Wood Screws: What to Know and How to Choose

The Wood Handbook names a screw's principal parts as the head, the shank, the thread and the core. The head carries the drive recess and bears on the work; the shank is the unthreaded part below it on a traditional screw; the thread is the helix that grips the wood; and the core, or root, is the solid body the thread winds round — on average about two-thirds of the shank's diameter. The point starts the screw. A traditional wood screw's number, its gauge, names the diameter of its shank, and the gauge and the length are chosen separately.

How a screw's length is measured depends on its head, and references do not agree for every head, so this page gives no single rule: take the length from the maker's own box or drawing. The figures by gauge are on the size chart.

• Flat (countersunk). The Wood Handbook's choice where a flush surface is wanted. The head is a cone that sits in a countersink. Head angles are set by each standard or maker, so match the countersink to the screw; a separate countersink is not always needed — SPAX's MULTI-Head has ribs under the head for a flush or countersunk finish in wood.
• Oval and round. Chosen for appearance, the Handbook says, and round heads where countersinking is objectionable; a round head sits on the surface.
• Wafer or washer. A wide, low head bearing on a larger area, for clamping parts together, as SPAX describes its wafer head.
• Trim. A small head that can be set just below the surface and filled; Simpson's Finish Trim screw adds threads under the head that it says help it countersink. A smaller head is a feature, not a measure of holding.

Counterbores, countersinks and the head sizes a countersink must clear are on the pilot hole chart.

The recess in the head decides the driver. The Wood Handbook describes wood screws as slotted, turned with a screwdriver. Cross recesses come as Phillips and Pozidriv, which are different interfaces, each with its own tip; square is the Robertson recess; and the six-lobe recess is standardised as the hexalobular driving feature. Some makers combine recesses — SPAX's Unidrive is a Phillips and square combination. The bit size depends on the screw: Simpson pairs T10 with its #7 Finish Trim screws and T15 with its #9, which is true of those screws only. Drives and tips are explained in the screwdriver guide; driving with power, and which bits a maker rates for impact, in the impact driver guide and the drill guide.

A traditional wood screw has a plain shank and then the thread. Modern screws carry their makers' own thread and point designs, and the claims for them belong to those makers and those screws. SPAX says its serrated threads reduce driving torque and that its 4CUT point prevents splitting and requires no pre-drilling in wood applications. GRK names a W-Cut thread for low torque, a CEE thread it says enlarges the hole to reduce splitting, and a Zip-Tip it says eliminates pre-drilling on R4 screws 1 in and longer. Simpson says the SawTooth point on its Finish Trim screw removes the need to predrill. None of these claims transfers to a different screw that looks similar.

A pilot hole prepares the wood for the thread, and a clearance hole lets the shank pass freely through the piece being fastened, so the screw draws the two pieces together. Pilot holes reduce splitting, keep a screw where it is meant to go and ease driving; whether a given screw needs one depends on its maker's instructions, the wood, the screw's size and how near the edge or end it goes. Sizes are on the pilot hole chart.

The Wood Handbook lists wood screws of steel, brass and other metals and alloys, with finishes such as nickel, blued, chromium or cadmium. Modern lines add coatings and stainless grades with stated uses: SPAX offers zinc, yellow zinc, HCR, HCR-X and 304 stainless, and approves HCR-X for treated lumber and freshwater environments; GRK offers its Climatek coating and 305 and 316 stainless, recommending 305 for ipe, treated lumber, cedar, white oak and hardwoods, and 316 within a mile of the coast. No material is best everywhere: the choice follows exposure, the wood, the appearance wanted and what the maker says the screw is for. Corrosion in treated wood and staining around metal fasteners are covered in outdoor woodworking projects.

The Wood Handbook's tests show what governs withdrawal from side grain: it rises with the square of the wood's specific gravity, and with the depth of the threaded part and the screw's diameter. In end grain the results were somewhat erratic and averaged 75% of side grain when splitting was avoided. That explains the choices — a denser wood, more thread in the second piece, avoiding end grain and splits — but it gives no load for a joint. No holding, shear or pull-out figure is given here: those depend on the exact screw and installation. Structural screws carry design values in code evaluation reports for named products — GRK's ESR-3201 covers wood-to-wood connections designed to the building code — and those values are for engineered connections, not a rating for furniture.