Lead flashing: why it fails and what to ask of a quote

A pitched roof is watertight because each slate or tile sheds water onto the one below it. That run is broken wherever something interrupts the slope — a chimney pushing through, a wall butting into it, two planes meeting in a valley, a parapet at the front of a butterfly roof. Lead covers those breaks.

It is used because it can be dressed. A sheet is beaten over a wooden stick until it takes the shape of a tile roll or turns a corner in one piece with no joint, and it holds that shape for decades. The same softness is why lead comes with rules attached: a metal you can form with a mallet keeps moving once it is up there.

Which of two jobs a piece is doing decides how thick it needs to be.

Rolled lead sheet is sold by code, and the code is thickness. It is set by BS EN 12588, and every roll is colour-marked at the mill so it can be checked on site before anyone cuts it: Code 4 blue, Code 5 red, Code 6 black.

Lead is bought by weight, so Code 4 in place of Code 5 saves a little on the material for a chimney. The sheet has to absorb the same movement either way, and thinner metal has less of itself to spread that strain across, so the same run of hot days does more damage to it. Code 4 in a valley crazes and splits while the same detail in Code 5 is still sound. It is invisible from the ground once the roof is finished, which is why the code belongs on the quote in writing.

Lead expands about 0.03 mm per metre for every degree Celsius. A south-facing flashing sits at freezing on a January night and reaches 50 to 60 degrees under direct July sun. A 1.5 metre piece therefore changes length by roughly 2.5 mm across the year, and by more than a millimetre between breakfast and mid-afternoon on a sunny day.

Given somewhere to go, it slides a fraction and slides back, and does that for a century without complaint. Held at top and bottom, or pointed in solid along its whole length, the movement has to be absorbed inside the metal instead. Each cycle hardens the grain along the line where the strain concentrates, and after some thousands of cycles a crack starts there and runs along it.

That is why failed flashings split in a straight line — parallel to the wall, or across the middle of an over-long piece — rather than corroding into holes. The metal did not let you down. The way it was fixed did.

The rules all exist to give lead room. Pieces of about 1.5 metres, no longer. Laps of at least 100 mm, more at a shallow pitch. Fixed along the top edge only, wedged into the chase at about 450 mm centres, with clips holding the bottom edge against wind lift but never pinning its length.

Almost every early failure we lift off a South London roof is one of these six. All six were decisions, not accidents.

Two other mechanisms account for most of the rest, and both catch people out because the lead looks perfect from above.

The first is underside corrosion. Lead in a closed, unventilated space — the boxed-in top of a dormer, a lining laid straight onto boards — gets condensation on its cold underside. Add organic acids off unseasoned softwood or oak and the metal converts to a white powder from below, thinning until it perforates while the top surface still looks like new work. The prevention is air behind the sheet and a separating layer, so lead never lies directly on timber.

The second is alkali attack. Fresh cement and lime mortar are strongly alkaline and eat into lead in contact with them, so the leg buried in a new chase should be painted with bituminous paint first. It takes minutes and is routinely skipped.

There is also a cosmetic one. New lead sheds a white carbonate as it weathers, which washes down the wall and stains it. Patination oil wiped on the day it goes up stops that forming; cleaning the stain off soft London stock brick afterwards is a job in itself.

And there is theft. Lead has a scrap value by the kilo, and a bay roof, a porch or a low parapet is a target — common enough across South London to weigh up before new metal goes anywhere reachable from a garden wall.

Lead substitutes are usually a polymer laminate over an aluminium mesh, sold in rolls and fitted with adhesive and sealant. They cost less, have no scrap value and so do not get stolen, and on a simple straight abutment a well-fitted one lasts something like fifteen to twenty years rather than a lifetime.

The limits are real. They cannot be bossed round a complex corner or a curved dormer cheek the way lead can. They rely on adhesive and sealed joints, which are consumable parts. And they are not accepted on a listed building or, usually, on a visible slope in a conservation area.

So there is an honest split. On a garage, an outbuilding, a rear extension abutment, or anywhere lead has been stolen once, a substitute is a sensible answer and we will price it. On a chimney, a valley, a parapet gutter or a front dormer it is a false economy: the labour of getting up there and stripping out is most of the cost, and you pay it again sooner.

A cement fillet, a paint-on coating and a roll of flashing tape are not alternatives at all. Each covers a junction rather than weatherproofing it, and each buys a winter or two.

The jobs below assume the work can be reached from a ladder or a tower. Scaffold is a separate cost, and a bigger one where a whole elevation must be reached rather than a single stack, or where the scaffold stands on the footway and needs a licence.

Access moves the price more than the lead does, and after that it is how much has to come off to reach the detail — slates lifted and re-hooked, a chase raked out and repointed, a tile course reset. A stack needing repointing is a job before the lead job. None of that prices honestly over the phone.

A quote that says only 'renew lead flashing' with a number after it gives you nothing to check. Four questions make it comparable: what code, how many pieces, how long is each, and is the chase being raked out and repointed or the lead going back into the old joint. Ask a fifth — is patination oil applied — because the answer tells you how the rest will be done.

There is no VAT on our quotes, because we are not registered for it. Our quotes name the code and the piece lengths, and we would rather you put the same five questions to everyone else quoting.