§1Fastening a fibrous parent
Every other fastener in this section enters an isotropic parent. Timber is different — a bundle of longitudinal fibres, far stronger along the grain than across it, and forever swelling and shrinking with moisture — and its fasteners are shaped by those facts.
Three properties of the parent drive everything on this page. First, direction: wood splits easily along the grain, so a driven fastener is a wedge that must part fibres without starting a split — which is why nails are slender, why points and pre-boring matter, and why fastening close to an end is the classic sin. Second, compliance: wood crushes locally and relaxes with time and moisture cycling, so the sustained clamping preload of the bolted-joint pages simply cannot be stored in a timber joint — a fact with the same consequence the wing-nut page reached by a different road: timber fasteners locate and hold, and the structure, not the preload, carries the load. Third, density: holding power rises steeply with how much wood there is per cubic centimetre, and §3 puts numbers on exactly how steeply. The two great families divide the work between them — the friction-gripping nail (§2) and the fibre-engaging screw (§5) — and the division is by load direction.
Contents§2The nail
A nail holds by friction alone: driving it wedges the fibres apart, and their elastic spring-back grips the shank. That grip resists sideways load well and axial pull poorly — the asymmetry the whole trade is built on.
Loaded laterally — the joint of the hero’s left panel — the nail works as a pin in bearing, the fibres crushing against its shank exactly as the rivet page’s plates bear on a rivet, and the joint is strong, tough and forgiving. Loaded in withdrawal, everything depends on that friction grip, and friction is precisely what timber gives up: fibres relax, moisture cycles swell and shrink the wood around the shank, and vibration walks the nail out — which is why the governing rule of nailing is stated in one line: never load a nail in withdrawal. Design detailing follows the rule — nails driven so service loads cross them, skew (dovetail) nailing so opposing nails convert pull into lateral load on each, and clinching where the point can be turned over. Where withdrawal cannot be designed out, the shank is modified to make friction mechanical: ring-shank nails whose annular barbs lock into the sprung-back fibres, and helical (screw-shank) nails that rotate as driven and hold like a coarse screw — the nail conceding, in effect, that the screw of §5 owns the axial direction.
| Fastener | How it holds | Where it serves |
|---|---|---|
| Plain wire nail | fibre spring-back friction on a smooth shank | general framing — lateral loads only |
| Ring-shank nail | annular barbs lock into the sprung-back fibres | sheathing, flooring, pallets — resists working loose |
| Helical (screw-shank) | rotates as driven — part-mechanical grip | decking and gun nailing with holding duty |
| Spike (≥6 mm) | as a nail, at structural scale | sleepers, poles, heavy timber — pre-bored about ¾d (§4) |
| Wood screw | thread engages the fibres mechanically | withdrawal loads, clamping, demountable joints (§5) |
| Coach screw | a spanner-driven structural wood screw | timber structure — the screw’s answer to the spike |
| Reading down the table is reading §2 and §5 in miniature: the further a fastener moves from smooth-shank friction toward positive thread engagement, the more axial duty it can honestly carry — and the plain nail remains what it always was, a superb lateral fastener that should never be asked to hold a withdrawal load. | ||
§3Holding power and density
The single strongest lever on holding power is the timber itself: withdrawal resistance rises much faster than density, so the species is worth more than the fastener.
The empirical withdrawal laws make holding power a steep function of specific gravity. Compare a hardwood at G = 0.70 with a building softwood at G = 0.45. For a nail: (0.70/0.45)2.5 = 3.02× the withdrawal resistance — the same nail, three times the hold, purely from the wood. For a screw: (0.70/0.45)² = 2.42×. The exponents also say something structural: the nail’s steeper power reflects how completely its friction grip depends on the fibres’ spring-back, where the screw’s thread engages fibres mechanically and leans on density a little less. Engagement length is the other axis, and it is linear — the sheet-metal logic of the self-threading page again — so penetration rules are stated in diameters: a nail wants at least ten diameters in the receiving member. A 90 mm nail of Ø3.75 through a 35 mm batten penetrates 90 − 35 = 55 mm = 14.7 diameters — comfortably inside the rule — wrapping the shank in π × 3.75 × 55 ≈ 648 mm² of gripping surface. Species, then penetration, then diameter: that is the order in which a timber connection is actually strengthened.
