Lead Flashing in Coastal Air: Patina, White Corrosion and Wear

What healthy lead looks like

New lead is bright and silvery, and it does not stay that way. Within weeks it reacts with the air and moisture and forms a thin, tightly adhering film of lead carbonate, the patina. That film is a dull, slightly mottled grey, and its job is to protect the metal beneath from further attack. Once it has formed, the lead is largely stable, which is precisely why it has long been used on exposed coastal buildings. A flashing that has gone from shiny to matt grey has not deteriorated; it has finished weathering.

Nor does a rough or slightly furrowed look mean failure. Lead is soft, and a flashing dressed tightly into a stepped chase will show the marks of the tool and the contours of the brick behind it. Those marks show the lead was properly worked into place, and they sit comfortably alongside a sound patina.

Look for evenness. A healthy patina is uniform over the sheet, with perhaps a little darker staining where water runs. Slight variations in tone, ripples from the original dressing, and a faint chalkiness on a surface that has just been laid are all within the normal range. What you are not looking for is texture: crusts, flaking, pits, or a surface that rubs off on a finger.

White corrosion and where it comes from

White corrosion is a different thing from the patina. It forms as a thick, powdery or crusty white deposit, and is a mix of lead oxides and carbonates produced when lead sits in damp conditions without the air it needs to form a stable film. It is most often found on the underside of lead, where it touches a damp substrate, or on new lead that has been stored wrapped up and wet before fitting. Organic acids can drive it too, which is why lead laid directly on certain damp timbers corrodes faster.

On a stack, white corrosion matters because it eats metal. A heavy deposit on the underside of a flashing or back gutter, visible where the edge has lifted, suggests that moisture is being trapped there. That deserves a look. A light bloom on the top surface of new lead, which weathers off, does not. If the white streaks you can see run down the brick or slates from a flashing, they are usually only run-off from the surface film; our note on efflorescence explains how to tell that from salt. Do not scrub either off.

Wear that has nothing to do with chemistry

On most roofs, lead fails mechanically long before it fails chemically. Lead expands and contracts with every temperature swing, and a long run laid without joints or allowance for movement slowly creeps downhill and eventually cracks, usually in a line across the sheet where it was bent or fixed too tightly. Cracks that follow the fold of a step or the corner of an apron are the classic sign. They let water in even when the rest of the sheet looks fine.

Wind is the other mechanical force. On an open coastal frontage, gusts get under any edge that is not held down, and a loose edge will flap, work its fixings and tear. Look for flashings that stand off the brick, for edges curling upwards, and for gaps where the pointing meets the lead. Mortar that has fallen out of the chase above a stepped flashing is another cause of lifted lead. Compare these with the points in Lead Flashing or a Cement Fillet? How to Tell Which You Have, which explains why lead is generally the better detail at a stack than a cement fillet.

Thickness codes, in general terms

Rolled lead sheet is sold in grades known as codes, and each code corresponds to a thickness. Lower codes are thinner and lighter; higher codes are thicker, heavier and greater in weight. As a general rule, the thinner codes are used for smaller details such as soakers and flashings, and thicker codes for aprons, gutters and larger areas that carry more water or see more movement. Which code is right for a given detail depends on its size, its exposure and how it is dressed, and a good written quotation names the code for each part.

Thicker is not automatically better, since a heavy sheet can be hard to dress into a tight corner and can tear its own fixings if it is badly supported. The design of the detail, with proper laps, drips and allowances for movement, matters at least as much as the number on the roll. Our guide to What Should Be on a Chimney Repair Quotation? covers what to look for.

When you read a quotation, expect the code to be written down against each item, along with how the lead is to be fixed and where the joints and laps fall. A quotation that only says new lead flashing, with no further detail, leaves you unable to compare one offer with another.

The parts that fail before the lead does

Near the sea, the weak links around lead are usually the things holding it on. Steel or galvanised nails and clips corrode quickly and stain the lead and the slates below, then let go. Lead wedges, stainless steel and copper are used instead, which is the practice we follow on any stack in chimney repairs in Hoylake. The pointing over the chase and any mastic or sealant used in place of a proper joint are the other early failures.

The same applies at the head of the stack. A cheap cowl or guard in poor-grade stainless will rust and streak the brickwork, and the staining can easily be mistaken for the lead. Pots, cowls and guards are discussed in Chimney Pots, Cowls and Bird Guards: Which Do You Actually Need?. If you notice any of the signs above from the ground, take a photograph and send it; please do not climb up to look. Roofline Solutions is fully insured and gives free written quotations.