Fault diagnosis
Failed Flashings: Where Abutments and Chimneys Let Water In
Flashings fail where two materials that move at different rates meet, which is why the junction between a roof and anything vertical accounts for far more leaks than the open slope ever does. Compare the failed flashing against the fault it is most often mistaken for, then book a survey. On this page: Flashing · Mortar fillet · Cover flashing.
Flashing is the sheet material weathering the junction between a covering and something it meets — a wall, a stack, an abutment. Most failures are not the lead. They are what somebody used instead of it.
A cement fillet Cement bonded to both the brick and the covering cracks as the two move at different rates, and then it admits water.
Flashing is specified by lead code — a numbered thickness grade — and individual pieces are limited in length so thermal movement does not tear them. A thinner substitute fatigues and splits.
Where do flashings fail?
Flashings fail at six locations: chimney stacks, parapet walls, valleys, dormer cheeks, the junction where a rear extension meets the main wall, and around roof windows. Every one is a meeting of a roof and something vertical.
- A flashing means sheet material weathering the junction between a roof covering and an abutting surface.
- Cement bonded to both brick and covering cracks as the two move differently.
- The correct detail of the cover flashing is sheet let into a mortar joint, dressed down over the upstand, free to move.
Why do cement fillets crack?
Cement fillets crack because they bond rigidly to two materials that expand and contract at different rates. Brick and roof covering move independently through every temperature cycle, and the mortar bridging them has no capacity to accommodate it.
A soaker means individual pieces interleaved with each course at a stack side; a front apron is the sheet weathering the upslope face of a stack. A back gutter is the detail carrying water around the downslope face of a stack.
What does a correct flashing look like?
A correct flashing is sheet metal let into a raked mortar joint, dressed down over the covering, and cut into lengths short enough that thermal movement cannot tear it. At a stack it combines soakers, a front apron and a back gutter.
- Individual pieces are limited in length so thermal movement does not tear them.
- The failed flashing has a leak signature of appears in driving rain from the direction of the abutment, dry otherwise.
- Lead code specification: flashing sheet is specified by code; a thinner substitute fatigues faster
How do you know a flashing is the source?
A flashing leak appears in rain driven from the direction of that abutment and stays dry in rain from other directions. Recording wind direction each time damp appears localises the fault to a single elevation.
Sealant over a cracked fillet lasts a season and conceals the defect.
- Abutment locations: chimney, parapet, valley, dormer cheek, rear extension wall, roof window
- Differential movement cracks a rigid mortar fillet and opens a water path.
Can a flashing be sealed rather than replaced?
No, not durably. Sealant applied over a cracked fillet lasts a season and hides the defect beneath it. The junction has to be re-formed with sheet that is free to move, which sealant does not achieve.
- A cover flashing is let into a mortar joint and remains a free to move.
- An oversized lead piece tears under thermal movement.
- Mastic over a crack conceals the defect without restoring the detail.
The junction is a detail, not a material.
Why the same junction fails twice
A cement fillet is bonded to two materials with different movement rates, so cracking is inevitable rather than a sign of poor workmanship. Pieces are limited in length so thermal expansion does not tear the sheet; over-long runs split at the fixing. Bay-fronted districts see the cycle run fastest. Each projecting bay adds two abutments to a single house, so cement at the bay junction leads the repair list on Harborne's Edwardian terrace and villa coverings, where the fillets have already been renewed more than once.
Getting an abutment inspected
A leak appearing only in rain driven at one elevation places the fault on that elevation's abutment. Sealant smeared over an abutment is evidence of a previous failed repair and of a defect still present underneath.
Flashing, and 8 more
- flashing
- mortar fillet
- cover flashing
- soaker
- front apron
- back gutter
- lead code
- abutment
- upstand
7 symptoms route to their own diagnosis: A damp patch on an upstairs ceiling, Water coming in with no visible damage outside, Slates or tiles slipped out of line, Water spilling over the gutter, Damp in the loft with no rain, A ridge line that dips in the middle and Moss across the slope.
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