The short answer
The white staining is lead carbonate, a powder that builds up when untreated lead meets damp air and the carbon dioxide in it. Rain then washes it down onto slates, tiles, brick or stone. It does no harm to the lead itself and fades once the surface has weathered, but it can mark masonry. A patination oil applied soon after fitting prevents most of it.
What the white powder is
Freshly cut lead is bright and silvery. Within hours it begins to react with the air, and within days it dulls to grey as a thin oxide skin forms. That skin protects the metal beneath. The trouble starts when the surface stays damp for long periods, for example where condensation gathers on the underside of a sheet, or when rain runs over metal that has not yet formed a stable layer.
Under those conditions the metal takes up carbon dioxide and water together, and the product is lead carbonate, a soft white powder. It sits on the surface until rain carries it off, and wherever the water runs it leaves a mark. That is the streak you see on the face of a chimney below a new flashing, or in the gutter below a new valley.
Is it a problem for the roof?
On its own it is cosmetic, not a sign that the roof is failing. The carbonate is the lead protecting itself, and the metal underneath remains sound. Most staining appears in the first year or two and then settles as a stable patina develops, usually without anyone needing to intervene at all.
There are two reasons it can still matter. The first is appearance: light staining on dark slate, red brick or pale limestone is obvious, and it can spoil a new job. The second is where the run-off lands. Lead carbonate is a poor thing to have in a garden water butt, so if a roof drains into one, divert the early rainwater elsewhere. For a broader look at whether lead is a safe material to have on a home, read is lead roofing safe.
Preventing it with patination oil
The most reliable prevention is to treat the lead before it can form the powder. A patination oil is a thin coating, applied by brush or cloth, that creates an even, matt surface and stops the carbonate forming in streaks. It goes on once the lead is fitted and clean, and before the first heavy rain.
The oil does not make the lead look new. It gives a uniform pale grey that darkens naturally over the coming months, and it avoids the blotchy look of untreated sheet. Our page on lead patination covers the products, and it is sensible to ask us to apply it as part of the job rather than leaving it to chance.
The best moment is straight after the leadwork is finished, while the scaffold is still up and every surface can be reached.
Cleaning stains that have already formed
If staining has already dried onto stone or slate, go gently. A soft brush and clean water will lift fresh deposits. Acids, wire brushes and a hard pressure wash all damage masonry far more than the powder did, and old stone and soft brick can be pitted by aggressive cleaning.
Once the lead has weathered, the staining often stops by itself, and rain will wash much of what remains. For stubborn marks on an old stone chimney, ask before treating, because mistakes on historic masonry are hard to reverse. Listed and conservation-area buildings need particular care, so check with Swindon Borough Council's planning team before changing a surface in a protected setting.
Where it shows most
Staining is most noticeable where lead sits above light masonry: a stepped flashing against a limestone chimney, a lead-covered dormer over rendered walls or a bay roof above a painted sill. It is also common after a repair that puts new lead beside weathered lead, because the new sheet reacts while the old does not. A lead-clad dormer is a typical case.
Flat roofs are a second hotspot, since water can sit on them for a while after rain. If a flat roof is already giving trouble, lead flat roof repair explains what can be mended, and lead or zinc compares the other metal people consider. The wider subject sits under lead roofing.
Staining on slates, tiles and gutters
Run-off follows the path of least resistance. On a tiled roof it fans out across the courses below a flashing, and on slate it can leave pale tide lines at the lap. In a gutter the powder collects as a grey-white sludge, which is worth flushing out gently so it does not clog the outlet.
Treat any mark on roof coverings the way you would treat the masonry: soft brush, clean water, no harsh chemicals. Most of it disappears within a season of ordinary rain. If it does not, send us a photograph, because persistent staining can mean water is sitting on the lead instead of draining off it.
After a theft or a re-lay
When thieves have taken the lead and a full replacement follows, the new sheet will go through the same early stage as any other. Our notes on stolen lead and insurance explain how the replacement fits with a policy. Whatever the reason for the new lead, the plan for the finish should be settled before the scaffold comes down.
When we fit lead, we clean the sheet, apply a patination oil if you want it, and tell you what to expect over the first few months. After the visit you receive pictures of the lead and a quote on paper, and the guarantee on our work runs for 10 years.