Key takeaways
- The drill flute must be longer than the total steel thickness, or the thread engages before the hole is finished and the screw binds.
- Point #2 covers most roofing sheet to purlin up to 2.8 mm; #3 and #4 handle light structural steel; #5 is for 6–12.5 mm hot-rolled sections.
- Never use a self-drilling screw on steel thicker than its rated drilling capacity. Pre-drill instead.
A self-drilling screw is three tools in one: a drill bit, a thread-forming tap and a fastener. The part buyers most often get wrong is the drill point. Choose one that is too small and the screw jams and snaps; choose one that is too large and installers waste time and break bits on thin sheet.
What the point number means
Points are numbered by the thickness of steel they can drill. The number does not describe the screw diameter. A #3 point can appear on a 4.2 mm screw and on a 6.3 mm screw.
| Point | Total steel thickness | Typical use |
|---|---|---|
| #1 | 0.5–0.9 mm | Light sheet, ductwork |
| #2 | 0.9–2.8 mm | Roof sheet to light purlins, steel studs |
| #3 | 2.8–4.4 mm | Cold-formed purlins and girts |
| #4 | 4.4–5.25 mm | Heavier cold-formed sections |
| #5 | 6.0–12.5 mm | Hot-rolled structural steel |
Add up every layer of steel the screw passes through, including the roof sheet and any brackets, then choose the point that covers that total.
The flute rule
The unthreaded section behind the tip, called the pilot section, has to clear the full material thickness before the first thread reaches the steel. If the thread arrives first, it tries to pull the screw forward faster than the drill can cut, and the screw seizes or shears. That is why a #5 screw has a long pilot and a #2 a short one.
Reading a size like ST5.5 × 32
Under DIN 7504 and ISO 15480 the first figure is the thread diameter (5.5 mm, roughly #12) and the second is the length under the head. For roofing screws with a washer, length is still measured from under the head, not from under the washer.
Driving speed matters
Drill points cut best at controlled speed. As a guide, run 4.2–4.8 mm screws at 1,800–2,500 rpm and 5.5–6.3 mm screws at 1,800 rpm or less. Impact drivers can overheat and dull the tip in thick steel; a screw gun with a depth-sensitive nosepiece gives better results, especially with EPDM washers.
Common mistakes we see
- Ordering one point for the whole job when the roof sheet goes into both light and heavy purlins.
- Using a self-drilling screw where the steel exceeds 12.5 mm. Pre-drill and use a tapping screw or bolt.
- Over-driving until the washer squeezes out sideways. Stop when the washer is compressed by about a third.
Not sure which point your job needs? Send us the steel thickness and sheet profile and we will recommend a screw and send samples to test on site.
Frequently asked questions
Can a #5 point drill thin sheet?
Yes, but it is slower and the long pilot can wobble in thin sheet. Use the smallest point that covers the full thickness.
Do bi-metal screws use the same point numbers?
Yes. Bi-metal screws are usually offered in #1 to #3, because the stainless thread section limits how much heavy drilling they should do.
