Coastal Corrosion on SA Roofs

Airborne salt is why a roof at Herolds Bay and the same roof in Oudtshoorn are not the same roof. Corrosion here is not a defect that appears one day — it is a continuous process that the specification either resists or does not.

Coastal Corrosion on SA Roofs — practical guidance from the team running metal roofing on the Garden Route. Below is how coastal corrosion actually works, what to specify near the sea, the mistakes that destroy an otherwise sound roof, and the maintenance that extends its life more than anything else.

How salt actually attacks a roof

Corrosion of steel is an electrochemical process, and it needs three things: the metal, moisture, and an electrolyte. Chloride from sea spray is an extremely effective electrolyte, and it is deposited on every surface within a few kilometres of the coast, continuously.

It is also hygroscopic — it attracts and holds moisture from the air. So a salt-laden roof surface stays wet longer after rain and dew than a clean one does, which extends the time corrosion is active each day. That, rather than any single dramatic event, is why coastal roofs age faster.

Distance from the sea matters enormously and it is not linear. The severity drops off sharply in the first kilometre or so, then more gradually. A roof at Glentana or Herolds Bay is in a materially harsher environment than one on the inland side of George, and the specification should reflect that rather than being uniform across a company’s whole area.

Sheltered surfaces are worse, not better. This surprises people. Rain washes salt off exposed surfaces; sheltered areas — under overhangs, the undersides of sheets, behind flashings — accumulate it and never get rinsed. Corrosion frequently starts where you cannot see it.

What to specify near the sea

The metallic coating comes first. Bare steel is protected by a metallic layer — zinc, or a zinc-aluminium alloy. Zinc-aluminium substantially outperforms plain zinc in a marine atmosphere. Plain galvanised sheeting near the sea is a false economy and we will not specify it there.

Coating mass matters as much as coating type. The figure quoted after the coating designation is the mass of metallic coating per square metre, and it is the most useful number on the spec sheet. Two sheets with the same trade name and different coating masses are not the same product, and the cheaper one is cheaper for exactly that reason.

A factory-applied paint system on top adds a further barrier and is what we specify on coastal work. The paint is the first line; the metallic coating is the backup when the paint is scratched.

Every other metal component needs the same thought — and this is where most coastal roofs actually fail. Fixings, clips, brackets, flashings, valley linings, gutter brackets. A premium sheet fixed with inadequate screws will have failing fixings long before the sheet shows anything, and the fixings are what hold the roof on.

Concealed-fix profiles have a real advantage here: no exposed fasteners means no exposed fastener heads and washers to perish.

The four mistakes that destroy coastal roofs

1. Dissimilar metal contact. Copper, lead or bare mild steel in contact with — or draining onto — coated steel sets up a galvanic cell and corrodes the steel rapidly. A copper geyser overflow discharging onto a steel roof will cut a hole through it. Check what is above and upstream of a roof, not just the roof.

2. Cutting swarf. Sheets cut with an angle grinder throw hot steel particles across the roof. Those particles embed in the coating and rust in place, producing rust spots across an otherwise new roof within months. Nibblers or shears, and sweep the roof clean at the end of each day. This is the single most avoidable coastal roofing failure and we see it constantly.

3. Unprotected cut edges. Every cut exposes bare steel. On a coastal roof, cut edges and drilled holes need treating, and sheets should be ordered rolled to length wherever possible so there are fewer of them.

4. Trapped debris and moisture. Leaves in a valley, sand behind a flashing, organic growth holding damp against the surface. All of it keeps the metal wet and salty, and all of it is removed by an annual clean that costs almost nothing.

Maintenance that actually extends roof life

On the coast, maintenance is worth more than specification upgrades — a mid-spec roof that is looked after outlasts a premium roof that is not.

Rinse it. Sheltered areas that rain does not reach — under overhangs, behind flashings, the underside of eaves — accumulate salt indefinitely. An annual freshwater wash of those areas removes the electrolyte and materially extends life. It is the highest-value and least-performed maintenance item on any coastal roof.

