Fastener selection on a roofing or cladding job is not a detail to sort out at the counter by grabbing whatever worked last time. The substrate underneath the sheet, whether that is a timber batten, a steel purlin of a specific thickness, or a concrete structure, dictates the fastener type, point geometry, coating and driving method. Get that wrong and you are trading pull-out strength for convenience, or sealing performance for speed.
This guide covers the three main substrate categories, the washer behaviour that determines whether a penetration seals or weeps, and the galvanic compatibility rules that matter most on coastal and industrial jobs.
Timber Substrates: Type 17 Screws and Why the Thread Matters
Timber battens and timber purlins require a Type 17 screw. The Type 17 designation refers to the point geometry: a sharp, single-fluted cutting tip that removes a plug of timber as the screw enters, reducing the splitting force on the batten and allowing the thread to bite cleanly without pre-drilling.
The thread form on a Type 17 is coarser than a metal thread, with a wider pitch designed to grip timber fibres rather than sheet metal. Using a fine-thread metal screw into timber produces poor pull-out values because the thread does not engage enough fibre depth. Conversely, using a Type 17 into steel does nothing useful: the cutting flute is not hardened for steel penetration and the coarse thread will strip in thin gauge material.
For roofing into timber, the standard screw is a 14g x 50mm or 65mm hex-head Type 17 with a neoprene-bonded steel washer, in Class 3 or Class 4 coating depending on the corrosion zone. Batten thickness matters: the screw needs sufficient thread engagement below the batten face, which AS 3566.2 addresses when specifying coating class, but the structural pull-out requirement is a function of the fixing design, not just screw length. Where wind uplift loads are specified by an engineer, confirm the embedment depth and fastener schedule before ordering.
Steel Purlins: Matching the Tek Point to the Steel Thickness
Self-drilling screws, commonly called tek screws after the original trade name, are designed to drill their own hole and form a thread in one operation. The key variable is the drill point number, which corresponds to the steel thickness the point can penetrate before the thread starts to engage.
If the thread engages before the drill point has cleared the steel, the screw jams. The sheet lifts, the washer gets crushed trying to compensate, and the fixing never clamps properly. This is not a torque problem; it is a point selection problem.
The common point ratings and their approximate steel thickness capacities are:
- Point 1 (short point): up to approximately 0.8mm total steel thickness
- Point 3: up to approximately 3.5mm
- Point 4: up to approximately 5.5mm
- Point 5: up to approximately 6.5mm
- Point 6: up to approximately 12.5mm
These figures are manufacturer-specific and should be confirmed against the fastener supplier's datasheet. When you are fixing through a sheet and a purlin flange, the combined thickness of both materials determines the point you need. A 0.42mm BMT Trimdek sheet over a 3mm purlin flange requires a Point 3 or Point 4, not a Point 1. Using a short point on thicker steel produces a stripped, spinning screw that has not engaged the purlin at all.
For structural purlins above 6mm, pre-drilling is often the more reliable approach, followed by a self-tapping hex head screw rather than a self-drilling point. If you are fixing into RHS or SHS sections where the combined wall thickness exceeds the point capacity, check the purlin specification and select accordingly.
Concrete Substrates: Mechanical vs Chemical Anchors
Fixing roofing or cladding back to a concrete structure, whether a tilt-up wall, a concrete purlin or a structural slab, requires either a mechanical anchor or a chemical anchor. Neither is universally superior; the choice depends on edge distance, spacing, base material condition and the load type.
Mechanical anchors (expansion anchors, screw anchors and undercut anchors) work by mechanical interlock or friction. Screw-type concrete anchors such as Tapcon-style fixings are suitable for lighter cladding loads into sound concrete where edge distances are adequate. Expansion anchors generate outward pressure to grip the hole wall, which means they are sensitive to edge distance and to cracked concrete, where the crack can relieve the clamping force. Always check the manufacturer's load tables for the specific concrete compressive strength on your project.
Chemical anchors use a resin system injected into a drilled hole to bond a threaded rod or rebar. They are preferred where edge distances are tight, where loads are high, or where the concrete is cracked. The resin requires a clean, dust-free hole, which means blowing out the drilling debris and, in some cases, wire-brushing the hole walls. Skipping the cleaning step is the most common reason chemical anchor installations fail to reach published load values. Cure time before loading is temperature-dependent and must be observed; driving a load onto an uncured anchor defeats the purpose of the system.
