A roof covering sheds water off a plane. Flashing is what deals with everything that is not a plane: a chimney, an abutment, a valley, a vent, the junction where an extension roof meets the house. That is why so few leaks are a hole in the middle of a roof and so many are at a flashing, and why the diagnosis matters more than the patch.
What flashing is, from lead to metal roof flashings
Flashing is a formed sheet that bridges a junction and directs water back onto the covering. Traditionally lead, dressed into a mortar joint and turned down over the roof, and lead remains the material most British roofs use at chimneys and abutments because it can be worked to shape and lasts a long time. Alternatives are lead substitutes, formed metal and, at the cheap end, a self adhesive tape, which has its place as a temporary measure and is not a repair.
A roof valley flashing carries two roofs of water
A valley is where two slopes meet and the water from both runs down one line, so the flow is far greater than anywhere else on the roof. Open metal valleys, lined valleys and tiled valleys each have their own failure: a split in the lining, debris damming the channel, mortar bedding cracking, or tiles cut so tight to the valley that leaves cannot pass. A blocked valley backs water up under the covering, and the stain inside appears several courses away from the fault.
Abutments and the lean to roof flashing
Where a lower roof runs into a wall, the detail has to stop water running down the wall from getting behind the roof. Done properly that is a two part arrangement: a soaker or upstand on the roof side and a cover flashing chased into the wall above it, with a cavity tray in the wall where there is a cavity. Rendering straight over the junction or relying on mastic is the commonest bad repair, and it fails within a couple of winters.
Why the stain is never where the leak is
Water entering a roof runs along battens, rafters and mortar before it drops, so a damp patch on a ceiling can be metres from the entry point and on the wrong side of the room. That is why a visit that simply seals the visible stain is wasted money and why the first job worth paying for is a proper inspection that works back up the path. Roof work is also the biggest single cause of fatal falls in construction, so access should be named in the quote rather than improvised.
Questions people ask about roof flashings
Is lead still the best, or is a metal roof flashing as good?
At chimneys and abutments it remains the standard because it can be dressed to shape and lasts. Substitutes are lighter and cheaper and are fine where they are detailed correctly.
Can flashing be repaired or must it be replaced?
A single cracked or lifted section can be re dressed and repointed. Where the lead is fatigued along a line or the detail is wrong, replacing the detail is the only lasting fix.
Is flashing tape a repair?
It is a temporary measure to stop water while a proper repair is arranged. It does not last and it hides the junction from the next person to look.
Why does my ceiling stain move?
Because water tracks along timber and mortar before it appears. The stain marks where it dropped, not where it entered.