Clear valleys, gutters and behind flashings. Debris holds moisture against metal.

Replace perished washers as a batch. On an exposed-fix roof the neoprene washers under the screws perish before anything else, and they are the most common cause of persistent minor leaks. Doing them all at once is cheap; doing them one leak at a time is not.

Touch up damage promptly. A scratch through the coating on a coastal roof is an active corrosion site, not a cosmetic mark.

Inspect annually and after any big front, and pay attention to the parts you cannot see from the ground — undersides, sheltered faces and the fixings.

Dealing with a roof that has already gone

Surface rust on an otherwise sound sheet — from swarf, scratches or cut edges — can be treated: remove the corrosion product back to sound material, treat, prime and coat with a compatible system. It is a genuine repair and it buys real time.

Perforation means the sheet has lost section and the roof is at the end of its life in that area. Patching perforated sheeting is a temporary measure, not a repair.

Corroded fixings with sound sheeting is the best of the bad outcomes — replace the fixings and the roof carries on. Worth checking before assuming a roof needs replacing.

Corroded structure — purlins, brackets, hangers — is the finding that changes the job. It is not visible from above and it needs a look from inside the roof space.

And when replacing, fix the specification rather than repeating it. A roof that corroded in eight years was under-specified for its exposure, and putting the same thing back gets the same result. That is the moment to correct the coating, the fixings and the dissimilar metal problem that may have caused it in the first place.

Frequently asked questions

Why do coastal roofs rust so quickly?

Airborne chloride from sea spray is deposited continuously on every surface, and it is hygroscopic — it attracts and holds moisture, so a salt-laden roof stays wet longer after rain and dew than a clean one. That extends the time corrosion is active every single day. Severity drops sharply in the first kilometre or so from the sea and then more gradually, which is why a roof at Herolds Bay and one inland of George should not carry the same specification.

What roof sheeting is best for coastal areas?

A pre-painted sheet over a zinc-aluminium metallic coating at an adequate coating mass. Plain galvanised is a false economy near the sea. Just as important is everything else made of metal — fixings, clips, brackets, flashings and valley linings — because those routinely fail long before the sheet does and they are what hold the roof on. Concealed-fix profiles have an edge, since there are no exposed fastener heads and washers to perish.

What is coating mass and why does it matter?

It is the mass of metallic coating applied per square metre of sheet, quoted after the coating designation, and it is the most useful number on a specification sheet. Two products with the same trade name and different coating masses are not the same product — the metallic layer is the sacrificial protection, so more of it means longer life. It is also the easiest thing for a cheaper quote to be quietly economising on.

Can rust spots on a new roof be the installer’s fault?

Frequently, yes. Cutting sheets with an angle grinder throws hot steel swarf across the roof; those particles embed in the coating and rust in place, producing rust spots across a brand-new roof within months. It is entirely avoidable — nibblers or shears rather than a grinder, and sweeping the roof clean at the end of each day. It is the single most avoidable coastal roofing failure and it is common.

Should I wash my roof?

On the coast, yes, and specifically the parts rain never reaches — under overhangs, behind flashings, sheltered eaves and undersides. Those areas accumulate salt indefinitely and never get rinsed, so corrosion often starts where you cannot see it. An annual freshwater wash of the sheltered areas is the highest-value maintenance available on a coastal roof and the least often done.

Does a rusted roof always need replacing?

No. Surface rust on sound sheeting can be treated back to bare metal, primed and recoated, and that buys genuine time. Corroded fixings with sound sheeting is even better news — replace the fixings and carry on. Replacement is the answer once the sheet has perforated and lost section, or where the supporting structure has corroded. It is worth having someone look inside the roof space before assuming the whole roof has to go.

Rust showing on a roof that should not have it yet?

We will look at the fixings and the sheltered areas as well as the sheet, and tell you whether it is treatable, whether it is the fixings, or whether it was under-specified for where it is.

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