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.
Roof flashing types: the kinds of roofing flashing, and where each 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.
Flat roof flashing at upstands and walls
On a flat roof the covering itself turns up the wall as an upstand. The flashing is the piece that covers the top of that upstand and tucks into a mortar joint above it, so water running down the wall is carried over the joint rather than behind it. Lead, a preformed metal trim or a cover strip in the roof membrane are all used.
Where flat roof flashing goes wrong
Three faults account for most flat roof leaks at a wall. An upstand finished with a bead of sealant and no cover over it at all. A cover flashing chased into a mortar joint that has since weathered out, so it hangs loose. And an edge or drip trim at the low side of the roof that lets water track back under the covering instead of dropping clear into the gutter.
How to fix roof flashing: fixing lead flashing back in when 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.
Chimney flashing repair cost: why the scaffold is worth it
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.
Step flashing roof details, apron flashing roof junctions and the names on a quote
An apron flashing is a single piece dressed over the junction where a roof meets a wall square on, such as the front face of a chimney or the top of a lean to. Step flashing is used where the roof slopes up a wall, so the lead is cut in steps to follow the mortar courses, usually with soakers under the tiles feeding water onto it. A back gutter runs behind a chimney to carry water round it. Cover flashing sits over an upstand to keep water out of the joint. If a quote names these, the contractor is describing the work; if it says flashing as a single word for a chimney, ask which of the four are included, because a stack usually needs three of them.
Why mortar fillets are the defect to look for
A mortar fillet is a triangular bead of mortar used in place of a proper flashing at an abutment. It is quick and cheap and it always cracks, because mortar is rigid and the two materials it joins move at different rates. It is one of the commonest findings on older British roofs and one of the commonest causes of a leak at a chimney or an abutting wall. Sealant smeared along the same junction is the modern version. Neither is a repair; both are signs that the correct lead detail was never there or has been covered over.
Roof penetration flashing: pipes, vents and rooflights
Every pipe, vent, flue or rooflight that comes through a roof needs its own flashing, because a covering cannot seal against a round object on its own. On a pitched roof that is a lead slate or a purpose made collar; on a flat roof it is a preformed collar bonded or welded into the membrane. Ones added after the roof was laid often leak, because they are sealed with mastic rather than flashed.
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.
Chimney lead flashing repair cost: can it 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.
How much does it cost to repair flashing on a roof in the UK?
Professional Roofers Ltd gives £200 to £600 for a flashing repair or repoint. Most of that is the cost of reaching it rather than the metal, because flashings sit beside chimneys, in valleys and against walls. Repointing a lifted upstand back in costs far less than re-dressing new lead. For scale, Arelite gives £300 to £700 for a basic chimney scaffold.
Is flashing tape a real flashing 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.
What is flashing on a roof?
The metal, usually lead, that weathers a junction between the roof covering and something else: a chimney, an abutting wall, a rooflight or a pipe. It carries water out onto the covering rather than into the joint.
Can flashing be repaired rather than replaced?
Sometimes, where lead has slipped from a chase or a single joint has opened. Where the lead is split from thermal movement because the lengths were too long, replacing it properly is the repair.