Key takeaways
- The coating on a roof screw has to suit the environment the roof sits in, because a screw that corrodes first will leak first, whatever the sheet is made from.
- The washer does the sealing, so check its condition and its compression at every fixing, and never reuse a screw whose washer has been damaged or cut.
- Choose the head and the length for the purlin or batten the design relies on, and confirm the penetration with the sheet manufacturer’s fixing schedule.
- Drive every screw square and to the depth that compresses the washer without distorting the sheet, because both over-driving and under-driving cause leaks.
- Keep dissimilar metals apart and check the fixings on any roof near the coast or an industrial site, where corrosion moves faster than most builders expect.
A profiled steel roof looks like one large sheet of material, but it is really hundreds of small fixings. Every sheet is held down at the crests or in the troughs by screws that pass through the metal and into a purlin or batten below. The roof keeps out the weather only as long as each of those fixings stays tight, sealed and free of corrosion. The sheet itself can last for decades while a single failed screw lets water in at the first heavy rain.
Roof sheeting screws are a small cost on a building job, and they are one of the first things to be chosen badly. The screw in the box is often the one that was on the shelf, or the one a previous job used, rather than the one the sheet manufacturer and the design call for. The consequence shows up later as a leak at a fixing, a rust streak down the side of a sheet, or a washer that has perished and let go. By then the roof is finished, the client has moved in and the fix is a costly one.
This article covers the choice and the use of roof sheeting screws: the coating, the head type, the washer, the length and the driving technique. It does not cover the metal flashings around the roof edges, which are dealt with in the metal roof flashing article, or the choice between roof materials, which is a separate decision. The subject here is the fixing itself, and why it matters more than most builders give it credit for.
Why the screw fails before the sheet does
Steel roof sheets are coated and designed to last a long time. The fixings are usually a smaller and less protected part of the system, and they are exposed to the same weather as the sheet. The head sits in the open, the shank passes through the sheet and into the purlin, and the washer is the only thing standing between the screw hole and the rain. When the coating on the screw breaks down, corrosion starts at the head and runs down the shank, and the washer may follow.
The two failure modes are different. Corrosion is slow and visible if anybody looks, with rust staining down the sheet below the fixing. A failed washer is quicker. The rubber or neoprene seal hardens, cracks or is distorted by an over-tight drive, and water finds a path down the screw into the purlin. That path can run for a long time before the leak appears inside the ceiling.
Both failures are the builder’s problem in the end. A roof that leaks through its fixings is a defect claim, and the defect is not hidden in the design. It is in the hardware the builder chose and the way it was driven. The cost of getting it right at the point of purchase is small compared with the cost of replacing fixings across a finished roof.
Choosing the coating for the environment
The coating is the first decision, and it has to match the environment. A roof in a dry inland suburb has a far easier life than a roof within sight of the surf, a roof over an industrial site or a roof in a sheltered valley where salt and moisture stay in the air. The sheet manufacturer’s fixing schedule will state the coating that suits each exposure, and that is the document to follow rather than the price on the screw box.
Zinc-coated screws are a common choice for mild environments. Zinc-aluminium coatings give better protection in harsher conditions, and stainless steel is used where the exposure is severe or the sheet is a material that would otherwise be attacked by the screw. Each coating has a different life, and the life depends on the exposure. A zinc-coated screw in a coastal setting will not last as long as the same screw inland, and the sheet manufacturer’s guidance is the place to check the match.
Do not mix coatings on the same roof without a reason. A roof with some screws of one coating and others of a different coating can corrode unevenly, and a quick substitution at the store on a busy day is how that mix happens. Keep the fixings for each roof together, labelled, and check that the delivery matches the schedule.
The head and the washer: where the seal is made
The head of a roof screw carries the load and provides the bearing surface for the washer. A hex head or a similar driving head is common, because it gives a secure grip for the driver and resists cam-out. The head should sit with its washer flat against the sheet, and the washer should be the bonded type the screw was supplied with, not a loose washer added on the job.
The washer is the seal. It is usually a rubber or neoprene component bonded under the head, and its job is to compress against the sheet crest or trough and stop water running down the hole. The washer’s life depends on the material, the exposure and the way it was driven. A washer that is too loose will not seal. A washer that is over-compressed will distort, split or pull out of the head. Both are common, and both are caused by the driving technique.
