A self-drilling screw drills its own hole, taps its own thread and fastens in one step — but only if the drill point is matched to the steel. Most installation problems with these screws are not quality problems; they are selection problems.
Drill point numbers
The point size is given as a number, commonly from #1 to #5. The higher the number, the longer and thicker the drill section, and the thicker the steel it can penetrate.
- #1–#2 points — thin sheet and light-gauge steel, such as sheet-to-sheet stitching.
- #3 point — the most common choice for roofing sheets fixed to steel purlins of light to medium thickness.
- #4–#5 points — heavy structural steel; #5 points are made for thick sections where a normal screw would snap or burn.
As a rule, the drill section must be longer than the total thickness it has to pass through, including all layers of sheet and the purlin wall. If the point is still inside the steel when the thread starts to engage, the screw will jam or break.
Thread and length matter too
Fine threads suit thicker, harder steel; coarse threads pull into timber and thin material faster. The screw length must let the thread fully engage the purlin while the washer seats on the sheet — too short and the fixing is weak, too long and it wastes money on every one of thousands of fixings.
The washer is part of the fixing
For roofing, the bonded EPDM washer seals the hole against leaks. Check its diameter and rubber quality: a washer that is too small, or rubber that hardens and cracks, turns a good screw into a leaking roof. In coastal or industrial areas, match the screw coating to the environment as well — a point covered in our earlier guide on zinc plated vs hot dip galvanized finishes.
A quick selection checklist
- Total steel thickness to be drilled (all layers combined).
- Base material: steel purlin or timber.
- Sheet profile and the washer diameter it needs.
- Environment: inland, coastal or industrial, and the coating it calls for.
Tell us the purlin thickness and sheet type you are fixing, and we will recommend the point size, length and coating before you order samples.