Tek Screws (Self-Drilling Metal Screws)

Tek screw sizes explained

Tek screws are sized as gauge and threads per inch (TPI). A 12-14 is a 12 gauge screw with 14 threads per inch; a 10-16 is 10 gauge with 16 threads per inch. The gauge sets the thread diameter:

Length is measured from under the head for hex, flanged, wafer and pan heads, and from the top of the head for countersunk, bugle and self-embedding heads.

Coarse or fine thread?

Rule of thumb: coarse thread for lighter steel, fine thread for heavier steel.

• •Coarse thread (up to 16 TPI): for lighter steel sections and timber, such as cladding, framing and roofing. Common sizes 10-16, 12-14 and 14-10.
• •Fine thread (over 16 TPI): metal threads for steel only, particularly steel from 2mm up, such as brackets to purlins and lapped purlins. Common sizes 8-18, 10-24 and 12-24.

Drill point types

• •Metal point: for steel sections such as purlins, top hats, roofing and cladding. The point length suits the steel thickness it is made to drill.
• •Series 500 (s500): the deep driller, for drilling and fastening into thick steel.
• •Winged point: for fixing timber or composite to steel. The wings ream a hole through the timber wider than the thread, so the thread cannot lift the timber, then break off when they reach the steel.
• •Type 17: for lighter steel and fixing with timber.
• •Needle point: for general purpose fixing.

Heads, seals and roofing features

• •Hex with washer or flanged hex: the everyday steel screw, driven with a hex socket. A bonded EPDM (neo) washer seals the hole watertight on roofing and cladding.
• •Wafer and button head: a low, wide head that sits nearly flush, for fixing sheet, brackets and trim where a hex head would be in the way.
• •Countersunk: sits flush in a countersunk hole, for a flat finished surface.
• •Scratchguard: protects the screw shank from scratching as it drives through roofing, improving corrosion resistance.
• •XGRiP: holds the roof sheet tight against the seal, so it stays watertight even when the roof is walked on.

Coatings and corrosion: zinc, Class 3, Class 4, X9 or stainless?

Match the coating to the corrosivity category of the site. Categories follow ISO 9223 and AS 4312:

• •Zinc yellow (ZYP): a thin 5 micron zinc coating for low corrosion or temporary fixing.
• •Class 3 (AS 3566.2): 21 micron coating for moderate environments, categories C2 and C3.
• •Class 4 (AS 3566.2): 38 micron coating for high corrosion, category C4.
• •X9 ProteXion (BDX): a coating rated for ISO 9223 category 4 environments.
• •R1000 and R1500: a zinc coating with a hard ceramic topcoat for abrasion resistance.
• •Bi-metal 304 and 316 (stainless with a hardened drill point): for fixing stainless and aluminium and for corrosive sites. Only 316 bi-metal is covered in very severe marine (C5) sites, and extreme (CX) sites need Hobson's written confirmation first. 410 and 550 stainless are also available.

Getting a full-strength fixing

• •In steel, at least three threads should come through the far side to reach full pull out strength.
• •In timber, embed at least 25mm for a 12 gauge screw and 35mm for a 14 gauge screw.
• •Choose a drill point that suits the total steel thickness, and drive at a steady speed without forcing the point.
• •Class 3 and Class 4 screws 10 gauge and larger carry a head stamp showing the maker and the class, so you can check them on the roof.

Tek screws in Adelaide

Allmat supplies tek screws, roofing screws and timber screws to builders, roofers and steel fabricators across Adelaide and South Australia. Add the screws you need to a quote with your sheet, steel and timber, and we will price the lot.

Questions