Screws and Fasteners for Wood: Sizing and Placement

A screw is a clamp that stays in place. In most furniture, glue carries the load and the screw exists to hold the joint closed while the glue cures and to resist the occasional knock afterwards. Understanding that changes how screws get used: the question is not how much the screw can hold, but whether it is closing the joint or holding it open.

Screw Anatomy

Each part of a screw does a specific job, and the failures come from mismatching them to the wood.

The head transfers load to the surface and determines how the screw seats. A flat head is designed to sit in a countersink; driven into an un-countersunk surface it compresses and splits the wood around it. A pan or button head bears on the surface and is used where no countersink is wanted.

The shank is the smooth section below the head on many wood screws. It passes through the top piece without gripping, which is what lets the head pull the joint closed.

The thread cuts into the lower piece and provides the grip. Coarse threads suit softwood and sheet goods; finer threads suit hardwood, where a coarse thread can tear out more fibre than it engages.

The point determines how easily the screw starts. A type 17 point, which has a small cutting flute at the tip, removes a plug of wood as it enters and reduces splitting in softwood.

Pilot and Clearance Holes

Getting the pilot wrong in either direction causes a distinct failure. Too large and the thread has nothing to hold, so the screw spins freely and strips. Too small, or absent, and the core acts as a wedge driven along the grain, which is what splits boards.

In hardwood, run the pilot the full depth the screw will reach. In softwood, a shallower pilot is often enough, but near the end of a board drill one regardless of species, because that is where splitting is almost guaranteed.

The clearance hole is the step most often skipped, and the symptom is a joint with a persistent gap. The thread has engaged both pieces and is holding them apart. Drilling the clearance hole afterwards fixes it, which is worth knowing when a joint refuses to close.

Where Screws Are the Wrong Answer

Screws fix two pieces of wood in a fixed relationship. Wood expands and contracts across its width. Where those two facts meet, something has to give, and it is usually the wood.

A solid wood top screwed rigidly to an apron on all sides will crack when the top tries to move and cannot. The standard answers are slotted holes, figure-eight fasteners or wooden buttons in a groove, all of which hold the top down while allowing it to slide. See wood movement for the amount of movement to expect and in which direction.

The same applies to a wide solid panel screwed to a frame, and to a breadboard end fastened along its whole length rather than pinned at the centre and slotted outward.

End grain is the second place screws underperform. The fibres run parallel to the screw, and the thread pulls out between them instead of across them. A cross dowel set behind the screw gives it long grain to bite, and it is the usual fix in a knock-down frame.

Countersinking and Plugs

A countersink cut to match the head angle lets a flat head sit flush without compressing the surrounding fibres. Cut it slightly deeper and the head sits below the surface in a counterbore, which can then be plugged.

Cut plugs from an offcut of the same board with a plug cutter and orient the grain to run with the surface. A matched plug disappears; a generic dowel plugging a counterbore reads as a dot of end grain.

Pocket Screws

A pocket screw enters at a shallow angle through a pocket drilled in one piece and into the face grain of the other, which is why it holds better than a screw into end grain would. The screw length is set by the thickness of the stock, and the jig’s drill collar is set to match, so the point stops short of the far face.

Pocket joints pull strongly across the joint line but offer little against racking, so they suit face frames and carcass parts where something else provides rigidity. Pocket hole joinery covers the setup in detail.

Screws in Sheet Goods

Plywood takes a screw well in the face and moderately in the edge. MDF edges split easily and need a pilot every time; fasteners designed specifically for the material hold considerably better than ordinary wood screws.

Particleboard holds screws worst of all, which is why flat-pack furniture uses cam fittings and threaded inserts rather than screws into an edge.

What People Get Wrong About This

The first mistake is sizing the pilot to the thread rather than the core. The screw turns, feels like it is tightening, then strips, and the hole is now too large for a second attempt.

The second is omitting the clearance hole and blaming the gap on the joint.

A third is driving a flat head into an un-countersunk surface. It works in softwood for a while and splits thin stock immediately.

A fourth is screwing a solid top down rigidly on all four sides. It looks solid, and the crack appears in the first dry winter.

And the last is choosing a screw by length alone. A long screw in a shallow piece breaks through the far face; a coarse thread in dense hardwood tears rather than grips.

Choosing Between Screws and Nails

Nails and brads flex with the wood, leave a small hole, and are quick. They suit trim, backs, and holding a part while glue cures.

Screws resist withdrawal, can be removed, and pull a joint closed. They suit assemblies that may be taken apart, parts that bear load, and anywhere a clamp cannot reach.

Where glue is doing the work, the fastener is a clamp and the smallest one that closes the joint is the right one. Where there is no glue, the fastener is the joint, and it should be chosen for the load it will actually carry rather than for what is in the drawer.