A proprietary multi-layer coating or stainless - ordinary zinc plating does not last in treated timber outdoors.

Several modern preservative chemistries are aggressive to zinc, and the coating fails in a season or two.

A corroding screw stains the board around its head, which is the most visible defect a deck can develop.

It also loses section at the surface, which is exactly where the fixing is worked hardest by board movement.

Stainless is the choice for hardwood, for coastal exposure and wherever staining is unacceptable.

The timber treatment supplier states which fastener finishes are compatible, and that statement is the one to follow.

They cut their own countersink so the head finishes flush without lifting a ring of fibre.

Driving a plain countersunk head into a softwood deck compresses the surrounding fibre, which then swells and stands proud.

Cutting nibs remove that material instead of compressing it, leaving a clean recess.

The result is a head that sits flush without the screw being over-driven.

Over-driving to force a head flush crushes the board surface and starts water ingress.

On hardwood and composite the head design is different again, and the manufacturer's recommendation should be followed.

Partially - the smooth shank is what lets the head pull the board down tight.

With a plain shank through the deck board, the thread engages only the joist, so tightening draws the two together.

A fully threaded screw engages the board as well and can hold a gap open permanently.

That is the usual reason a board sits slightly high no matter how hard it is driven.

The unthreaded length should at least equal the board thickness for the effect to work.

Fully threaded screws have their place in structural timber work, but not here.

Two per joist is the general rule, with sensible edge distances - one allows the board to cup and rotate.

Boards move across their width continuously, and a single central fixing lets the board twist around it.

Two fixings hold the board flat and share the movement between them.

Edge distance matters because a screw too close to the edge splits the board along the grain as it moves.

Near board ends a pilot hole is worth drilling even where the screw does not otherwise need one.

Spacing between boards for drainage and expansion is a separate decision and depends on the material and its moisture content at installation.

Not usually in softwood; almost always in hardwood, and often near board ends.

Decking screws are designed with sharp points and cutting threads to start without a pilot in softwood.

Dense hardwoods can exceed the screw's torsional strength, and the screw shears before it seats.

Hardwood also splits readily, so a pilot protects the board as much as the screw.

Board ends are the highest-risk position for splitting in any species.

Where a pilot is used, its diameter should suit the screw's core rather than its outside diameter, or withdrawal capacity is lost.

Yes - ordinary screws cause mushrooming, and composite decking generally specifies its own fastener.

Driving a standard screw into composite pushes material up around the head in a raised ring.

Screws for composite have a reverse thread or a cutting feature under the head that clears that material as it drives.

Head profiles are often different too, since the surface behaves differently from timber.

Many composite systems use hidden fasteners clipping into a groove in the board edge instead of face fixing.

The board manufacturer states which fixings maintain the warranty, and that governs the choice.

Board movement rather than load - the timber works the screw until the hole enlarges or the board splits.

Decking carries modest loads and enormous cyclic dimensional change.

Every wet and dry cycle moves the board across its width, and the screw resists that movement at a single point.

Over time the hole in the board enlarges, the fixing loosens and the board begins to rattle.

Corrosion accelerates the process by reducing the screw section exactly where it flexes.

Correct spacing, two fixings per joist, appropriate edge distance and a durable coating address most of it.