For cladding rails fixed back to concrete, the anchor design is typically an engineering matter. CSA supplies materials; the structural adequacy of the fixing pattern and the anchor specification should be confirmed by the project engineer.
Sealing Washers: The Detail That Determines Watertightness
Every exposed-fixed roofing and cladding screw relies on a sealing washer to prevent water ingress at the penetration. The washer is a bonded assembly: a metal backing plate with a neoprene or EPDM pad vulcanised to it. When the screw is driven correctly, the pad compresses against the sheet surface and forms a seal around the hole.
Two failure modes are common, and both are caused by driving technique.
Over-driving crushes the neoprene pad. The metal backing plate contacts the sheet, the pad extrudes outward and loses its sealing geometry, and the screw head can dimple or crack the sheet around the hole. On colour-coated steel like COLORBOND, an over-driven screw also damages the paint system at the penetration point, creating a corrosion initiation site. The fix is not to back the screw out; the washer is already deformed and the hole is oversized. The penetration needs to be moved and the original hole sealed.
Under-driving leaves the neoprene pad partially compressed or not compressed at all. The washer rocks under foot traffic or thermal movement, the seal is intermittent, and water tracks down the thread into the batten or purlin. This failure is less visible than over-driving but equally damaging over time.
Correct driving torque is the control point. Most roofing screw manufacturers publish a recommended torque range. On site, the practical method is to drive until the washer is flush and the neoprene is compressed to the edge of the backing plate, with no extrusion visible. Screw guns with adjustable clutch settings or depth-stop nose pieces make consistent driving achievable across a large roof area. Setting the clutch on a scrap piece of the same sheet material before starting a run is standard practice.
Inspect washers periodically during installation. Neoprene hardens and cracks over time in UV exposure, which is a maintenance issue rather than an installation defect, but washers that arrive on site already cracked or delaminated from the backing plate should not be used.
Galvanic Compatibility: Coating Class and Mixed Metals
The coating on a roofing fastener is not decorative. It determines how long the fastener resists corrosion and whether it accelerates corrosion in the surrounding sheet material.
AS 3566.2 classifies roofing screws by corrosion resistance:
- Class 3: mechanically galvanised or hot-dip galvanised coating, suitable for interior and sheltered exterior applications in low-corrosion environments
- Class 4: a heavier or more corrosion-resistant coating (typically hot-dip galvanised to a higher specification or stainless steel), required in marine, industrial and high-humidity environments
- Class 5: austenitic stainless steel, for severe marine environments within one kilometre of surf coastline or in aggressive industrial atmospheres
BlueScope's published guidance for COLORBOND and Zincalume products specifies the minimum fastener class for each corrosion zone. Using a Class 3 screw in a zone that requires Class 4 is a warranty and durability issue, not just a specification shortcut.
Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, typically moisture. The less noble metal corrodes preferentially. On a coastal job, a zinc-coated steel screw fixing an aluminium cladding panel creates a galvanic couple that accelerates corrosion of the aluminium around each fixing point. Stainless steel screws into aluminium are a lower-risk combination because the potential difference is smaller, but the aluminium remains the anodic material and some preferential corrosion still occurs.
The practical rules for coastal and industrial jobs:
- Match fastener coating to the sheet material and the corrosion zone, not to the price bracket
- Do not use copper or brass components anywhere in contact with steel or aluminium roofing
- Avoid mixing stainless and plain carbon steel in the same fixing assembly
- Where dissimilar metals must contact, use an isolating washer or tape to break the electrical path
For Zincalume substrates, the zinc-aluminium alloy coating is compatible with standard zinc-coated fasteners because the potential difference is small. For COLORBOND, the paint layer provides some isolation, but the hole created by the screw exposes the base metal, so fastener coating class still applies.
Ordering the Right Fastener
Fastener selection is a function of four variables: substrate type and thickness, sheet material and coating, corrosion zone, and the load case. Treating any of these as a secondary consideration produces failures that are expensive to rectify after the roof is complete.
CSA stocks roofing and cladding fasteners across the standard Type 17 and tek screw ranges, in Class 3 and Class 4 coatings, alongside the sheet materials they are designed to fix. If you are specifying a job and want to confirm point selection against purlin thickness or coating class against a specific corrosion zone, the trade desk can work through the combination with you. Visit constructionsupplies.group/au to view stocked lines or to get in touch.