A screw is four decisions, not one. Get the head, the drive, the thread and the point right and the gauge and length follow almost automatically. Get them wrong and you strip drives, split timber, or hang a job on a fastener that was never made for the substrate.
The head decides how the screw finishes against the surface, and how much bearing area it has to pull against.
| Head | Sits | Use it for |
|---|---|---|
| Countersunk (CSK) | Flush or below | Hinges, hardware plates, anywhere the head must not foul a moving part or a finished surface |
| Pan | Proud, flat underside | Sheet metal and general fixing where flush is not required — the broad bearing face resists pull-through |
| Button / dome | Proud, rounded | Exposed work where the head is part of the look, and where a snag hazard matters |
| Hex / hex flange | Proud | High torque without cam-out — roofing, framing, anything driven with a socket |
| Bugle | Flush, tapered | Plasterboard and fibre cement — the curve pulls in without tearing the paper face |
| Wafer | Near flush, wide | Thin sheet and battens where you need bearing area but not a countersink |
Countersunk heads need a countersunk hole. In steel or hardwood that means countersinking; in softwood and chipboard the head will usually pull its own recess, which is exactly what you want for a self-countersinking chipboard screw and exactly what you do not want under a hinge leaf.
Drive choice is really a torque decision. Slotted is fine for light hand work and traditional detailing but cams out under a driver and will not centre itself. Phillips centres and is universally available, but it is designed to cam out — useful in 1935, less so on a cordless impact driver. Pozidriv adds four secondary ribs, holds more torque than Phillips and needs a PZ bit, not a PH one. Square and Torx hold the most torque and let you drive one-handed overhead.
The practical rule: match the bit to the recess exactly. Most "stripped screws" are a PH2 bit spinning in a PZ2 recess.
| Thread type | Holds in | Notes |
|---|---|---|
| Self-tapping (sheet metal) | Thin steel, aluminium, plastic | Fully threaded, forms its own thread in a pilot hole. Needs the right pilot — too big and it spins, too small and it snaps. |
| Wood screw | Solid timber | Partial thread with a plain shank. The unthreaded shank lets the top member pull tight against the bottom one. |
| Chipboard / construction | Particleboard, MDF, timber | Fully threaded, coarse, sharp. Fast in soft sheet material. Full thread means no clamping action — drill a clearance hole in the top piece if you need the joint pulled up. |
| Drywall | Plasterboard to timber or steel stud | Coarse thread for timber, fine thread for metal stud. They are not interchangeable. |
| Machine screw | A nut or a tapped hole | Parallel shank, constant thread. It does not form its own thread — it needs one waiting for it. |
| Carriage bolt | Through timber, with a nut | Square neck under a dome head pulls into the timber and stops the bolt turning while you tighten the nut. |
If a screw is threaded all the way to the head and both members are threaded, the screw cannot pull them together — it just holds them where they already are. Any gap you started with stays. Either use a partially threaded screw, or drill the top member to clearance so the thread only bites the bottom member.
Screw gauge is a nominal shank diameter, not a millimetre measurement. Ours run:
| Gauge | Approx. shank | Typical use |
|---|---|---|
| 2g | 2.2 mm | Small fittings, electrical plates, light cabinetry |
| 4g | 2.9 mm | Hinge leaves on light doors, hardware, sheet metal |
| 5g | 3.2 mm | General cabinet and joinery work |
| 6g | 3.5 mm | The workhorse — general fixing, drywall, light framing |
| 8g | 4.2 mm | Door hardware, medium framing, heavier sheet |
| 10g | 4.8 mm | Structural timber connections, heavy hinges, brackets |
| 12g | 5.5 mm | Heavy duty, batten and roofing work |
| 14g | 6.3 mm | Heaviest hardware and structural fixings |
For length, the usual working rules are:
Hardwood, near an edge, or anything you cannot afford to split — drill a pilot at roughly the screw's root diameter (the shank minus the threads), and a clearance hole at the full shank diameter through the top member. Softwood and chipboard usually need neither, but a hardwood door stile absolutely does.
Everything above assumes the screw survives its environment. Mild steel with a zinc plating is an interior fastener. Outside, and anywhere near the coast, the material matters more than the head does — which is a separate conversation, and one we have written up in stainless or mild steel in coastal Queensland.
Every gauge, length and pack quantity we stock is listed in the screw range on the home page, and in the screw catalogue. If you are specifying volume and want a sample before you commit, send us the specification.