Self-drilling screws drill their own hole and form their own thread in a single operation — no pilot drilling, no separate tapping. That efficiency made them the default fastener for steel framing, roofing and cladding, and a small family of standards defines exactly what they must do. This guide covers those standards and how to read a self-driller callout.
Self-drilling is not self-tapping
A self-tapping screw forms a thread in a hole that already exists. A self-drilling screw carries a fluted drill point — shaped like a jobber drill bit — that cuts the hole first, then the tapping thread behind it forms the mating thread. The two are often confused in purchasing; on drawings and RFQs, the governing standard removes the ambiguity.
The governing standards
| Standard | What it covers |
|---|---|
| ISO 10666 | Mechanical and functional properties: heat-treated (case-hardened) steel, drilling capability, torsional strength |
| ISO 15480 | Hexagon washer head self-drilling screws — the roofing/cladding staple |
| ISO 15481 / 15482 / 15483 | Cross-recessed pan / countersunk / raised countersunk head self-drilling screws |
| DIN 7504 | Legacy German product standard, still common on drawings: form K (hex), N (pan), P (countersunk) |
| AS 3566.1 / 3566.2 | Australian building standard: Part 1 mechanical requirements, Part 2 corrosion resistance Classes 1–4 |
| SAE J78 · ASME B18.6.3 | Inch-series mechanical requirements and product dimensions for US projects |
All the ISO product standards share the ST (spaced) thread defined in ISO 1478 and the mechanical requirements of ISO 10666. A screw marked only "DIN 7504K" and one marked "ISO 15480" are, for practical purposes, the same product.
Reading the size
A self-driller callout carries three figures — gauge, threads per inch, length: 12-24 × 40mm is a 12 gauge screw (5.5mm major diameter), 24 TPI, 40mm long. The TPI matters here more than on any other screw type, because the same gauge is made in coarse and fine pitches with different jobs: coarse threads (up to ~16 TPI) suit thin sheet under about 2.4mm; fine threads (above 16 TPI) suit thicker steel. Length is measured point to underside of head, except countersunk heads, which measure overall. For gauge-to-metric conversions, see our unit converter.
Drill points set the capacity
The point number describes the drill flute length, and the flute must fully penetrate the steel before the thread engages — otherwise the thread outruns the drilling and the screw jams or snaps. A short No. 2 or No. 3 point handles common sheet and purlin thicknesses; a long No. 5 point is needed from around 6mm steel upward, with heavy-section screws drilling to 12mm. Winged points add wings that bore a clearance hole through timber, then break off against the steel below — the standard detail for fixing decking or cladding over steel framing.
Corrosion class
Because most self-drillers live outdoors, AS 3566.2 assigns Classes 1 to 4 of corrosion resistance, verified by salt-spray testing — Class 3 is the usual specification for external building work, Class 4 for severe and coastal environments. Mechanically galvanised, zinc-flake and zinc-ceramic systems (Ruspert and similar) are the common routes to Class 3 and above; the trade-offs between them are covered in our coatings guides.
Specifying one, unambiguously
12-24 × 40mm hex washer head self-drilling screw, No. 5 point, ISO 15480, AS 3566 Class 3
Size, head, type, point capacity, product standard, corrosion class — six items, no follow-up questions from the supplier. Leave out the point number or the class, and you've left the two most failure-prone decisions to chance.