Sheet Metal Flashings: Lead, Copper, Zinc
Roofs almost never leak in the middle of a slope: they leak at the junctions. Flashings are five per cent of a roof's area and ninety per cent of its leaks, and everything hinges on two things — letting the metal move, and never letting it touch the wrong metal.
Overview
TAV. 00Sheet metal work is the trade that resolves the points where the roof covering stops: eaves, ridge, chimney, rooflight, abutment wall, valley. These are all places where water concentrates and changes direction, and where an error of a few centimetres produces a leak that will appear metres away. The three historic metals have very different characters: copper is stable and long-lived but aggressive towards metals downstream of it; titanium zinc costs less and works well but tolerates neither contact nor restrained expansion; lead is the only one that can be dressed with a mallet over impossible geometries, and in exchange creeps slowly under its own weight if the pieces are made too long. Over all of them one rule governs: metal expands, and must be left free to do so.
The site score
TAV. HThe detail of the craftthe edge that decides where the water goes
The substrate and the levels
Checking the boarding is continuous, setting the eaves levels and gutter falls, and surveying the singular points.
Outer edge lower than the inner one. If the gutter has to overflow, let it overflow into the garden.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The rule that saves roofs
Which metal may sit above which
When two dissimilar metals touch in the presence of water, or when water runs from one onto the other, a battery forms: one of the two sacrifices itself and corrodes. The direction of the damage is not random and follows the nobility of the metals. On a roof this translates into an extremely simple practical rule: copper must never sit upstream of zinc, aluminium or galvanised steel, not even without direct contact, because run-off alone is enough. Stainless is compatible with almost everything, which is why it is the standard choice for fixings.
A qualitative scale as commonly used in roofing. Real compatibility also depends on the environment — marine air, industrial atmosphere — and must be checked against the manufacturer's guidance before bringing two dissimilar metals together.
Three metals, three trades
Copper, titanium zinc and lead compared
They are not interchangeable, and the choice is not only about cost or colour. Copper is the most stable and the longest-lived, and its green patina is a protection, not decay. Titanium zinc costs less and works very well, but it is the most sensitive to wrong contacts and restrained expansion. Lead has no rival on complicated geometries because it can be dressed cold like putty, but that very ductility makes it creep under its own weight if used in long pieces.
A qualitative comparison. The use of lead is subject to specific requirements on handling, protective equipment and waste management: these must be checked against current regulations before specifying it.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QWhy does a new gutter stain the facade?
Because in the first months the metal has not yet formed its patina and releases oxidation products that the water carries onto the wall below. It is especially evident with copper, which leaves green streaks, and with lead, which leaves a very visible white carbonate on pale surfaces. The remedy is applied beforehand, not afterwards: lead is treated with patination oil immediately after fitting, and in every case the drip edge must be designed so the water leaves the sheet instead of running down the wall. Once the stain is there, it fades but rarely disappears.
Can lead still be used?
On roofs yes, and it is still irreplaceable in certain junctions and in restoration work, because no other metal adapts to irregular geometry with the same ease. What has changed is the context: handling, operator protection and waste management are subject to specific requirements, and any contact with water intended for human consumption must be excluded — so never in a rainwater harvesting system for drinking use. In a specification it is worth checking current regulations before prescribing it, because this is an area that keeps being updated.
Can flashings be welded on site?
It depends on the metal and the location. Zinc and lead are traditionally soldered or welded, and it is a correct technique when carried out by someone who knows it; the problem is a different, conceptual one: a weld is a rigid restraint, and putting one where the piece needs to slide simply moves the failure point. The practical rule is to weld short pieces and details, and to entrust long runs to seams and expansion joints. On site, moreover, flame work over a timber roof has fire implications that must be managed, not improvised.
How big should a gutter be?
It is sized on the rainfall intensity of the area and the roof area served, not on appearance or habit. The most neglected point, though, is not the gutter section but the number and position of the downpipes: a generous gutter served by a single downpipe at the end of a long run still fails, because the outlet is the bottleneck. It is worth remembering that intense events have become more frequent: sizing on historic minima today means designing for overflow, which is why the lower outer edge matters so much.