Check the washer on every fixing before the sheet goes on. A cracked, cut or missing washer is a fixing that will leak. Do not reuse a screw from a previous job or a pull-out that has been lifted and set back. Replace it with a new fixing that has a sound washer, because the seal on a used fixing cannot be relied on.
Choosing a roof screw for the job in front of you
| Mild inland exposure | Coastal or industrial exposure |
|---|---|
| Coating matched to the sheet manufacturer’s schedule for an inland site | A more protected coating, as the schedule requires for salt or industrial air |
| Bonded washer in sound condition | Bonded washer checked for cracks and distortion before use |
| Length set by sheet thickness and purlin depth | Length set the same way, with a check for timber or steel purlins |
| Driven with a clutch set before the first fixing | Driven with a clutch set and the first fixings checked against a sample |
| Dissimilar metals checked and kept apart | Dissimilar metals and run-off checked with extra care |
Length, embedment and the purlin underneath
A screw has to go far enough into the purlin or batten to hold, and not so far that it strips the thread or breaks through the other side. The length is set by the thickness of the sheet, any packing or spacers, and the depth of the member underneath. The sheet manufacturer’s fixing schedule and the design documents set the embedment, and the builder should check the length against the purlin before the screws are bought in bulk.
Timber purlins and steel purlins take the screw differently. A timber member needs a screw with a thread that bites the timber without splitting it, and a steel member needs a screw that is designed to drill and tap into the steel. Using the wrong type of screw for the member is a common reason for a loose fixing, because the thread does not engage properly and the screw turns freely in the hole.
Check the purlins for any splitting or movement before the fixings go in. A timber purlin that has checked or shrunk will not hold a screw as the design assumes, and the fix is to correct the member rather than to drive a longer screw into it. The frame underneath the sheets, and the tie-down it relies on, is covered in the roof truss bracing article, and a roof that has been checked there is easier to fix on the sheeting side.
Driving technique: the depth that matters
Driving decides whether the washer seals. The screw should go in square to the sheet, at a steady speed, and stop when the washer is compressed just enough to seal the hole. A driver set too high will strip the head, distort the washer and crack the coating, which exposes the metal to corrosion. A driver set too low leaves the washer proud, and the seal does not take. Use a driver with a clutch, set it before the first fixing on each roof, and check the first few fixings against the sample before the rest are driven.
Keep the driver square. A screw driven at an angle will bind, cam out of the head and sit crooked, and the washer will not seat evenly across the sheet. Hold the driver perpendicular to the sheet, let the tip seat in the head, and drive at the speed that keeps the screw from walking. A tilted fixing is a fixing that will leak.
Fix in the pattern the design calls for, which usually means in the crest of the profile for a particular sheet and in the trough for another. Fixing in the wrong place can distort the sheet or leave the washer sitting on a curved surface where it cannot seal. Check the pattern with the sheet manufacturer’s drawing, and mark the fixing positions before the drive begins.
The over-driven washer. The mistake that costs the most is a fast crew driving every screw on the roof at full torque. The washers split, the coating cracks at the head, and the leaks show up across the roof in the first heavy rain, long after the crew has left.
Dissimilar metals and the run-off below
Corrosion does not only depend on the coating. It also depends on what the screw touches. Steel screws in contact with copper, brass or certain other metals can corrode quickly, because the two metals form a cell in the presence of moisture. Water running off a copper downpipe or a copper flashing can carry the metal down onto the sheets and the fixings below. Check the roof for dissimilar metals before the fixings are chosen, and separate them with an appropriate barrier where the design allows.
Look at the run-off as well. A roof that drains over the top of a fixing, or a gutter that overflows onto the sheet, keeps the screw wet and the coating under attack. The fixing has a better life where water runs clear of it, and the gutter and downpipe layout is part of the fixing performance.
Retention money is a useful reminder that defect exposure has a commercial edge. Where a head contractor holds retention money, the NSW Government guidance on retention money explains in general terms how that money is held against the defects period. A roof that starts leaking through its fixings can turn into a dispute over that money, so the fixing decision is as much a commercial one as a technical one. The NSW Government guidance on contracts for home building work is worth reading when the roof is part of a building contract.
