Who this guide is for
- Owners replacing flashings, gutters or fixings on an existing building
- Readers specifying a metal roof, cladding or rainwater system
- Renovators adding a new metal component to an elevation that already has one
- Anyone in a coastal or industrial setting where exposure is severe
- Readers preparing questions about compatibility before components are ordered
Contact is a question about the position, not only the pairing
The material records consistently qualify contact by what the joint is like. Galvanised steel is listed as needing review where the zinc surface would be in contact with a dissimilar metal with moisture present. Stainless steel is described as needing review where it will be in direct contact with another metal in a wet position without assessment. Aluminium is described as sensitive in contact, with wet cement-based materials, some timbers and other metals in damp conditions all able to affect the surface, which is why isolation appears as a standard part of the detail.
The condition that recurs in all three is moisture at the joint. The galvanised record adds that the places worth designing out are the ones that hold water against the coating, such as ledges, tight laps and unventilated hollow sections, and that where sections lap or bolt together the joint holds moisture for longer than the open face does.
- A joint that drains and dries is a different condition from one that holds water
- Laps, bolted connections and hollow sections stay wet longest
- Isolating components at a contact are part of the detail rather than optional
- Whether a pairing is acceptable is assessed case by case, not by family
Coatings are interrupted where the work is made
A coated metal arrives protected and is then cut, drilled and fixed. The galvanised record separates the two production routes for exactly this reason: dipping after fabrication coats everything the zinc reaches, including cut ends and holes, while coil coating protects the faces but leaves anything cut afterwards bare. It adds that coil-coated sections are bare at every cut end, and those ends are often where rust first shows.
The assemblies made this way say the same from the other side. The sheeted metal wall record notes that the wall is built by cutting and piercing coated metal on site, so what happens at those points is a property of the installed assembly rather than of the delivered sheet, and that every cut edge, hole and fastener is a point where the coated system has been interrupted. The light steel frame record makes the consequence positional: because the frame is thin, the loss of protective coating at a cut, a drilled hole or a dissimilar-metal contact matters more than it would on heavier steel.
The coating and the metal underneath are different questions
Not all protection works the same way, and the difference decides what a cut means. A zinc coating is described as a separate layer that is consumed over time and is simply absent wherever it has been cut through, protecting small scratches and cut edges sacrificially for a time. Stainless carries its resistance right through the section, so a cut end behaves much like the face, with the trade-off that stainless is a family of alloys with different resistance and choosing within that family is a specification decision.
Aluminium sits differently again, with its own oxide film usually finished further by anodising or by powder coating, and the records note that coating damage can sometimes be touched in while damage to an anodised surface cannot be repaired in place in the same way at all. What a cut edge means, therefore, depends on which kind of protection is present.
Run-off carries the question downstream
Contact is not the only way two metals meet. The copper record states that rainwater running off copper carries traces of the metal with it, which is why staining of surfaces below, and the choice of metals downstream of the run-off, are both design questions. It asks where run-off from the roof goes and what surfaces it passes over on the way, and whether run-off from another material lands on the copper from above.
The other records ask the same question in their own terms. The galvanised record asks whether run-off from another metal higher up the building will land on this galvanised surface. The stainless record asks whether run-off from another material will land on the stainless surface and leave deposits behind. The seamed metal roof record puts it as a design question of its own: which metals will the run-off from this roof pass over before it leaves the building.
- Run-off carries traces of the metal it has run over
- The path matters, not only the point where two components touch
- Gutters, hoppers and downpipes are part of the same path
- Both directions count: what lands on a surface as well as what leaves it
Fixings are the component most often chosen last
The records single out fasteners repeatedly. The galvanised record notes that bolts, screws and washers are frequently a different metal from the component they hold together, and that fixings and washers are chosen alongside the component rather than picked up on site. The stainless record notes that stainless fixings and stainless sheet do not automatically come from the same alloy family.
The assemblies inherit that. The tiled covering record states that the metal of nails, hooks and clips has to be compatible with the covering and with any flashing metal it will sit against, or corrosion becomes the failure route, and asks who is confirming compatibility between the fixings, the flashings and the rainwater goods. The light steel record notes that contact between the sections and an incompatible metal fixing can shorten the life of a frame that will never be inspected again.
