Coastal zones and warranties
The zone is not a label on a product — it is a property of the address. Establishing it is the first thing you do on a coastal job, and it decides the warranty you can promise.
Corrosion severity is set by distance to breaking surf: mild at 5 km or more, moderate 500 m to 1 km, severe 100 to 500 m, very severe 25 to 100 m. The zone decides which products can be warranted at the address and for how long. It is established before the material is specified, not after — and the warranty that results is conditional on installation detail and on maintenance, both of which sit with you to get right and with the owner to keep up.
The four zones
| Zone | Distance to breaking surf | What it means on the job |
|---|---|---|
| Mild | 5 km or more | The full range is available and warranties are at their longest. |
| Moderate | 500 m – 1 km | Pre-painted, and the coating tier starts to matter. |
| Severe | 100 – 500 m | Coastal-grade coating. Warranty periods shorten. |
| Very severe | 25 – 100 m | Premium coating, heavier gauge worth considering, and read the warranty statement before you promise anything. |
Note what the measurement is: breaking surf, not water. It is the airborne salt off the break that does the damage, so a sheltered harbour edge is a milder environment than an open surf beach at the same distance from the sea.
Establishing the zone for a real address
The zone is a site assessment, not a lookup. Four things go into it:
- Distance to the nearest breaking surf, in a straight line. Measure it off a map before you go, then look at what is actually between the house and the water.
- Shelter. A hill, a stand of trees or a row of houses between the building and the coast changes what reaches the roof. So does being in a bay rather than on an open coast.
- Prevailing wind. A house downwind of an open coast sees more salt than one the same distance away in the lee.
- Anything else corrosive nearby. Geothermal activity, industrial discharge, and animal housing all have their own effect and they are not about the sea at all.
Where a site sits between two zones, the honest move is to price the more severe one and say why. It is a conversation worth having in front of the customer, because it explains the price difference between the quote you have written and a cheaper one they may be holding.
What the warranty is, and what it is not
A steel roof usually carries more than one:
- The perforation warranty — the sheet will not rust through. This is the long number, and it is the one that shortens with the zone.
- A surface or appearance warranty — the coating will not chalk or fade beyond a stated limit. Usually a shorter period.
- Your workmanship warranty, which is a separate thing entirely and comes from you, not the manufacturer.
Indicative figures: premium pre-painted lines reach up to fifty years' perforation cover in mild zones; plain unpainted Zincalume carries a twenty-year non-perforation warranty; cheaper pre-painted ranges sit around thirty in mild environments. All of them come down as the zone gets worse.
Check the current published warranty for the product, the tier and the specific zone before you write a number in a quote. Manufacturers revise them, they vary by colour and coating, and a warranty quoted from memory is one you are personally underwriting.
What voids one
This is the half of the subject that gets skipped, and it is the half that turns into an argument in year seven. A warranty is conditional on the roof having been installed and looked after properly, and the usual conditions are:
- Compatible fixings and flashings. The screws, rivets and flashings have to belong to the same system as the sheet. A cheaper fastener is the classic way to void an expensive warranty.
- No dissimilar metals. Copper, lead or unprotected steel in contact with, or draining onto, the roof will corrode it. That includes a copper pipe discharging onto a steel roof and a lead flashing left in place from the old one.
- Swarf removed. Drilling and cutting swarf left on a sheet rusts into it, and the marks are permanent long before they are structural. Sweep as you go, not at the end of the day.
- Unwashed areas washed. Anywhere the roof is sheltered from rain — under an eave, behind a parapet, under a solar array — accumulates salt that rain would otherwise rinse off. Those areas need washing down periodically, and the obligation is the owner's.
- No unprotected cut edges where the specification requires them treated, and no storage of the sheets wet or stacked in the weather before they go up.
Put it in the quote
Three lines protect everybody:
- The zone you have assessed and the basis for it — the distance and the exposure. If the roof is later assessed as a worse zone than you priced, this is the record of what you were told and what you saw.
- The product, tier, gauge and colour, in full. Not "new Colorsteel roof" — that is a description a cheaper steel also fits, and it loses on price to something that is not the same product.
- The maintenance the warranty depends on, in plain words, so the owner knows on the day they sign that washing the sheltered areas is part of owning the roof.
Which product suits which zone is covered in Colorsteel grades compared.
What comes up most.
How do I work out the coastal zone for a house?
Measure the straight-line distance to the nearest breaking surf, then adjust for what is actually between the building and the water — shelter, terrain, prevailing wind, and any other corrosive influence like geothermal activity or industry.
Mild is 5 km or more, moderate 500 m to 1 km, severe 100 to 500 m, very severe 25 to 100 m.
Why is it distance to breaking surf and not distance to the sea?
Because the salt that corrodes a roof is airborne, and it comes off the break rather than off flat water.
An open west-coast beach at 300 m is a far harder environment than a sheltered harbour edge at the same distance. The measurement is to where the waves break.
What voids a roofing warranty?
Incompatible fixings or flashings, contact with or drainage from dissimilar metals such as copper and lead, swarf left on the sheets, and failing to wash down areas the rain never reaches.
The last one is the owner’s obligation rather than yours, which is exactly why it belongs in the quote in writing rather than in a conversation on the day.
What is an unwashed area?
Anywhere on the roof that rain does not reach — under a wide eave, behind a parapet, beneath a solar array, under a carport overhang.
Salt and dirt build up there instead of being rinsed off, and those are the places a coastal roof fails first. Manufacturers require them to be washed down periodically for the warranty to stand.
Can I use a mild-zone product 300 m from the coast if the customer wants to save money?
You can physically fit it, and the warranty will not cover it. That means when it fails you are the one holding the problem, because the manufacturer will point at the specification.
Price the correct product, explain what the zone is and why, and let the customer decide with the information in front of them. Put the assessment in the quote either way.
Does a heavier gauge help in a coastal zone?
It is worth considering. 0.55 mm is standard practice in severe zones and gives more metal between the weather and the inside, as well as stiffness underfoot.
It is not a substitute for the right coating, though. Gauge and grade are separate decisions and the coating is the one that fights corrosion.
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