A worked example: the coastal roof with the wrong screws
Consider a builder in a coastal suburb on the mid north coast of NSW who is finishing a single-storey house with a steel roof. The screws were bought from a general hardware supplier with the sheets, and they were the standard zinc-coated type that the store stocks for everyday work. The crew started driving them on a windy afternoon, and the supervisor noticed that some screws were going down quickly while others were stripping the head.
The builder checked the sheet manufacturer’s fixing schedule and found that the site, being close to the sea, called for a more protected coating than the screws on the job. The supervisor also noticed that the driver had been set on its highest torque for the whole job, and that several washers were split on the first run of sheets. The builder stopped the crew, pulled the wrong screws and reordered the correct coating and head type.
The re-fix cost more than the original screws, the crew lost a day, and the first sheets had to be lifted and re-fixed where the washers had split. The roof was then sealed and passed the final walk-through without a leak. The client never knew how close the first run had come to a problem.
The lesson is that a roof screw is not a commodity. The exposure, the coating, the head, the washer and the driver all have to be right, and the job is easier to get right at the start than to fix at the end.
Inspecting the fixings before handover
Before the roof is handed over, walk the surface and check the fixings. Look for stripped heads, washers that are proud or split, screws that sit crooked, and any sign of rust running down from a fixing. Check the fixings near the edges, the ridges and the penetrations most closely, because those are the places where wind loads and water concentrate.
Record the screw type, the coating, the head and the washer on the job file, together with the sheet manufacturer’s schedule the crew worked to. A future roofer who needs to add a penetration or replace a sheet will need that information, and a record that shows the correct fixings were used protects the builder in any later dispute.
The steps below set out a practical order for selecting and driving roof sheeting screws. They are a builder’s checklist, not a substitute for the sheet manufacturer’s installation manual, which governs the specific product on the roof.
Fixing the sheets so the washers seal
- 1Confirm the scheduleCheck the coating, head, length and fixing pattern against the sheet manufacturer’s document.
- 2Inspect the membersCheck the purlins or battens for splitting, movement and the right fixing type for the material.
- 3Set the driverSet the clutch before the first fixing and check the first few fixings against a sample.
- 4Drive squareHold the driver perpendicular, seat the tip in the head and drive at a steady speed.
- 5Inspect the sealCheck every washer is compressed and undamaged, and replace any screw that is split, stripped or crooked.
Roof work is a place where good detail earns its keep, and the professional development that supports that work is set out in the PointCPD guide to NSW builder CPD. The course catalogue lists the building materials and fixings modules that cover the same ground from the builder’s side.
The fixing is small and the consequences are large. Choose the coating for the place, check the washer, drive each screw square to the right depth and keep the metals apart. Those four habits stop most of the leaks that builders are called back to fix.
Questions builders ask
What coating should roof sheeting screws have?
The coating should match the exposure and the sheet manufacturer’s fixing schedule. A mild inland site can use a standard protected coating, while a site near the sea or an industrial area needs a more protected coating or stainless steel. Do not choose by price alone.
Can I reuse a roof screw that has been pulled out?
No. A pulled-out screw may have a damaged washer, a stripped thread or a damaged coating, and the seal cannot be relied on once it has been removed. Replace it with a new fixing that has a sound washer and the correct coating for the site.
How tight should roof screws be driven?
Drive each screw until the washer is compressed just enough to seal the hole, without splitting the washer or distorting the sheet. Use a driver with a clutch set before the work starts, and check the first fixings against a sample. Over-driving and under-driving both cause leaks.
Does the washer really matter that much?
Yes. The washer is the seal that stops water running down the screw into the purlin. A cracked, cut, missing or distorted washer is a leak waiting to happen, even if the screw itself is sound. Check every washer before the sheet goes on.
Why do screws rust on a roof that is new?
Usually because the coating does not suit the exposure, the screw touches a dissimilar metal, or water runs over the fixing from a gutter or a downpipe. Check the coating against the site, separate the metals, and check the run-off before blaming the sheet.
What should I check on a roof before handover?
Look for stripped heads, split or proud washers, crooked screws and any rust running down from a fixing, paying most attention to the edges, ridges and penetrations. Record the screw type and the schedule the crew worked to on the job file.
Sources
- Continuing professional development for builders and swimming pool builders · NSW Government
- General building work licensing · NSW Government
- Home Building Act 1989 (NSW) · NSW Legislation
Published 19 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
