Materials for coastal architectural exterior

Last verified 19 August 2026. Every candidate below has a property page with sourced values; nothing appears here that is not sourced there.

What decides it

Distance from the sea, and what the surface does with salt. A marine atmosphere deposits chloride and the surface either gets rinsed or it does not. Sheltered elevations, soffits and the underside of handrails collect salt and never wash, and those are where discolouration starts, on the same building where exposed faces stay clean. This is a geometry problem wearing a materials label.

Surface finish, which is not a grade property. A smoother surface holds less deposit and drains better, and the practical rule in coastal work is that finish moves the outcome further than a step up in grade. Nothing on the tables below captures it, and no property page on this site holds it, which is worth saying plainly on a page ranking grades.

PREN, but only for the ordering. 304 near 18, 316L and 316Ti near 24, the lean duplex 2101 in between, 2205 near 35 — the ranking is right and, as with every application on this site, the number ranks and does not qualify.

Strength, when the element is structural. A balustrade, a canopy or a facade support is sized, and a duplex at roughly twice the proof strength of an austenitic makes a lighter section possible. In cladding, which is not structural, that advantage buys nothing and the duplex is harder to form.

The ferritics are here for cost, and they carry a real limit. 441 and 444 are stabilised ferritics used in exterior work where the budget rules. 444 is roughly a molybdenum-bearing ferritic equivalent to 316L on paper. What ferritics do not have is the forgiving formability of an austenitic, and 441’s proof strength is directional, which has its own note.

Candidates

Every figure carries its evidence level, and on this site that level is E3 throughout — real values from resolvable supplier documents, none of them verified against the standard they cite. What the levels mean, and what our data does not cover. These are screening figures. None of them is a design allowable.

What usually goes wrong

The failure mode that actually appears: tea staining — a brown surface discolouration that removes no metal and ends the conversation about whether the right grade was chosen.

Nobody loses a coastal facade to perforation. What happens is tea staining: a light brown discolouration on the surface, cosmetic, removing no measurable section, appearing within the first year or two, and utterly unacceptable to the client who paid for stainless because it was going to look like stainless.

Three things make the argument that follows worse.

It is not a grade failure, so upgrading does not fix it. Tea staining occurs on 316L. It occurs less than on 304, and less again on a smoother finish, and hardly at all where rain washes the surface — but a specification that moved from 304 to 316L and changed nothing else has bought a smaller improvement than the price difference suggests.

The maintenance regime is part of the specification and is never written down. Coastal stainless is designed on the assumption of periodic washing. If nobody agrees who washes it and how often, the material has been specified for a condition that will not exist.

Contamination during construction is permanent. Carbon steel dust from cutting or grinding nearby embeds in the surface and rusts, and the resulting stain is indistinguishable to the client from a material failure. It happens on site, after the grade decision, and it is the single most common cause of a stainless facade looking wrong in its first winter.

The engineering judgement on this page carries no source

The property values are sourced and the reasoning around them is not. Which failure mode dominates, which grade is forgiving to fabricate, which screening number misleads — those are domain knowledge, and this page states them without a citation because we have not found one worth citing. They are the author’s judgement, held to the same standard as everything else here: if one of them is wrong, it is wrong in public and we will say so in the changelog.

Treat the table as evidence and the prose as an opinion from someone who has read the datasheets.

What this page cannot tell you

• Tea staining is not in our data. There is no property row for it here or anywhere else, because it is a surface and exposure phenomenon rather than a material constant. Everything this page says about it is judgement, not measurement.
• Surface finish, which we argue is the deciding variable, is absent from every property page on this site.
• Atmospheric corrosion rates and distance-from-shore categories. ISO 9223 classifies atmospheres for exactly this purpose and we hold no data against it.
• Fixings, brackets and the carbon steel behind the cladding, which is where staining and galvanic problems actually start.
• Formability and finish availability — a grade that cannot be had in the finish the architect drew is not a candidate, whatever its PREN.
• Whether any of these suits your elevation. Distance from the sea, prevailing wind, shelter and washing regime decide this, and none of them is a material property.

Take this further

If you are choosing for a real part, the useful output is not a grade — it is a record of why you chose it that survives the review in eighteen months. Build a decision record, or start from the selection process if the requirements are not settled yet.

Generated from data/applications/architectural-exterior-coastal.yaml. Regenerate rather than editing this file by hand.