Tek Screw Sizes Explained: Chart, Points & Uses

They're called Tek screws, self-drilling screws, roofing screws, cladding screws — but on the packet, what actually matters is three numbers: the diameter, the length, and the drill point. Get the first two wrong and the fixing's under-spec; get the drill point wrong and you'll burn out tips, snap screws or spin holes that never tighten up.

Here's the short version, then the full chart.

Quick reference — the three numbers

Number	What it tells you	Example
Diameter (mm or gauge)	Thread thickness — drives strength and drill capacity	4.8mm (No.10 gauge), 5.5mm (No.12), 6.3mm (No.14)
Length (mm)	How far it reaches — sheet + at least a solid bite into the frame	5.5 × 32mm = light-section steel fix
Drill point (No.1–No.5, or Type 17)	The built-in drill bit tip — matched to the metal thickness	No.5 point for heavy section steel, Type 17 for timber

The drill point chart — what the numbers mean

The point number is the real spec: it tells you the thickest steel the screw can drill and tap in one pass. Match this before anything else.

Point	Typical drilling capacity	What it's for	At Fastco
No.1 (reduced point)	Sheet to sheet only	Stitching overlapping cladding sheets together — no frame underneath	Stitcher screws
No.2	Steel up to ~2.5mm	Thin sheet metalwork and light trims	Stocked within the light-section range
No.3	Steel 1.2–5mm	Zed purlins, rails and light-section steel — the roofing and cladding workhorse	Sheet to light-section steel
No.5	Steel 6–12.5mm	Heavy section steel, RSJs and structural frames	Sheet to heavy-section steel
Type 17 (gash/slash point)	Timber — not for steel	Steel sheet into timber purlins: the point cuts the timber as the thread forms	Sheet to timber

The classic mix-up: a No.3 point will start in heavier steel but stall and snap before it taps the thread — and a No.5 point in thin sheet will tear the hole instead of cutting it. If you're fixing to RSJs or thick columns, go No.5. If you can dent the frame with a screwdriver, No.3 is the answer.

Gauge vs millimetres — the conversion

Tek screw sizes jump between gauges and millimetres depending on the brand. The common ones:

Gauge	Diameter	Typical use
No.8	4.2mm	Light trims, stitchers, small sections
No.10	4.8mm	Light steel, small cladding profiles
No.12	5.5mm	The standard for roofing and cladding to steel
No.14	6.3mm	Timber purlins, fibre cement, heavy duty fixings

If a job sheet says "5.5 tek" and the packet says No.12, they're the same screw.

Choosing the length

The length includes the point — it has to pass through your top sheet and leave enough thread biting into the frame:

Into steel: all the way through the sheet, plus at least three full threads (roughly 10mm) into the section. For a 0.7mm sheet on a 3mm purlin, that puts a 25mm or 32mm screw in range.
Into timber: aim for 25–30mm of embedment past the sheet. A 6.3 × 45mm screw through 22mm board leaves a solid hold in the purlin.
Don't over-length it: too much protrusion means thread engaged where it isn't doing anything — and on roofs, a longer screw is more likely to be driven at an angle and weep.

Our most-used sizes at a glance (all live on site):

Application	Sizes we stock
Light-section steel	4.8 × 16mm · 5.5 × 19 / 25 / 32 / 38 / 43mm
Heavy-section steel	5.5 × 25 / 32 / 38 / 60 / 80 / 100mm
Sheet to timber	6.3 × 25 / 32 / 45 / 60 / 80 / 100mm
Stitchers (sheet-to-sheet)	6.3 × 22 / 25mm
Fibre cement to timber	6.3 × 130mm

That's the everyday core of the range — from 16mm stitchers up to 130mm+ for cement board on purlins.

Driver, washer, finish

Drive: hex head Tek screws use a socket adapter, not a standard bit — our Tek screw adapters keep the screw square to the surface and stop the skittering that rounds off heads. This is also the answer to "what bit do Tek screws need" — an 8mm hex adapter covers the standard sizes.
Sealing: on anything weather-exposed, use bonded washers — 16mm (G16) suits most profiles, 19mm (G19) for wide-crown profiles. Most of our roofing packs come with washers included.
Finish: colour caps match the screw head to the cladding and add a weather seal on top.
Speed matters as much as the point: slow the drill down for thicker steel — a No.5 point driven at full trigger temperature is the most common way to kill one mid-fix.