Metal Roofing: What the Options Are and What to Check Before You Buy

A metal roof is a decision a household lives with for decades, and the choices that matter are which profile, which coating, and how well the details are built. Those three choices decide how the roof drains, how it ages, how loud it is in a hail storm and whether the warranty means anything in year twelve. This article sets out what each option is for, then the checks that separate a roof that lasts from one that leaks where it was skimped.

What each profile is for

Two profiles cover most houses, and they suit different jobs.

Corrugated sheet is the default. The familiar wave gives it stiffness for its weight, which lets it span further on lighter battens, and it drains well at the pitches most Australian houses use. It is fixed with screws through the sheet into the battens, which makes it straightforward to install and straightforward to repair. Its exposed fixings are the trade: each one is a potential leak point if the screw, the washer or the setting is wrong, and that is where an installer’s care shows.

Standing seam is the premium answer. The panels are joined by a raised seam with concealed clips, so there are no screws through the sheet face, and the roof can be laid at pitches too low for corrugated. The concealed fixing also lets the sheets expand and contract under the clips instead of working the screw holes. The look is cleaner, the installation is slower, and the cost is higher. Where the design calls for a low pitch or a long, uninterrupted plane, it earns the difference.

Other profiles exist between those two, and any of them works when the pitch, the span and the fixing method agree. A reader who cannot say which profile is proposed, and why, has not yet had the question answered.

The coatings, and what the brand names mean

Colorbond and Zincalume are trade marks rather than materials, and the names are worth translating before they appear in a quote. Colorbond names a painted steel finish: a metallic-coated base with a factory-applied paint system, in a fixed colour range. Zincalume names a zinc-aluminium alloy coating that protects bare steel without paint.

The difference matters in two places. A painted finish gives a colour, and colour is the part that fades, chalks or scratches, though the range is designed for exterior exposure. An unpainted metallic coating has no colour to lose but takes on a dull grey over time. Where the house sits in a corrosive environment, near the coast or near industry, the coating and the wash-down regime both matter more than the colour chart, and the specification should be chosen with that in mind rather than after the budget is set. Speak to a roofing supplier about which coating suits a coastal or high-exposure site.

Where metal roofs fail

Almost no leaks begin in the middle of a sheet. They begin at the details: the flashings, the valleys, the penetrations for pipes and flues, and the ridge capping. These are the places where the roof is cut, joined and redirected, and they are where water is deliberately concentrated.

Flashings are the shaped metal that closes the roof at edges, walls, chimneys and changes of direction. A flashing that is dressed to fit the building and fixed with the right laps will outlast the sheets; one that is stretched, underlapped or sealed with mastic alone will fail within a few seasons. Ridge capping closes the top and seems simple, but it takes the force of the wind and needs the same care with laps and fixings as anything else. The check for a homeowner is to ask, before the work starts, how the flashings will be formed and whether the sealing compound is the main measure or the last reserve.

What goes under the sheet

Three layers matter beneath the metal, and they are chosen at the start or not at all.

Sarking or underlay sits over the battens and under the sheets, and its job is to manage condensation and provide a second line against wind-driven rain. Insulation controls the temperature of the rooms below. An acoustic blanket, a layer made to absorb sound, changes what a hail storm sounds like from inside, which is a real consideration for a metal roof over a bedroom. The three duties overlap, and a product that is doing one is not automatically doing the others, so the specification should say which job each layer is for.

Retrofitting any of them later means removing the roof, which is why the decision belongs to the quoting stage. A homeowner weighing the cost of an acoustic blanket should compare it with the cost of the only other way to add it.

Fixings and movement

Metal expands in the sun and contracts as it cools, and a long run of sheet moves more than a short one. A roof fixed as though it were static will loosen its fixings, crack its seals and open its laps over time, and that is the mechanism behind most premature metal roof problems.

The correct fixing method differs by profile. Corrugated sheet wants screws with sealing washers set in the crest or the valley as the manufacturer directs, with the setting allowing slight movement, and the pattern to the manufacturer’s spacing. Standing seam wants clips that hold the seam while letting the sheet slide. The check a buyer can run is to ask how movement is being allowed for, and where the answer is nothing, the quote is not finished. Walking on a finished metal roof has its own rules, and the installer’s guidance on access, for servicing or cleaning, is worth keeping.

Metal on the house, sheet on the verandah

Metal is a fair answer for the main roof of a dwelling: it is light for the area it covers, it handles the weather, and the profiles suit both traditional and modern designs. Where the cover is a verandah, carport or pergola, the decision changes, because those structures are usually not insulated and light is often welcome. Where a translucent sheet suits better, and where it does not, is covered in the guide to polycarbonate roofing, including the heat and noise trade-offs that decide the sheet type.

The questions to put to the installer

• Which profile is proposed, and what about the pitch and the span led to it?
• How will the flashings be formed, at the edges, the valleys and the penetrations, and who forms them?
• What fixings are specified, at what spacing, and how does the design allow for thermal movement?
• How will the sheets be cut to length, and where, given that the method affects corrosion around cut edges?
• What underlay or blanket is specified, and which of the three jobs, moisture, insulation or sound, is it there to do?
• What does the warranty cover, who honours it, and what does it require of the installation and the owner?
• How will access for maintenance and cleaning be handled once the roof is on?

The scope of the quote should answer those questions in writing. A quote that lists sheets and a rate, and nothing about flashings, fixings or underlay, is priced for a different job.

What a finished roof will not show you

From the ground, one well-built metal roof and one that will leak in five years look much the same. The difference is in the layers and the details, and after the sheets go on, the evidence is covered. The practical response is to have the stages that matter inspected or photographed while they are open: the battens and their spacing, the underlay, the flashing work at the valleys and penetrations, and the fixing pattern. None of that is unreasonable to ask, and an installer who does careful work is usually willing to show it.

Before you commit

Settle the profile against the pitch, settle the coating against the environment, and hold the details, flashings, fixings, underlay and movement, to the same standard as the sheets themselves. Metal roofing is a long-lived choice on the strength of those decisions, and each of them is cheapest to make before the first sheet is ordered.