Ask for a “tek screw” on a site in Brisbane and you will be handed a self-drilling screw. Ask for a “self-tapper” and you might be handed the same thing. They are not the same thing, and the difference decides whether the fixing holds for twenty years or backs out in two. This note sets out what each screw actually does, where each one belongs, what the Australian standards say about them, and the handful of mistakes that account for most of the failures we are asked about.
A self-tapping screw forms or cuts its own thread in a hole that already exists. You drill or punch a pilot, drive the screw, and the thread shapes the material around it. The screw does one job: threading.
A self-drilling screw has a drill point — a short fluted tip shaped like the end of a drill bit — ahead of the thread. Under a power driver it drills the hole, then the thread behind the point forms the mating thread, in one continuous operation. No pilot, no second tool, no change of bit. It does two jobs, and it is only as good as the drill point.
Every self-drilling screw is therefore also self-tapping. The reverse is not true, and that is where the confusion on site comes from.
The drill point exists for metal. Specifically, for driving through steel or aluminium where pre-drilling every hole would double the labour: roofing and wall sheet to purlins and girts, aluminium section work, and sheet fixed to sheet. On a roof with several hundred fixings per sheet run, the saved operation is the whole reason the screw exists.
The constraint is thickness. The drill point has to finish drilling before the thread starts engaging; if the thread arrives while the point is still cutting, the screw stalls or strips the hole it has half-made. So the combined thickness of what you are drilling through has to be less than the length of the drill point. A point sized for sheet-to-purlin will not drill a thick flange, whatever the driver does.
| Our self-drilling line | Detail |
|---|---|
| Line | MS CSK Phillips Self Drilling Screw — mild steel, plated, countersunk head, Phillips drive |
| Gauges | 6 (3.5 mm), 7 (3.9 mm), 8 (4.2 mm) |
| Lengths | 13 mm to 75 mm, 20 sizes |
| Packs | 1,000 for the short lengths, 500 and 200 as the length increases |
| Intended for | Roofing sheet fittings, aluminium section work, joining sheet metal to sheet metal |
The scale is worth stating. The ABS counted 205,249 dwellings approved across Australia in 2025–26, up 9.2 per cent on the year before, and Queensland alone had 4,841 dwelling units approved in June 2026 on the seasonally adjusted series. The Queensland Government Statistician’s trend estimate was running at about 4,460 approvals a month by August 2026. Most of those roofs are sheet metal, and every sheet is held on by screws. The fastener is the cheapest component on the roof and the one that decides whether the roof stays on.
That is literally the test in the north of the state. Under the National Construction Code, buildings in wind regions C and D — the cyclonic coastal strip that takes in Mackay, Townsville and Cairns — must meet Specification 4, which requires that metal roof cladding, its connections and the immediate supporting members “must be capable of remaining in position notwithstanding any permanent distortion, fracture or damage” under a sequence of cyclic pressure loads. In those regions the roofing screw is a structural element with a test regime behind it, and the manufacturer’s cyclonic fixing table — gauge, spacing, washer — is not advisory.
Corrosion has its own standard. AS 3566.2 is the Australian standard for self-drilling screws in building and construction, and it defines four classes of corrosion resistance, tested by neutral salt spray and other accelerated methods, so that a screw can be matched to its exposure. When a roofing specification says “Class 3” or “Class 4”, that is what it means. Which class a site needs comes from AS 4312, the atmospheric corrosivity map of Australia: the Galvanizers Association’s summary puts metropolitan Brisbane and the Gold Coast in category C3 (coastal), the first 200 metres back from a surf beach in C5, and the strip from there to about a kilometre inland in C4. By that distance rule the surf suburbs of the Sunshine and Gold Coasts fall in C4 and C5; much of suburban Brisbane is C3; a few bayside suburbs on quiet water are C2.
And the climate keeps the steel wet. The Bureau of Meteorology’s Brisbane station averages 1,056 mm of rain a year over 127 rain days, with mean relative humidity of 63 per cent at 9am and 52 per cent at 3pm. Time of wetness is what drives atmospheric corrosion, and Brisbane has a lot of it.
Everywhere the hole is already there, or can be made better by a drill than by a screw point. Timber battens and framing. Plastics and trims. Pre-punched or pre-drilled brackets. Thin sheet that would tear under a drill point. Anywhere swarf matters, because a drill point throws steel filings onto whatever is below it, and on a painted or plated surface those filings rust in a week.
It is also where the range is. Our self-tapping lines run from 2 to 12 gauge and 4 mm to 100 mm in mild steel — 65 sizes per line — and from 4 to 12 gauge, 6 mm to 100 mm in stainless. Four head and drive combinations in each: countersunk or pan, slotted or Phillips. The self-drilling line, by comparison, is one head, one drive, three gauges. That is not a shortcoming; it reflects what the screw is for.
The thread letter tells you what the screw was designed to bite into. We stock six thread styles across the range; these are the four that matter for this decision.
| Thread | What it is | Bites best into |
|---|---|---|
| AB Type | Spaced thread, sharp gimlet point | Thin sheet metal, timber, soft materials with a pilot |
| B Type | Spaced thread, blunt point | Thicker sheet, non-ferrous castings, plastics — where a sharp point would wander |
| BT Type | Wider-spaced, thread-cutting flutes | Plastics and brittle materials that would crack under a forming thread |
| Drill Point | Thread behind a fluted drilling tip | Steel and aluminium without a pilot |
| You are fixing | Use | Because |
|---|---|---|
| Roof or wall sheet to steel purlin or girt | Self-drilling, to the AS 3566.2 class the site needs | No pilot through the sheet and the purlin; one operation per fixing |
| Aluminium extrusion or section | Self-drilling, short length | Drills clean; keep the length to the section wall plus thread engagement |
| Sheet to sheet, pre-punched | AB self-tapping | The hole exists; a drill point adds swarf and nothing else |
| Timber batten or frame | AB self-tapping, or a wood screw | A drill point tears fibres and loses hold; a tapping thread forms cleanly |
| Plastic trims, PVC, acrylic | BT self-tapping | Thread-cutting flutes relieve the stress that cracks plastic |
| Hardwood, or near an edge | Self-tapping with a pilot | See the pilot-hole note in the screw selection piece |
A self-driller into timber. The point drills a hole wider than the thread root, the thread has little to grip, and the fixing feels tight and then is not. Use a tapping or wood thread.
Overdriving thin sheet. Once the head seats, further torque strips the one or two threads the sheet has to offer. The hole is now oversize and the only fix is the next gauge up. A driver with a clutch, set low, pays for itself on the first job.
The wrong corrosion class for the exposure. Our self-drilling line is plated mild steel, suited to interior and sheltered work. It is not a Class 3 or Class 4 roofing screw and we do not sell it as one. For an exposed roof in a C4 or C5 zone — anywhere within a kilometre of surf — specify to the class the roofing manufacturer calls for, or pre-drill and use the stainless self-tapping lines, and read the coastal Queensland note before you choose the grade.
A drill point wants steady, moderate speed with firm pressure — it is cutting, not punching. Run it too fast and the point overheats and dulls before the hole is through; stop pushing the moment the head seats. If the screw spins without advancing, the point has given up: back it out and start a fresh screw rather than forcing it.
Every gauge, length and pack quantity we stock is listed in the screw range on the home page, and in the screw catalogue. If you are specifying volume for a roofing or sheet-metal program and want a sample before you commit, send us the specification.