How a screw keeps water out
Most residential metal roofs are held down with self-drilling screws that pass through the sheet into a timber or steel batten. Under the head of each screw sits a sealing washer. When the screw is set correctly, the washer is compressed enough to seal the hole without being crushed or split. Manufacturers such as Lysaght publish installation manuals that cover which screws and washers suit their sheets, where screws go and how they should be set. Those instructions are the reference for any replacement.
There are a few ways this simple joint fails. The screw can be overdriven, which crushes the washer and can dimple the sheet so water pools at the hole. It can be underdriven, leaving the washer loose. It can miss or strip the batten, so wind lifts the sheet and works the screw loose over time. The washer itself can age, crack or harden. And the screw can corrode, especially if it is not the right class of fastener for the roof or the location.
Screw position matters too. On many profiles, roof screws go through the crest of the rib so the hole sits above the water that runs along the pans. A screw placed in a pan, or an old hole left in a pan after something was removed, sits in running water every time it rains. That is a much harsher job for a washer or a dab of sealant, and a common source of leaks that owners attribute to the roof in general.
Read the signs from the ground and the ceiling
From the ground or an upstairs window, look for rust staining that starts at screw heads and runs downhill, lines of discoloured screws, and screws that sit higher than their neighbours. Binoculars help. Do not climb on the roof. A photograph taken with a phone zoomed in from a safe spot is often clear enough for a roofer to see whether screw heads are corroded or lifting.
Inside, record whether the stains form a line and how that line sits relative to an outside wall or gutter. That pattern gives the roofer a location to investigate, but water can travel along framing or insulation and a ceiling mark cannot identify a failed screw by itself. Note whether drips occur throughout steady rain or only with particular wind directions. Ask for the covering, laps and flashings to be assessed alongside the fixings before a repair is proposed.
Listen as well. Sheets that rattle, bang or creak in wind can indicate loose fixings or screws that have lost their grip in the batten. That is worth fixing even before it leaks, because a sheet that moves in wind works its screws further loose and can eventually lift. If you notice a new noise after a storm, record the wind direction and which part of the house it comes from.
| What you notice | Possible cause to investigate | What to ask for |
|---|---|---|
| Rust streaks running down from screw heads | Corroded screws or aged washers | Replacement screws matched to the sheet and finish |
| Drips in a line parallel to the gutter | Failed screws along one batten | Inspection of that batten line and fixings |
| Sheets rattling or banging in wind | Loose or stripped fixings | Check of fixings and battens before it leaks |
| Orange spots scattered on the sheet | Swarf from drilling or cutting | Clean-off and assessment of any staining |
| Leak near a vent, wall or skylight | Flashing or sealant joint rather than screws | Check of the flashing and its fixings |
Other causes that look like screw leaks
Side laps and end laps are the next thing to check. Where two sheets overlap along a rib, or where a short sheet laps over a longer one down the slope, water can be driven through by wind or drawn in by capillary action. The leak may sit right next to a screw line and be blamed on the screws. A roofer should look at the lap, the fixings along it and whether the ends of the sheets are turned up at the top as the manufacturer intends.
Flashings are another. Ridge cappings, barge cappings, apron flashings against walls and flashings around vents and skylights all have their own fixings and seals. A cracked sealant joint on a flashing produces a leak that can appear several metres away. If the leak is near a wall junction, a chimney, a vent or a skylight, mention that detail. Our skylight guide covers leaks around roof openings.
Finally, check for damage to the sheet itself. Holes from old antenna brackets, satellite dishes or removed equipment are sometimes filled with sealant and forgotten. Rust through the sheet, often starting in areas that do not get washed by rain such as under eaves overhangs or behind solar panels, can let water in without any screw failing. A quote to replace screws will not solve a hole in the sheet, so ask for the full cause to be identified.
