New metal roofs

Zincalume is the aluminium-zinc coated base steel. Colorbond is that same steel with a baked-on paint system over it. The paint is not decoration, it is the outer layer of corrosion protection, which is why a Colorbond roof holds up better than bare Zincalume in the same location.

Where the building sits changes which grade is appropriate. Standard Colorbond is fine for most of suburban Melbourne. Closer to breaking surf, or near heavy industry, the specification moves to Colorbond Ultra, which has a heavier metallic coating built for severe marine and industrial environments. Manufacturer warranties are written against distance from those conditions, so putting standard material in a severe environment does not just wear out faster, it voids the warranty that was supposed to cover it.

There are also performance variants worth knowing about. Colorbond with thermal technology reflects more solar radiation in darker colours, which measurably reduces heat load in summer. Matt finishes trade a little durability for a much softer look that architects favour. None of these are upsells if the building actually calls for them, and none of them are worth paying for if it does not.

A metal roof is a cold surface. Warm, moist air from inside a house rises, hits it, and condenses. Managed badly, that condensation drips back into insulation, soaks it, and quietly destroys ceilings and framing over years while everyone blames a leak that does not exist. The National Construction Code has tightened considerably on this, and it is now one of the most common defects found in new builds.

The answer is a properly specified sarking or insulated blanket laid over the battens, with the vapour permeability and the thermal break the building actually needs, and ventilation paths that let the roof space breathe. Anticon blanket does thermal and acoustic work at the same time, which is why rain on a modern metal roof is a soft rush rather than the racket people remember from an old shed.

Batten type and spacing then follow the profile's span tables and the wind classification of the site. This is not a place for judgement calls. A profile rated to span 900mm on a sheltered suburban block may be rated to far less on an exposed hilltop, and battens set to the wrong centres are the reason roofs peel in storms.

Fasteners are specified by corrosion class, and the class has to match both the environment and the sheeting. Class 3 covers most suburban work; coastal and industrial sites need Class 4. Mixing a cheaper fastener into a Colorbond Ultra roof puts a rusting steel point every few hundred millimetres through the most expensive part of the building.

Crest fixing versus valley fixing matters too. Screws through the crest of the rib keep the fastener out of the water path. Screws through the pan sit in the flow, and rely entirely on the sealing washer staying compressed and intact for decades. Torque is part of it as well: over-driven screws crush the washer and dimple the sheet, under-driven screws let it lift and breathe.

The other quiet killer is dissimilar metals. Copper or lead running off an upper roof or a flue onto Zincalume or Colorbond will corrode straight through it. So will unwashed contact with treated pine, or debris left sitting in a valley. A roof can be perfectly installed and still be destroyed by what someone attaches to it two years later.

Sheeting rarely leaks in the middle of a sheet. It leaks at the ridge, at the barge, in the valleys, where a wall meets a roof, and around every pipe, flue, vent, skylight and aerial that punches through it. Those are the details that take the time, and they are the first thing to look at on any quote that seems cheap.

Long runs bring in thermal expansion. Steel moves with temperature, and a long sheet fixed rigidly at both ends will either buckle or tear at the fixings. Expansion joints, correctly sized flashings with proper laps, and fixings that allow movement are all part of a roof that is still tight in twenty years.