How do I tell if roof flashing has failed?

Where flashings are and what failure looks like, how to inspect from the ground, and matching the internal leak to the external cause.

Difficulty: beginner
Time: 45 min
Read: 4 min
Safety: restricted

Short answer

Look with binoculars for lifted, split, corroded or missing lead where the roof meets a wall, a chimney or another roof. Cement fillets that have cracked away, lead that has slipped out of its mortar chase, and staining below an abutment are all failure signs. Repair is roofer's work.

Flashings are the weatherproofing at every junction — chimney, abutment wall, valley, roof window, soil pipe. Almost every roof leak that is not a slipped tile is a flashing leak, because junctions are where water is deliberately steered rather than simply shed.

Safety

This guide covers inspection from the ground, from a window and from a ladder at eaves level only. Do not go onto a roof. Flashing repairs at a chimney, an abutment or a valley are roof work carried out at height on a sloping surface, and belong to a roofer with proper access equipment. Falls from roofs are a leading cause of serious DIY injury and death. If the flashing has failed, get a roofer.

Step by step

Learn where the flashings are.Around chimneys (front apron, side step flashings, back gutter), where a roof meets a wall, in valleys between two roof slopes, around roof windows, and around soil vent pipes. Those are the leak points.
Inspect from the ground with binoculars.You can see far more than people expect. Look for lead that has lifted at the edge, a stepped flashing that has come out of its mortar chase, cracked cement fillets, and any patch of a different colour.
Look for cement fillets instead of lead.A mortar fillet where a flashing should be is a common cheap repair. Cement is rigid, the roof and wall move at different rates, and it cracks. A cracked fillet is a certain leak point.
Look for lead fatigue.Old lead that has been in place for decades develops splits, particularly at the folds and where a piece is too long without a joint. Lead flashings should be in lengths of no more than about 1.5m for this reason.
Check the pointing at the chase.Stepped flashing is tucked into a mortar joint and wedged, then pointed. Failed pointing lets the flashing pull out. This is visible from the ground on a clear day.
Match the internal stain to the external junction.In the loft, find where the water is coming in and work out what is above it. A stain at a chimney breast, at a party wall, or below a valley points straight at the flashing there.
Look after heavy driven rain, not after any rain.Flashing leaks often only show with wind-driven rain from one direction. Note which conditions produce the leak — it narrows down which face of which junction is failing.
Check the chimney itself while you are looking.Loose pots, cracked flaunching, spalled brickwork and open joints all let water into the stack, which then shows up inside and gets blamed on the flashing.
Get a roofer for the repair.Flashing work needs access to the roof slope, lead-working skills and the right materials. It is one of the clearest examples of a job that is cheap to have done and very expensive to get wrong.

Tips

A phone camera on a pole, or a drone if you have one, gives a good look at a chimney without leaving the ground.
Photograph the flashings when the roof is known to be sound. Comparison photographs years later are extremely useful.
Self-adhesive flashing tape is a legitimate temporary measure on a small split, and a poor permanent repair. It buys time until a roofer can attend.

Common mistakes

Repairing a lead flashing with a cement fillet — Cement is rigid and the junction moves. It cracks within a couple of years and then holds water in the crack against the wall. Lead is used because it moves.
Going onto the roof to look — Roof surfaces are slippery, tiles break under weight, and there is nothing to stop a fall. Everything you need to see can be seen from the ground with binoculars or with a camera on a pole.

If it doesn't work

Damp patch on a chimney breast inside

Cause: Failed flashing, cracked flaunching or a defective stack — Fix: Inspect the stack and flashings from the ground, then get a roofer. Internal damp at a chimney is nearly always coming in at the roof line.

Leak only in driving rain from one direction

Cause: Flashing on that elevation lifting or a gap at a step — Fix: Note the wind direction and inspect that junction specifically. Give the information to the roofer — it saves a lot of searching.

Green or white staining on the wall below a flashing

Cause: Water running down the wall from a failed junction — Fix: It marks the leak path. Follow the staining upwards to find the entry point.

Cement fillet cracked all along a roof abutment

Cause: Rigid mortar at a moving junction — Fix: It needs replacing with a proper lead flashing. Repointing the fillet buys perhaps a year.

Questions people ask

Can I repair roof flashing myself?

Inspection, yes, from the ground. Repair means working on a roof slope at height with lead-working skills, and it is not a sensible DIY job. Flashing tape can hold a small split temporarily while you arrange a roofer.

How long does lead flashing last?

Correctly installed lead lasts many decades — often the life of the roof. It fails early when the pieces are too long, when it has been fixed with cement instead of being dressed and wedged, or when the pointing at the chase fails.