Screws and the screw driver that fits them: types and drives

Match the screw driver to the drive

The recess or shape on top of the head is the drive, and it decides which screw driver or bit you need. Stahlwille’s guide to screw drives describes the main ones:

• Slotted. The traditional single slot. It is hard to centre the blade, and the tool slips out easily.
• Phillips (PH). A cross that centres the tip, but its tapered flanks push the tool out under torque, which is called cam-out. Common sizes are PH0 to PH3.
• Pozidriv (PZ). A cross with parallel flanks and small extra notches between the arms. Common sizes are PZ0 to PZ4.
• Torx (TX). A six-lobed star. Stahlwille notes there is no axial force and no cam-out.
• Hex socket and external hex. Sized across flats; external hex heads are turned with a spanner or socket.

A Phillips screw and a Pozidriv screw look alike, but Stahlwille is clear that the tools are not compatible and that using the wrong one damages the fastener head. Check the marking on the screw bits: PH and PZ bits of the same number are different tools.

Types of screws

The thread and point, not the head, decide what a screw can go into. Most types of screws fall into a few families:

Types of screws

Type	What it is	Where it goes
Wood screws	sharp point and coarse thread; round or flat head for a flush finish	wood, also plastic
Coach screws (lag screws)	large wood screws with a sharp point, coarse thread and usually a hex head for more torque	beams, posts, outdoor fixings, a trellis on a brick wall
Self-tapping screws	cut their own thread in a pre-drilled hole	sheet metal, plastic, wood
Self-drilling screws (tek)	drill-shaped tip that makes the hole and then the thread	steel and aluminium sheet, metal to timber
Concrete screws	heat-treated carbon steel with a hardened tip that cuts its own thread	concrete, driven fully home for maximum load
Machine screws	small fasteners chosen by size, thread pitch and head	panels and components; brass, stainless or zinc-plated steel

The descriptions come from Konnect Fastening Systems’ guide to screw types, Atlas Fasteners and JC Fasteners. A countersunk screw, in Wilson-Garner’s definition, has a head that sits level with the surface once driven; a pan or hex head stays proud. In joinery, screws often work together with PVA wood glue, which holds the joint once it has set.

Self-tapping or self-drilling screws

Self-tapping screws, or self tappers, form a thread but need a hole first. Self-drilling screws drill the hole as well: Atlas describes a tip shaped like a drill bit, so no pilot hole is needed, which suits metal-to-metal and metal-to-wood fixing.

JC Fasteners adds the points that decide whether a self-drilling screw will work:

• The length of the flute, the groove behind the tip, sets the thickness of metal the screw can drill.
• Screws made entirely of 304 or 316 stainless steel cannot be hardened, so they drill only softer metals such as aluminium; bi-metal screws with a carbon steel point drill steel.
• Hex flange heads under DIN 7504 K are used with an EPDM washer for a leak-proof fixing on roofing, for example roofing sheets on a steel frame.
• Winged screws are recommended when fixing wood thicker than 12 mm to metal.
• Drive them with a drill, not an impact driver or hammer drill, to spare the point.

Wall plugs: the Fischer screw fixing

A screw in brick or concrete needs a plug, and many people simply ask for a Fischer screw, after the German maker. Fischer’s SX Plus plug data shows how the parts are matched: the drill size is the plug size, and each plug takes a set range of screw diameters.

Wall plugs: the Fischer screw fixing

Plug	Drill	Minimum hole depth	Screw diameter
SX Plus 5 × 25	5 mm	35 mm	3.0–4.0 mm
SX Plus 6 × 30	6 mm	40 mm	4.0–5.0 mm
SX Plus 8 × 40	8 mm	50 mm	5.0 mm (5.0 × 55 mm in the kit)

Fischer gives the screw length as plug length plus the thickness of the part being fixed plus the screw diameter. A 6 × 30 plug holding an 18 mm board with a 4.5 mm screw therefore needs a screw of at least 52.5 mm (our calculation: 30 + 18 + 4.5). Fischer lists concrete, solid and perforated brick, aerated concrete, natural stone and solid gypsum board among the base materials; for hollow gypsum board partitions, check that the plug is rated for that base.

How screws are measured

Wilson-Garner, a US fastener maker, measures screw length from the flat of the head to the point, so a countersunk screw is measured over its whole length, while the major diameter runs from the crest of the thread on one side to the crest on the other. Screws are sold as diameter × length in millimetres, as in the 5.0 × 55 mm screw in the Fischer kit.

What to check when you buy

• Drive. PH, PZ or TX on the box, and a screw driver or bits to match.
• Thread and point. Wood thread, self-tapping, self-drilling or concrete thread for the material you are fixing into.
• Coating or steel. Zinc-plated carbon steel or stainless; for self-drilling into steel, a hardened or bi-metal point.
• Size. Diameter × length, with the length worked out for the plug and the part being fixed.
• Plugs. A plug sized to the drill and the screw range, rated for the wall you have.
• Nuts and bolts. Konnect stresses the right size and thread pitch for machine screws, so buy screws and nuts together.