Why Is Lead Staining My Roof Tiles?

Bright new lead does not stay bright. Within weeks of being fitted, its surface reacts with moisture and carbon dioxide in the air and forms a thin, pale crust of lead carbonate. That crust is what gives weathered lead its dull grey look, and once it has formed all over the sheet it protects the metal underneath.

The trouble is the stage before that. While the crust is still forming, the first heavy showers wash loose carbonate off the sheet and carry it down the slope. Wherever the water runs, it leaves a pale trail: across the tiles below a chimney flashing, down the face of a dormer cheek, or over the brickwork under a parapet gutter. Dark slates and brown concrete tiles show the marks most clearly, which is why the stains look worse than they are.

Run-off staining is a sign that the lead is doing what lead does. It is not a sign of a leak, a poor fit or a faulty sheet. If the marks are dark and damp rather than pale and dusty, though, something else is wrong, and our answer on white residue on lead flashing explains how to tell the two apart.

Patination oil is a thin, clear coating brushed or wiped onto new lead before the weather gets to it. It sits on the surface and slows the carbonate from forming and running, so the lead weathers gradually and evenly instead of shedding in the first few storms. The oil also stops the sheet looking blotchy and streaked while it settles.

It works best when it goes on at the time of fitting, on clean, dry lead, before the first rain. Lead that has already been rained on and has started to run is harder to treat, because the oil needs a clean surface to hold. The oil is a temporary aid rather than a lasting coat, and the lead still ages underneath. It buys time while the natural crust forms, and it does not need to be reapplied once the lead has weathered.

Ask whoever fits your lead whether they oil it, and whether they protect the roof covering and walls below while they work. On a roof where the stain would show badly, such as a dark tiled slope under a long run of flashing, it is a small step that saves a cleaning job later.

Start gently. Once the lead has weathered and stopped shedding, most carbonate marks wash away with clean water and a soft brush. Let the lead settle first, because cleaning a surface that is still being streaked only starts the job again.

Do not use a pressure washer on roof tiles. Marley, a tile maker, steers owners away from jet washing, since the force of the jet can scour the surface of a tile or slate and cut short its working life. Do not reach for strong acid cleaners either, which can mark the tile and stain the brick. Where marks have dried hard, a roofer can work on them with a soft brush and water from a safe position, rather than you leaning off a ladder.

Lead compounds should not be swallowed or breathed in, so treat the residue with care. Wet the area before brushing, rather than dry-sweeping it, wear gloves, wash your hands afterwards, and send children and pets indoors while you work. Rinse the water into a drain or a soil area you are not growing food in, and not into a garden used for vegetables.

Not every mark under a lead detail is carbonate. A brown or green tide mark on the ceiling below, a wet patch beside the chimney or a dark line of damp down a wall suggests water getting past the flashing instead of off it. That is a repair question, and the answer lies in how the lead meets the masonry and overlaps the tiles.

It is also worth looking at the flashing itself. Lead that has lifted from the wall, split along a fold or gaps where the mortar has fallen out will let rain in, and the water often shows up well away from the real fault. Our answer on lead flashing coming away from the wall covers the usual causes.

When we survey a roof with lead staining, we look at the lead, the tiles beneath and the ceiling inside before we say whether it is cosmetic. We send photos of the lead, the tiles and the ceiling, with a single written price for whatever needs doing. Full details of the trade are on our lead work page.