Metal Rainscreen Cladding: Weathering Steel and Brass
Weathering steel protects itself, but only if it is allowed to dry: the patina does not form in damp, it forms through wet-and-dry cycles. And for the first three years it runs — whatever sits below the facade has to be designed beforehand, not cleaned afterwards.
Overview
TAV. 00A ventilated metal cladding follows the same constructional logic as any rainscreen — sub-frame, insulation, windtightness, cavity, panels — but the material shifts the problems to three precise points. The first is the patina: in weathering steel the compact oxide layer is the protection itself, and it stabilises only where the surface gets wet and then dries; in permanently sheltered or permanently damp areas it never forms, and there the steel keeps corroding. The second is run-off: for the first years the not-yet-bound oxide is washed away by rain and indelibly stains the stone, concrete and paving below. The third is the chemistry of contacts: one wrong fixing short-circuits two metals and consumes one of them. Brass follows a different curve — from gold to brown — and forces a declared choice: let it age, or lacquer it and accept the maintenance.
The site score
TAV. HThe detail of the craftthe screw that eats the panel
Brackets, sub-frame and galvanic pairs
Fixing the brackets with thermal isolators, mounting the profiles and checking the compatibility of the metals in contact.
Check the screws before the panels. A galvanised screw on weathering steel is wasted time dressed up as a saving.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The commonest misunderstanding
The weathering steel patina forms by drying, not by getting wet
It sounds like a paradox and it is not. The protective oxide compacts only through cycles: the surface gets wet, then dries, and with each cycle the layer becomes denser and less permeable. Where that alternation is missing — under an overhang that always shelters, in a joint where water sits, against an element that holds damp — the cycle never closes and the steel keeps losing section, indefinitely. Designing in weathering steel is therefore first of all designing how water arrives and how it leaves.
Schematic. Weathering steel is not suitable for chloride environments (marine air, salt spray, de-icing salts): there the patina does not stabilise and corrosion continues. The check must be made against the site's actual exposure.
Two metals, two biographies
Weathering steel and brass: how they change over time
Both are materials that visibly age, and in both cases the real question is not aesthetic but contractual: does the client know what colour the building will be in five years? Weathering steel goes from bright orange to deep brown and then stabilises; brass starts golden and moves towards brown, unevenly wherever it is touched. The third route — lacquering — freezes the appearance but introduces maintenance that did not exist before.
Indicative representation: the speed and final tone depend on exposure, orientation, pollution and proximity to the sea. An on-site sample, left exposed for a few months before installation, is the only way to align designer and client.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QCan weathering steel be used near the sea?
No, or rather: not with this expectation. In the presence of chlorides — marine air, salt spray, road de-icing salts — the patina cannot stabilise and corrosion continues instead of stopping. It is the material's most important limit of use and also the most often ignored, because the defect appears only after years. In those contexts you move to stainless steel of an adequate grade, or to painted steel with a marine-grade system, accepting that it becomes a surface to maintain.
How much extra thickness should be allowed for corrosion?
The right concept is a sacrificial allowance: a portion of thickness the material will lose while forming its patina, and which must not be counted in the strength calculation. Its magnitude depends on how aggressive the environment is and must be taken from the manufacturer's data for the site's exposure class, not from a generic figure. The practical point is a different one: for non-load-bearing cladding that loss is almost irrelevant, while for a structural element in weathering steel — a footbridge, a load-bearing parapet — it becomes a real design item, and it must be declared.
Should brass be lacquered or left to age?
It is a design decision with different consequences over time, and it is worth putting in writing. Left natural, brass moves from golden to brown over a few years, unevenly: areas touched by hands stay lighter, areas sheltered from rain go darker. Lacquered, it keeps its initial tone but introduces maintenance that did not exist before: the lacquer wears first at the contact points, and local touch-up shows. On handles and handrails natural is almost always the better choice, because use itself polishes the metal.
How do you get the client to agree on the final colour?
With an exposed sample, not a brochure. Two or three panels are mounted on site, oriented as they will be in the finished work, and left there for a few months: it is the only way to show the real speed of that particular site, which depends on exposure, orientation, pollution and proximity to the sea. It costs little and prevents the most frequent complaint about these materials, which is not about installation but about appearance — “that isn't the colour I was shown” — when the facade is already finished and the material is doing exactly what it should.