Where does rust begin on a Melbourne metal roof?

The coating fails in dots and cuts, not in a blush

Factory-painted steel is protected on the broad face. Every time the sheet is cut, drilled or lapped, that protection is interrupted. Melbourne weather then works on the interruption. A screw is a drilled hole with a washer asked to stay elastic for decades. A gutter line is a cut edge sitting in whatever the gutter did not drain. A side lap is two edges and a capillary gap. Rust is the steel announcing those details. The broad pale fade people notice from the street is usually the paint chalking, which is a different mechanism from a pit.

Read the pattern. Regular orange dots in rows are fixings. A continuous orange line above the gutter is the cut edge or the overhang sitting in leaf mould. Orange inside a lap, or a stain that seems to come from under the sheet, is water held in the joint. Random scabs in the middle of a pan, especially under a tree, are where debris sat long enough to stay wet after every shower. Roof Revival Melbourne photographs the pattern before naming a remedy. A single word, “rusty”, is not a scope.

Older galvanised corrugated iron rusts with less warning once the zinc is gone. Modern Colorbond and Zincalume take longer to show a pit and then show it at the same classic sites. The address changes the speed. Salt air around the bay hurries every cut. A dry, steep inland roof can show screw freckles and almost nothing on the pan.

Screws and the small inlet that becomes a hole

The washer cracks, the shank loosens, and water sits in the drive of the screw. The head goes orange. For a while the pan around it is fine. Then the hole elongates as the sheet moves, the washer no longer covers the slot, and the ceiling stain lines up with the screw rows. Replacing washers and screws on a sound pan is a repair. Once the steel around the hole is thin or split, the new washer is capping a wound. That screw is no longer the whole story. The pan is.

Dissimilar metals hurry the same spot. A fitting, a swarf line left from a cut, or a fastener of the wrong family can start a local pit that looks like “just one rust spot” and is actually a galvanic edge. Swarf is the fine steel dust from cutting that welds itself to the paint and rusts as a pepper of dots. It is a reason to be careful about later trades on the roof. It is not a reason for you to grind or treat the sheet from a guide. Point it out. Have it assessed. Do not scrub a metal roof with whatever is in the shed and call it maintenance.

Edges, laps and the leaf that holds the water

Cut edges at hips, valleys and eaves are unpainted steel from the day they were sheared, even when a touch-up was applied. Touch-up is a thin courtesy. Years of wet leaves defeat it. Gutters that are not cleared hold that wet against the edge all winter. The first perforation is often a pinhole just above the gutter, invisible from the garden, obvious once the rust scab is lifted. A roof can be replaced at the bottom course of sheets and nowhere else, if the upper pans are honest. A quote that replaces everything because the eave is orange may be painting with a crane. A quote that paints the orange and leaves the pinholes will be back.

Laps fail when water cannot drain out of them. Low pitch, a blocked pan, or a bead of failed sealant turns the lap into a tray. Rust then grows from the inside of the joint toward the face, so the top still looks painted while the edge is gone. Open the question on site. From the ground you will only see a stain at the lap line and, later, a drip. Valleys are the same geometry with more water. A valley full of gum litter is a rust factory for the sheets either side.

The underside, where driveways cannot look

Warm moist air from the house condenses on the back of a cold sheet. Without a dry, intact underlay and a way for the space to breathe, the back of the pan scales. White or grey oxide under Zincalume, and orange under older steel, can be well advanced before the top film fails. Penetrations drip from above and keep the back wet in one patch. Roof Revival Melbourne looks up from a safe point in the roof space when the house allows it. A lifespan or a rust call that refuses the underside is guessing.

If the back is scaling and the top is only faded, the conversation may still be repair of the cause — moisture, a duct dumped into the roof, a blocked eave — plus monitoring or replacement of the worst sheets. If the back is perforated, the sheet is finished regardless of how even the colour looks from the street. Do not pressure-wash the top to “see if it is bad”. Water driven under laps is a good way to invent a leak while you investigate one.

What each location is allowed to cost you

Freckles on screws, sound pans: fixings, and a wash or a specified film only if the steel has earned it. Orange at the gutter edge with sound upper sheets: the edge detail, the gutter, and the bottom sheets if they are holed. Rust inside laps on a low pitch: often those sheets, and a honest look at whether the pitch and profile can drain. General perforation or widespread underside failure: replacement of the affected roof, with sarking and battens named if they share the blame.

This page does not give a rust-treatment recipe, a converter mix, or a wire-brush method. Those are how sound paint gets scratched back to steel. The useful act is to name the location and match the scope to it. Send a close photo of the orange, say whether it is in rows, along the gutter, or in the pans, and name the suburb so salt is part of the guess we then check on the roof.

A short map to bring to the inspect

Rows of dots mean fixings. A line at the eave means the cut edge and the gutter. A line at the lap means the joint. Pepper on an otherwise new sheet means swarf or a fitting. Stains with no top-side pit mean look underneath. Roof Revival Melbourne will confirm the map on the roof and say which of those sites are still a repair. When the map is mostly holes, replacement is the clean reading, and the colour of the new sheet is a later choice.