The short answer
The white powder on new lead is lead carbonate, a salt that forms on the surface when bright lead meets moisture and carbon dioxide in the air. It shows up in the first weeks or months, most often in damp, mild weather or when the sheet was left unprotected before fitting. It is not a sign of poor lead or a leak. It is part of the weathering that eventually leaves a stable grey patina, though it can leave white streaks on brick and stone.
Why lead goes white
Freshly rolled lead has a clean, bright surface. As soon as it meets damp air, the outer layer starts to oxidise. That first film is thin and invisible. In the presence of carbon dioxide, which rainwater carries in small amounts, the oxide turns into carbonate. When conditions are right, the carbonate forms a loose, fluffy white deposit rather than a tight grey skin.
The worst cases happen when sheets are stored in a damp place, stacked tightly together, or left wrapped with moisture trapped between layers. The white bloom on those is sometimes heavy, because condensation has sat on the surface for days. Lead fitted in a dry spell and left to weather in sun usually shows much less.
Is it a problem?
For the lead itself, usually not. The carbonate layer does not eat into sound metal, and when it settles it forms a protective patina. If the bloom is thick, it may come off in a rainstorm, and the layer underneath continues to build up slowly. After a year or so the surface is generally an even, dull grey.
The visible problem is the run-off. Rain washes the loose powder down the roof, and it leaves a white stain on whatever sits below: brickwork under a flashing, stone coping, painted fascia or tiles. The marks are cosmetic but stubborn. Cleaning is easier in the first weeks, before the powder dries into the brick.
The powder is also toxic if swallowed, so it should not be brushed up into a cloud or left where children and pets play. Our page on whether lead flashing is dangerous covers the sensible precautions.
How it differs from the grey patina
It helps to know what good weathering looks like, so you do not worry about the wrong thing. Healthy lead goes through three stages. At first it is bright and metallic. Within a few weeks to months it dulls, and a matt grey bloom spreads across the surface. Over a year or two this tightens into an even mid-grey, which is firm to the touch and does not rub off on a finger.
The white powder is a different, looser deposit that can be wiped away with a dry cloth. If you run a finger over it and it comes off in a chalky smear, it is carbonate and not the stable grey layer. A powdery surface that keeps coming back means moisture is sitting on the sheet, and that usually has a cause worth finding.
Moisture trapped under the sheet
One cause that people miss is condensation underneath. On a flat lead roof or a gutter, damp air in the roof space can condense on the underside of the sheet, which corrodes the metal from below. White deposits on the underside of lead are a warning that the deck needs ventilation. If you notice them while the lead is lifted, tell your roofer, because they can change how the job should be done.
How to prevent it
Prevention begins before fitting. Lead should be stored flat, dry and off the ground, and never left wrapped in damp polythene. On the roof, patination oil brushed onto clean, dry lead reduces the early reaction and helps the surface weather evenly. Our page on patination oil explains how it is applied.
Design helps as well. A drip that throws water clear of the wall, a flashing that sits close to the masonry and a short sheet length all reduce how much run-off lands on the brick. Where a stained wall would be conspicuous, the area beneath can be masked during the early weeks.
What to do if it has already appeared
Do not scrub it hard or pressure wash the lead. Water jets drive powder into the brick pores and can force water behind the flashing. If the deposit is light, wait: rain and time often clear it. If it is thick, brush it off gently while dry, wearing a mask and gloves, and collect it in a bag. Wash down the stained brick with plenty of clean water, then leave it to dry.
If white deposit keeps forming after the first season, ask why. It can mean the lead is permanently wet, because a gutter is blocked, a sheet is bedded on damp timber or a flashing is holding water. That is a detail worth looking at, and we check it during the survey. The rest of our lead services sit under wider lead roofing services.
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