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Fasteners & specification guides · TSR Europe

Three stainless grades come up constantly in fastener specifications — SS410, SS304, SS316 — and they are not interchangeable, even though all three are called “stainless”. The difference that matters most is not corrosion resistance, it is whether the screw can self-drill at all.

Why SS410 self-drills and SS304/316 do not

SS410 is martensitic stainless: its carbon content allows it to be quenched and tempered to HRC 40–50, the hardness a self-drilling point needs to cut through structural steel. SS304 and SS316 are austenitic — they cannot be hardened by heat treatment, so a solid screw made from either simply is not hard enough to self-drill. That is the whole reason bi-metal fasteners exist: a hardened alloy point does the drilling, an austenitic body provides the corrosion resistance the point cannot.

The magnet test does not work

Magnetism is a property of crystal structure, not quality: SS410 is magnetic because it is martensitic, SS304/316 are non-magnetic because they are austenitic. A magnetic screw is not necessarily inferior, and a non-magnetic one is not necessarily genuine SS316 — some manufacturers use SS304 with a passivation process that reduces surface magnetism. The only reliable verification is a Mill Test Certificate, batch marking, or an independent salt-spray (NSS) lab report.

Material comparison

Grade	Self-drills	Magnetic	Corrosion resistance	Typical role
SS410	Yes (HRC 40–50)	Yes	Good, can surface-stain in coastal air	Solid self-drilling screw
SS304	No	No	Excellent	Body/shank of bi-metal screws
SS316	No	No	Superior — resists chloride pitting	Body/shank of bi-metal screws, marine exposure

Which one for which project

• Dry inland, low corrosivity: solid SS410 — the most cost-effective option with adequate protection.
• Standard coastal / high humidity: SS304 bi-metal — hardened point for drilling, SS304 body for rust resistance.
• Marine or heavy chemical exposure: SS316 bi-metal — the molybdenum content is what stops chloride pitting in these conditions.

As a rule of thumb: SS410 alone is fine away from the coast; anywhere within reach of sea spray, specify a bi-metal fastener rather than a solid stainless screw.

Technical background adapted from TGR/Tsingri manufacturing specifications for the European market.

Fasteners & specification guides · TSR Europe

A roofing fastener that fails first shows up as a leak — a functional problem, usually found before it does much damage. A cladding fastener that fails first shows up as a rust streak on the facade at eye level — a visual problem that a building owner notices immediately. Cladding is driven at angles rather than straight down, is visible rather than hidden under laps, and has far less tolerance for a downgraded surface treatment. All of that changes how the fastener should be specified.

Hex washer head or pan head: not interchangeable

The two most common head types are not alternatives for the same job — each is built for a different fixing point.

Hex washer head with EPDM washer

The standard for exposed fixing of profiled metal cladding. The hex drive delivers higher torque than Phillips or Torx without cam-out, which matters because the bonded EPDM washer needs to be compressed to precisely the right degree: under-driven and it does not seal, over-driven and the washer deforms and the seal fails. A hex-indented variant, with a recessed underside, spreads that compression more evenly on curved or profiled panel faces.

Phillips pan head

A low-profile, flat-underside head used for structure fixing to heavy steel — backing rails, brackets, internal framing — where a flush head is wanted and the fixing is not weather-exposed. It is not the right choice for an exposed cladding face: without an EPDM washer it does not form a weatherpoint, and if it must be used somewhere visible it needs a separate EPDM-faced backing washer.

Matching drill point to substrate

Point	Substrate range	Typical use
#3	0.8–7.0mm steel	Standard cladding to secondary framing, purlins, Z-girt
#5	6.0–12.5mm steel	Heavy structure fixing, backing rails to steel columns/beams

Fine thread (higher engagement per length) generally outperforms coarse thread once the steel substrate is over 2.0mm; coarse thread remains standard for light-gauge steel, timber and valley fixing.

Coating: match to exposure, not just to spec sheet minimums

Zinc plating on carbon steel typically fails within 3–5 years outdoors. A multi-layer Ruspert coating (zinc flake + binder + sealant) is a different order of protection, and is the sensible minimum for any externally exposed cladding fastener — zinc plating should be reserved for internal or sheltered fixings only. For facades within a kilometre of the coast, step up to the higher Ruspert grade or specify SS410; in direct salt-spray or marine exposure, specify stainless or bi-metal even where the base substrate is ordinary carbon steel, because the coating alone will not carry the design life.

Where the fastener head will be visible, colour-matched finishes are available against standard RAL references — confirm the code against the panel spec and allow extra lead time, since colour-matched fasteners are made to order.

Before you finalise the spec

• Panel type and whether the fastener head will be visible on the finished facade
• Substrate and thickness — decides #3 vs #5 point and fine vs coarse thread
• Exposure zone — inland, coastal or marine
• Coating: Ruspert grade, SS410 or bi-metal
• Washer requirement for weather sealing at exposed heads
• Driver compatibility with the installation crew’s tooling

Technical background adapted from TGR/Tsingri manufacturing specifications for the European market.