Swarf, other trades and incompatible materials
When anyone drills or cuts on a metal roof, small steel filings called swarf land on the sheets. If they are not swept off, they rust and leave orange spots that can look like the roof itself is corroding. Record any recent drilling or cutting on the roof so the roofer can consider surface contamination as well as corrosion of the sheet itself. Solar installers, air conditioning technicians, antenna installers and plumbers all create swarf on roofs they work on.
Other trades also walk on the roof. Walking in the wrong place on the sheet can crush ribs, crack washers or loosen fixings. If a leak appeared soon after another trade worked on the roof, tell the roofer who was up there and what they did. That is not about blame. It tells the roofer where to look first, and it helps if you need to raise the problem with the other contractor.
Materials that touch or drain onto the roof matter as well. Some metals and materials are not compatible with coated or zinc and aluminium coated steel, and runoff from them can shorten the life of the sheet. Manufacturers list compatible fixings, washers and flashing materials. When screws are replaced, ask for a type that matches the manufacturer's recommendation for your sheet and finish, not whatever is on the truck.
Spot repair, refix or replace
A spot repair may suit a small number of failed screws on an otherwise sound roof. The roofer needs to assess the sheet, hole and support beneath each affected fixing, then choose a repair compatible with the roof manufacturer's requirements. A larger screw is not automatically an adequate answer to a damaged hole or deteriorated batten. Ask what will be repaired and how the replacement fixing will obtain a suitable hold and seal. Surface sealant alone can obscure the underlying defect.
A refix is worth considering when many screws across a roof are corroded, loose or have perished washers. It involves replacing or supplementing fixings across whole runs, and checking battens where screws have lost their grip. On an older roof, it is sensible to ask the roofer whether the sheets are in good enough condition to justify refixing, or whether the screws are failing because the roof as a whole is at the end of its life.
Replacing sheets or reroofing becomes the conversation when the sheets are rusting through, when laps are corroded, or when the fixing pattern cannot be restored because the battens are deteriorated. That is a much larger decision with building and possibly approval implications, covered in our reroof approval guide. A good quote separates what is needed now to stop water entry from what is recommended for the longer term.
Mistakes that make fixing leaks worse
The first is sealing everything. Running sealant over every screw head and lap on a roof can trap water, look untidy and make it hard to find the real leak later. It also tends to be offered as a quick fix after a storm when the actual cause has not been found. Ask for the specific failed points to be identified and photographed, then repaired properly.
The second is overtightening replacements. Excess compression can damage a sealing washer or distort the sheet, while insufficient compression may leave a poor seal. The contractor should follow the product's installation requirements and suitable tool settings. Ask for close completion photographs instead of climbing up to inspect the fixings. If the photographs show distorted sheets or unusual washer positions, ask the roofer to explain whether the detail matches the manufacturer's requirements.
The third is ignoring what is around the leak. A single failed screw near a solar mounting, a vent or a flashing may be part of a bigger detail that needs attention. When the roofer is up there, ask for photographs of the surrounding area so you can see whether anything else needs planning. It is cheaper to plan work while access is already set up than to call someone back.
Hypothetical example: a garage roof in Wilsonton
A homeowner in Wilsonton notices drips on the car bonnet after rain, in a line about a metre in from the gutter. From the driveway, several screw heads on the garage roof look rusty and one appears to sit proud. The garage roof is older than the house extension, and an old television antenna was removed a few years ago. They photograph the roof from the street and the drip marks on the garage floor, and note that the drips appear in any steady rain.
The roofer finds several corroded screws along one batten line, one screw that has stripped its hole, and an old antenna bracket hole in a pan that was filled with sealant that has since cracked. The quote lists replacing the corroded screws with fasteners matched to the sheet, repairing the stripped fixing location after checking the supporting batten, and patching the old bracket hole with a method suited to the sheet. It notes that the sheets are otherwise sound.
Without the photograph, the roofer might have treated this as a simple screw replacement and missed the bracket hole. Without the roofer's inspection, the owner might have assumed the whole garage roof needed replacing. The combination led to a small, specific repair.