof flashing seals the top of a single slope against the house wall. Each is a different shape solving a different geometry.
How lead fails, and how mortar fails
Lead is the traditional British material and it fails in two ways: it fatigues and splits when a piece is too long and cannot expand, and it lifts when the mortar joint holding its upstand has weathered out. The second is far commoner and far cheaper to fix, because repointing the flashing back in is an afternoon rather than a re-dressing. Cement fillets used instead of proper flashing crack within a few years by design, and a roof with a fillet where a flashing should be will keep leaking until it is replaced.
Why a flashing repair is worth the scaffold
Flashings sit at the awkward places on a roof: beside a chimney, at a valley, against a wall. Reaching them safely is most of the price of fixing them, which means a flashing repair done properly on a tower or a scaffold looks expensive against the size of the metal involved and is still the cheapest leak on the roof to cure. It is also the repair most often done badly from a ladder with a tube of sealant, which lasts one winter.
Questions people ask about roof flashing
How long does lead flashing last?
Correctly sized and dressed lead is one of the longest lived materials on a roof and often outlasts the covering around it. What fails first is nearly always the mortar joint holding it into the wall rather than the lead.
Can flashing be repaired without replacing it?
Often yes. Raking out and repointing a lifted upstand, or dressing a lead apron back down, cures a great many chimney leaks without new metal.
Is flashing tape a real repair?
It is a temporary measure. Self adhesive flashing tape over a split will keep water out for a season and it does not solve the reason the flashing moved, so it is worth using while a proper repair is arranged rather than instead of one.