Fasteners & specification guides · TSR Europe

A timber deck is built to last decades outdoors, but the timber itself is rarely what fails first. Modern pressure-treated lumber (ACQ, MCQ) contains copper-based compounds that are aggressively corrosive to bare steel, and a fastener that stains or weakens before the boards do turns a good installation into a callback.

Decking screws are a different product from general-purpose wood screws: they need corrosion resistance against treated timber chemicals, a drive that tolerates high torque without cam-out, and a point that will not split the board on the way in. The material you choose decides how long the fixing lasts; the drive and point decide how good the installation looks.

Material: SS304, SS316 or coated carbon steel

Material	Corrosion resistance	Best for
SS304 (A2)	Excellent in standard outdoor exposure	Inland decking, residential landscapes, freshwater lakefronts
SS316 (A4, marine grade)	Superior — molybdenum resists chloride pitting	Coastal projects, docks, piers, anything exposed to sea spray
Coated carbon steel (Ruspert)	Good, finite service life vs. solid stainless	Budget inland projects, internal structural framing not on view

Within roughly 5km of a coastline, SS316 is the only specification that reliably avoids pitting corrosion. Coated carbon steel is workable for framing that stays dry and out of sight, but it should not be treated as equivalent to stainless for anything exposed.

Torx drive: fewer stripped heads, less operator fatigue

Phillips drives cam out under the torque needed to drive a decking screw into hardwood — the bit slips, the head strips, and the driver bit wears out faster. The Torx (6-lobe) drive’s vertical sidewalls keep the bit seated at full torque, which means faster, cleaner installation and noticeably less strain on the installer over a full day. TSR decking screws standardise on T20 or T25 Torx.

Type 17 point: no split boards

A screw driven into solid timber without a pilot hole displaces wood fibres and builds up internal pressure, which is what splits a board near its end. The Type 17 point adds a cutting gash to the first threads that removes fibre rather than pushing it aside, effectively drilling its own pilot hole as it drives — useful on any board, essential near board ends and on denser hardwoods.

Specifier’s checklist

• Within 5km of the coast? Specify SS316.
• Fixing into ACQ or MCQ pressure-treated timber? Specify stainless, not coated carbon steel.
• Dense hardwood (ipe, merbau)? A Type 17 point helps, but pre-drilling is still advisable.
• Crew tooling: confirm T20/T25 Torx bits are on site before the delivery arrives.

Technical background adapted from TGR/Tsingri manufacturing specifications for the European market.

Fasteners & specification guides · TSR Europe

A utility-scale solar array is designed for a 25-year service life. In practice, the part that limits that lifespan is rarely the panel or the inverter — it is the joint between the aluminium mounting rail and the steel substructure underneath it, and specifically, the screw that holds the two together.

Aluminium rails (typically 6005-T5 or 6063-T5) are fixed to galvanised steel purlins or cold-formed steel framing. Put two dissimilar metals in electrical contact with rainwater or condensation between them, and you get a galvanic cell: aluminium is the more active metal, so it corrodes to protect the steel fastener. The first sign is a white powdery deposit around the fastener head; left alone, it progresses to pitting and then to an oversized, oval hole. Once the hole is bigger than the fastener head, the rail can shift under wind load.

Why a single-material screw cannot solve this

A fastener hard enough to self-drill through steel purlins has to be either carbon steel or martensitic stainless (SS410) — and both sit far enough from aluminium on the galvanic series to drive the corrosion described above. A screw with better galvanic compatibility with aluminium (SS304 or SS316) is not hard enough to self-drill structural steel on its own.

The bi-metal answer

Component	Material	Role
Body & shank	SS304 or SS316 (austenitic)	Galvanic compatibility with the aluminium rail
Drill point	SCM435 alloy steel, HRC 50–55	Drills 2.0–3.0mm steel purlin at carbon-steel speed

The stainless body keeps the potential difference against the aluminium low enough that the rail stays sound for the design life of the array; the heat-treated alloy point supplies the hardness the stainless body cannot.

Three specification details that matter

• Double thread. The first thread start engages the aluminium rail, the second engages the steel purlin, giving a dual-locking fix that resists the vibration loosening caused by wind-induced panel flutter.
• SS304+EPDM bonded washer. A washer chemically fused to a stainless backing seals permanently where a loose rubber washer would perish or slip during installation.
• Coated drill point. The SCM435 tip is still carbon-alloy steel, so it needs a Ruspert or equivalent coating to stop rust starting at the tip during installation or from trapped condensation.

Worth the premium

Bi-metal fasteners cost more up front than coated carbon steel. On a live array, replacing corroded fasteners means decommissioning strings, arranging access, and in many cases voiding the racking or panel warranty for having used an incompatible fastener in the first place. Specifying bi-metal at the outset is closer to an insurance decision than a materials decision: the smallest component in the system should not be the reason the largest investment fails.

Technical background adapted from TGR/Tsingri manufacturing specifications for the European market.