Metal Roofs: Profile, Gauge, Coating

Three specifications decide how long a steel roof lasts, and only one of them is visible from the ground. Profile is the choice people argue about. Gauge and coating are the ones that determine the outcome.

Metal Roofs: Profile, Gauge and Coating — practical guidance from the team running metal roofing on the Garden Route. Here is what each of the three actually controls, and how we specify them on Southern Cape roofs.

Profile: shape, and what it is for

Profile is the shape rolled into the sheet. It does two jobs — it stiffens a thin piece of steel enough to span between purlins, and it channels water down the slope.

IBR, the inverted box rib, is the workhorse: trapezoidal ribs, pierce-fixed through the crown, straightforward to detail around penetrations and changes of plane. Corrugated is the older sinusoidal profile, still common on outbuildings and agricultural work. Concealed-fix profiles such as Klip-Lok clip onto brackets with no fastener through the sheet.

Profile also sets the minimum pitch. Deeper ribs and concealed-fix systems handle lower pitches; shallow corrugated needs more fall. Getting this wrong is a common cause of leaks at end laps on near-flat roofs that should never have had a lapped profile in the first place.

Profile is the least important of the three for durability, and the one that gets the most attention.

Gauge: the thickness that decides how it behaves

Gauge is the thickness of the steel, quoted in millimetres — commonly 0.47, 0.5, 0.53 and 0.58 for roofing.

Thicker steel spans further between purlins, resists foot traffic and hail denting better, and holds its shape under wind load. Thinner steel is cheaper and lighter and, on a roof with close purlin centres and modest exposure, entirely adequate.

Where gauge earns its cost is exposure and span. On a long unsupported span, or an exposed coastal plane, going from 0.47 to 0.53 changes how the roof behaves in a gust and how it feels underfoot for anyone maintaining it later.

Gauge does not, by itself, determine corrosion life. A thicker sheet with a poor coating corrodes at much the same rate as a thin one — it just takes marginally longer to perforate. Buying thickness as a substitute for coating is the wrong trade.

Coating: the specification that actually decides the life

This is the one that matters most on this coast and the one most often left unspecified.

There are two layers to think about. The metallic coating on the steel — galvanised zinc, or aluminium-zinc such as AZ150 — and the paint system on top of it, such as a coated finish in the Chromadek family.

In marine conditions, aluminium-zinc substantially outperforms plain galvanised. The gap widens the closer you are to the sea, and on the Garden Route coast it is not a marginal preference. A sheet specified for inland use, installed within sight of the water, has a materially shorter life.

The paint system on top provides the colour and the UV protection, and it is what fades or chalks over time. A quality coated finish holds colour and film integrity far longer than a cheap painted sheet, which begins chalking within a few years.

Cut edges, fasteners and the weakest link

A coated sheet is protected on its faces. Every cut exposes bare steel at the edge.

That means sheet ends, site cuts around penetrations, and any trimming done on the roof are small starting points for corrosion. Ordering sheets to length rather than cutting on site removes most of them. Where cuts are unavoidable, treating the edge is worth the few minutes it takes.

Fasteners are the other weak link, and they are routinely under-specified. Putting a low-grade screw into a well-coated sheet in coastal air gives you a roof whose life is set by the cheapest component on it. Class 4 fasteners with quality sealing washers are the baseline near the sea, and they cost very little more.

Dissimilar metals matter too. Copper or brass in contact with steel, or run-off from a copper element onto a steel roof, sets up galvanic corrosion that eats the roof at that point regardless of how well it was specified.

Insulation, condensation and the layer under the sheet

A bare steel roof is a condensation surface. Warm moist air from inside meets cold steel, and water forms on the underside — which drips onto ceilings and looks exactly like a leak.

That is what insulated or foil-backed products address, along with the thermal comfort issue. On a habitable building a steel roof without an appropriate underlay or insulation layer is asking for condensation complaints, especially in the cold still weather this coast gets in winter.

The distinguishing sign is timing. A roof that drips on cold still mornings and stays dry in rain has a condensation problem, not a water ingress problem, and no amount of sealing the outside will fix it. Ventilation into and out of the roof space is part of the answer.

How we specify a coastal steel roof

Coating first, driven by distance from the sea. Near the water, aluminium-zinc substrate with a quality coated finish, and matched Class 4 fasteners.

Then gauge, driven by span and exposure rather than by budget. Then profile, driven by pitch, geometry and how many penetrations the roof has to accommodate.

Then the fixing pattern, densified at eaves, ridge and corners where uplift concentrates, rather than a uniform pattern taken off a generic table.

And single-length sheets wherever the roof allows it, because removing end laps removes the second most common leak path after the fixings themselves.

Frequently asked questions

What gauge roof sheeting should I use?

Common roofing gauges run 0.47 to 0.58 mm. Thicker steel spans further, resists denting and holds shape under wind load. Span and exposure should decide it — on a long unsupported span or an exposed coastal plane, step up. Gauge does not substitute for coating quality, though; a thick sheet with a poor coating still corrodes.

What is the best roof sheeting for coastal areas?

Aluminium-zinc coated substrate such as AZ150 with a quality coated paint finish, matched with Class 4 fasteners, and cut edges minimised or treated. That combination matters more to the roof’s life than the profile does. Standard galvanised near the sea has a substantially shorter life.

What is the difference between Chromadek and galvanised?

Galvanised refers to the zinc coating on the steel. Chromadek is a coated paint finish applied over a metallic-coated substrate — it provides colour, UV protection and an additional barrier. They are not alternatives to each other; a Chromadek sheet still has a metallic coating underneath, and what that coating is determines corrosion performance.

How long does a metal roof last in South Africa?

Anywhere from around fifteen years to several decades, driven almost entirely by coating specification and distance from the sea. On this coast, the fixings and flashings usually fail well before the sheet does, which is why re-fixing is a common and worthwhile repair on roofs that still have plenty of sheet life.

Why is my metal roof rusting at the screws?

The sealing washers have perished and the fastener heads have corroded, most often because lower-grade fasteners were used, or because the screws were over-tightened and crushed the washers on installation. Re-fixing with new Class 4 fasteners and fresh washers is the standard repair.

Do metal roofs cause condensation?

They can. Warm moist air meeting cold steel forms water on the underside, which drips and looks like a leak. The signature is dripping on cold still mornings rather than during rain. An appropriate insulated underlay plus ventilation into and out of the roof space is the fix — sealing the outside will not help.

Is a thicker roof sheet always better?

Not necessarily. Thicker steel helps with span, denting and wind performance, but it does not meaningfully improve corrosion resistance — the coating does that. Spending on gauge while economising on coating is the wrong way round, particularly near the sea.

Getting a steel roof specified right?

Coating for the exposure, gauge for the span, fixings that match — and a pattern designed for where the roof actually sits rather than off a standard table.

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