Movement is the underlying cause
A flashing is the metal strip sealing the line where a slope meets brickwork, a stack or glazing. It sits outdoors, so it heats up on a bright day and cools overnight, and each cycle makes it grow and shrink very slightly. Lead is soft enough to take that for a long time, but not for ever.
The movement has to go somewhere. A piece that is free to slide at its lower edge relaxes with every cycle. A piece that is pinned at both ends cannot, so the strain collects in the metal, and in time a hairline crack opens where the strain is greatest, often at a fold or a corner.
That is why the cracks matter less than the question they raise: was the lead allowed to move? A crack that follows a bend across the flashing tells a different story from one running along a nail line.
Walls move too, if only a little. A chimney that has settled, a gable that leans or a bay that has shifted pulls on the lead fixed to it, and a flashing in the way of that movement is the first part to show it.
The fitting faults that cause cracks
Many of the preventable causes sit in how the lead was cut and held. In general terms they are these:
- Pieces that are too long. A long run has more metal expanding in one direction, so the strain is greater and the cracks come sooner. Short lengths with proper laps move without tearing.
- Fixings in the wrong place. Nails driven through the face of the lead pin it down and tear it as it moves. The top edge should be held in its chase, and the lower edge left free.
- Lead too thin for the position. A heavily exposed or hard-worked spot needs more metal than a sheltered one. The thickness is called the lead code, and it is set in the specification.
- Sharp folds. A tight crease is a place where the metal is already stressed, and a crack starts there first.
None of these shows on the day the job is finished. They show after a few winters, which is why asking how the lead was fixed matters more than asking how it looks.
Repair, patch or replace
Once a flashing has cracked, there are three routes, and the fair one depends on the cause.
A single crack from a knock or a stray nail in otherwise supple lead can be mended, as whether lead can be repaired explains. Cracks along the same run, torn edges or lead that has thinned are a sign that the whole piece has done its work. A patch on lead that has tired only moves the failure to the edge of the patch, and sealant over a crack does not stop the metal from moving.
Replacement means the old flashing comes off, the wall and roof edge are checked for damp, and new lead is cut in shorter pieces with laps, held at the top in a chase with wedges and left free to move below. The Royston Lead brochure says Code 3 is for soakers only. For a flashing the specification sets the code, and it should be written in your quote. Photograph the cracks before anything is touched, because a clear picture of each one helps whoever surveys the roof.
What to do while it waits
A cracked flashing seldom causes a flood, but it does let water in every time it rains, and a damp wall or timber is slower and dearer to put right than the lead was.
Meanwhile you can do a few useful things from the ground, without ever stepping onto the roof:
- Check the ceiling and wall beside the flashing after heavy rain for stains or a musty smell.
- Look for lifted or sagging lead with binoculars and note where.
- Keep gutters and downpipes near the flashing clear, so overflow is not forced against the lead and the wall behind it.
Do not climb up to smear sealant along the crack, however simple it looks from the pavement. Working at height on a wet slope is a real danger, and the sealant is a short-lived cover for a problem that needs a proper answer.
Smaller jobs are covered under lead flashing repairs. For the longer view of what a good job can deliver, how long lead flashing lasts is worth reading. The service overview is lead roofing.