Contamination that looks like corrosion
Not every rust mark is a failure of the component it appears on. The stainless record states that rust marks on stainless are often contamination from other steel rather than the stainless itself, and that grinding dust from nearby steel work settles on stainless surfaces and stains them later. It describes particles of ordinary carbon steel left behind by tools or grinding dust as making a sound component look as though it is corroding.
Other appearances are equally easy to misread. White deposits on newly delivered galvanised work are described as usually coming from wet storage rather than from failure, and a brown discoloration on stainless in coastal or polluted atmospheres is described as something to assess rather than to assume in either direction. Who assesses whether a discolouration is cosmetic is a question the record asks directly.
Exposure and washing change the answer
Where the components sit changes what the compatibility question is being asked against. Coastal, industrial or chemically aggressive atmospheres are named as needing assessment in their own right, and chlorides from sea air, de-icing salt or pool atmospheres are described as able to attack a passive film locally. Splash from road salt, pool water or run-off from another surface appears in the exposure questions of more than one record.
Rain washing matters as much as exposure. The galvanised record states that the coating is used up faster in sheltered damp corners than on faces that rain washes clean, and the stainless record that sheltered surfaces which rain never washes can perform worse than fully exposed ones. That makes the sheltered, hidden joint the one worth asking about rather than the one that looks most weathered.
Questions about metals meeting on an envelope
- 1List every metal on the elevation or roof, including fixings, trims and rainwater goods
- 2Ask which of them touch one another, and whether that joint stays damp
- 3Ask whether an isolating component is included at each contact
- 4Establish how each component is protected, and whether that protection goes through
- 5Ask what happens at every cut edge, drilled hole and welded area, and who does it
- 6Check whether coated sections are being cut after coating
- 7Trace the run-off path and note which metals it passes over before it leaves
- 8Ask whether run-off from anything above lands on a metal below it
- 9Establish whether fixings were selected alongside the component or separately
- 10Ask whether the fixing alloy matches the sheet it is holding
- 11Establish what the exposure is, including salt, pollution and pool atmospheres
- 12Identify the joints that never dry, and treat those as the ones to ask about
- 13Ask who confirms compatibility across the fixings, flashings and rainwater goods
- 14Ask what remedial materials exist for site damage and whether any are left on site
Common mistakes to avoid
- Choosing fixings after the component they hold rather than alongside it
- Assuming a family name settles compatibility without checking the specific pairing
- Cutting and drilling coated sections on site without the specified remedial treatment
- Considering contact only where components touch, and ignoring run-off paths
- Reading rust marks on stainless as corrosion of the stainless itself
- Treating a sheltered joint as the safe one because it looks unweathered
- Adding one new metal component to an elevation without asking what it now meets
When to involve a professional
- Ask which metals are in contact in this detail and what separates them
- Ask what exposure the protection on each component was chosen for
- Ask how cut edges, holes and welds are treated and by whom
- Ask what the run-off path is and which surfaces it passes over
- Ask whether the fixings, flashings and rainwater goods were confirmed as a set
- Ask who assesses discolouration if it appears, and on what basis
Frequently asked questions
Questions readers ask about this topic
Is it always a problem for two metals to touch?
The records do not treat it as a yes or no question. They describe contact between different metals with moisture present as a compatibility question for a qualified professional in each case, and note that the condition of the joint matters as much as the pairing: laps, bolted connections and unventilated sections hold moisture far longer than a rain-washed face.
Why do cut edges matter so much?
Because a coating is only where it was applied. The records describe coil-coated sections as bare at every cut end, note that those ends are often where rust first shows, and observe that on a thin section the loss of protective coating at a cut, a drilled hole or a dissimilar-metal contact matters more than it would on heavier steel.
How can run-off be a compatibility question?
Because it carries traces of the metal it ran over. The records state this of copper explicitly, and each of the other metal records asks whether run-off from another metal higher up the building lands on the component in question. The path through gutters, hoppers and downpipes is therefore part of the same question.
Does rust on stainless steel mean the stainless has failed?
Not necessarily. The records state that rust marks on stainless are often contamination from other steel rather than the stainless itself, and describe grinding dust from nearby steel work as settling on stainless surfaces and staining them later. Who assesses whether a mark is cosmetic or something more is a question worth asking directly.
What should I ask before replacing a gutter or a flashing?
What the new component is made of, what it will touch, what will run onto it and what it will discharge over. The records treat the covering, its fixings, its flashings and the rainwater goods as one compatibility path, and ask who is confirming that set rather than each item on its own.
Keep reading