The short answer
The white or pale grey powder on lead flashing is usually lead carbonate, a natural crust that forms as lead weathers. Left alone it protects the metal underneath. It can streak the roof below in the first months, and a coat of patination oil on new lead slows it. Avoid dry-sweeping it, wear gloves, and wash your hands after touching it.
What the pale powder on your lead is, when it is harmless, when it hints at a problem and how to deal with it safely.
Why lead turns white
Freshly cut lead is shiny and silver. Out in the weather, the metal takes up moisture and carbon dioxide from the air and gradually turns dull, and in places a pale, chalky bloom develops on it. Rain, dew and condensation all feed the change. That bloom is lead carbonate. The trade calls the whole slow process weathering, and a sheet that has been up for some time usually settles into an even matt grey.
The white powder is most obvious in the early months, and on surfaces that stay damp, such as the underside of a flashing, a sheltered corner or a spot where water sits after rain. Lead in a damp, shaded gutter can show more than lead on an open, breezy slope, because moisture lingers longer.
In most cases the residue is a normal feature of the metal, not a fault in the flashing. It is not a sign the lead is failing, and a thin even layer is part of what protects the sheet. Where the powder is washed down onto the tiles or brick beneath, it leaves streaks, and our answer on lead run-off staining deals with those. A flashing that stays damp underneath, behind a gutter or beside a valley, can build up a thicker, whiter bloom than one that dries quickly, even when the lead is perfectly sound.
When the white deposit needs a closer look
Most pale powder is harmless, but a few patterns are worth a second look. Thick, heavy white crust that builds up quickly can mean the lead is staying wet, perhaps because water is trapped behind it or the flashing has no free fall for water to leave. A pitted or flaking surface under the powder suggests the lead is wearing away.
Check also for other signs next to it: gaps along the wall edge, lead that has lifted, splits at the folds or damp staining on the ceiling beside a chimney. These point to water getting in, and the powder is just the visible tip. Our answer on lead flashing coming away from the wall covers that group of faults.
If you can only see the flashing from the ground, take a zoomed photograph and compare it over a few months. A roofer can judge from the photo whether the white is a normal bloom or something heavier, and can check closely on a survey, with photos to show you what is there.
Patination oil and what it changes
Patination oil is a thin coating applied to new lead before it has been rained on. It slows the formation of the carbonate so the metal weathers more evenly, and so there is less loose powder to wash down the roof. It is a finishing step at the time of fitting, not a repair for lead that has already gone white.
On lead that is already weathered, there is little to be gained from oiling. The crust that has formed is doing the protecting, and removing it only exposes fresh metal, which will start to weather again. Resist the temptation to scrub a flashing back to shiny silver, as it will simply bloom white once more.
Do not paint or coat weathered lead with household products either, because they tend to flake as the sheet expands and shrinks. A bare, matt grey flashing is the finish the metal was meant to have, and it is what a well-weathered roof looks like.
If the bloom bothers you on a visible flashing, the answer lies in how the lead was treated when it was new, not in a coating applied years later. That is a question for the roofer who fits the next piece.
Handling it safely
Lead compounds are not something to eat or inhale. The pale powder is one, so a little care is sensible, though simply living under a weathered lead roof is not a cause for alarm. Never dry-sweep it into the air, and keep children and pets from touching or mouthing it.
If you need to clean an area, damp it first, wear disposable gloves, and use a damp cloth or soft brush. Wash your hands and any tools afterwards. Do not wash the residue into a vegetable plot or a place where children play. For more on how risky leadwork is, our answer on is lead flashing dangerous sets it out.
A roofer working with lead should handle offcuts and dust with the same care, and clear them away properly after each visit. We survey your roof first, take photographs of the lead, and name the price as one written total afterwards. If the residue is the only worry, send us a clear photo of the flashing and where it sits, and we will say whether it needs a visit. You can read more about the trade on our lead work page.
More on this in Lead Work & Lead Flashing in Crawley
Two more questions on white lead
Should I clean the white off my flashing?
Usually not. A thin, even bloom is the lead protecting itself, and scrubbing it off exposes bare metal that will bloom again. Clean only where run-off marks the tiles or wall below. Use plenty of clean water and a soft brush, damp the area first, and avoid pressure washers and strong acids.
Does white residue mean the lead needs replacing?
On its own, no. A pale bloom is normal weathering. It is worth a survey if it comes with splits, pitting, lifted edges or damp indoors, because then the lead or its fixing may be at fault. Close photos taken during the survey show which case you have.