§4Spikes and driven practice
A spike is simply a nail grown to structural size — about 6 mm diameter and upward — and at that scale the splitting risk that shadows all nailing becomes the governing concern.
Spikes fasten heavy timber: sleepers, wharf and bridge decking, pole framing, landscape structures — work where the “nail” is a forged pin driven with a maul. The wedge mechanics of §2 scale with it. A slender nail parts a few fibres; a 6–10 mm spike displaces enough wood to split a member outright unless the wood is given somewhere to go, so pre-boring becomes standard rather than optional: a lead hole of roughly three-quarters of the spike diameter, which surrenders a little friction grip in exchange for an unsplit member — an excellent trade, since a split timber holds nothing at all. The same logic, scaled down, is ordinary good nailing practice: pre-bore hardwoods and dense sections, keep end distances generous, blunt a point before nailing near an end (a blunt point crushes fibres ahead of it instead of wedging them apart — the carpenter’s oldest trick, and sound mechanics), and stagger nails across the grain so no single fibre line collects them. Driven practice, in short, is split management; the holding arithmetic of §3 only applies to wood that is still in one piece.
Contents§5The wood screw
The wood screw takes the direction the nail concedes: its thread cuts into the fibres and engages them mechanically, so it holds in withdrawal — and it can do what no nail can, which is clamp two pieces together.
Its anatomy is purposeful. The threaded portion — traditionally about two-thirds of the length — carries a deep, coarse thread whose flanks bear on wound-in fibres: withdrawal resistance by positive engagement, not friction, which is why it survives the moisture cycling that loosens nails and why it can be removed and refitted. The plain shank above it is not laziness but function, the hinge of §6’s clamping discipline. The head — countersunk to finish flush and centre itself, round or pan to bear on the surface — is driven through the slot, cross-recess or Torx forms of the machine screws page, with the same cam-out hierarchy applying and mattering more, since driving into hardwood takes sustained torque. Around the classic form the family has specialised: self-drilling wood screws with cutting points that tap their own way (the self-threading page’s idea, in timber), coach screws — hex-headed, spanner-driven screws for structural timber, the wood screw’s answer to the spike — and double-threaded and shank-milled patterns tuned for decking and modern engineered boards. Pilot holes remain the screw’s pre-boring: about the root diameter in hardwoods, smaller or none in soft — enough relief that the thread cuts rather than wedges, and the §4 split never starts.
Contents§6The clearance-hole discipline
One drilling decision decides whether a wood screw clamps or jacks: the member under the head must be drilled so the thread cannot bite in it.
Watch the geometry of the hero’s lower panel. For the screw to pull the two members together, its thread must engage only the far member while the head bears on the near one; the near member must be free to slide along the shank as the screw draws it down. Give the top member only a pilot hole and the thread bites in both: the screw now fixes each member to itself at the pitch spacing the thread happens to catch, and whatever gap existed between them when the thread crossed the joint line is locked in permanently — the screw jacks the boards apart with exactly the authority it should have used to close them. Hence the discipline, unvaried: clearance hole in the near member, sized to the shank so the thread passes freely; pilot hole in the far member, sized to the root so the thread bites fully; countersink to suit the head. It is the timber restatement of a principle running right through this section — from the bolt gripping on its unthreaded shank to the stud loading only its nut end — that a fastener clamps only across the joint, never within a member. Modern part-threaded and shank-milled screws build the clearance into the screw itself, which is convenient; it is not a repeal of the rule, merely its mass production.
Contents§7Quick reference
The working core of the page on one card rack.
Directions
nail: lateral strong
withdrawal — never
Density
nails ∝ G2.5 → 3.02×
screws ∝ G² → 2.42×
Penetration
≥ 10 diameters in
90 mm nail → 14.7d ✓
Splitting
pre-bore spikes ~¾d
blunt the point near ends
Screw rule
clearance near · pilot far
thread only across the joint