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Wood Screws
A comprehensive engineering reference for joiners, site carpenters, furniture manufacturers and procurement teams — covering traditional and modern wood screw types, gauge-to-metric conversion, head profiles, drive systems, tapered vs parallel shank design, pilot and clearance hole sizing, holding strength in softwood and hardwood, coating and material selection for indoor, outdoor and treated timber applications, and coach screw specifications.
Wood Screw Types, Gauge Classification
& Thread Design Principles
Traditional Gauge · Tapered Shank · Fully Threaded
Traditional vs Modern Wood Screws
The term “wood screw” covers two distinct product generations with different thread and shank geometries:
- Traditional wood screw (the original British and American standard) — tapered shank that reduces in diameter from head to tip; fully threaded only on the lower two-thirds; coarse single-start thread; available in slotted or cross (PZ) head; sold by gauge number (No.4 through No.14, where gauge = nominal head diameter in eighths of an inch — No.8 = 1" head = 4.17 mm shank). The tapered shank helps draw two pieces of wood together when the screw passes through the top piece without engagement and threads into the second piece.
- Modern wood/chipboard screw (the contemporary standard, largely replacing traditional screws) — parallel shank with consistent diameter along its full length; coarse thread to approximately two-thirds of length (partial thread, unthreaded shank zone draws pieces together); sharper thread angle and deeper thread profile than traditional; available primarily in PZ2 and TX drive; sold by metric diameter and length. Modern parallel shank screws drive faster, hold better in most materials and are better suited to power driver installation.
Traditional wood screw gauges use the formula: Shank diameter (inches) = (Gauge × 0.013) + 0.060. The most common gauge-to-metric equivalents are: No.4 ≈ 3.0 mm; No.6 ≈ 3.5 mm; No.8 ≈ 4.2 mm; No.10 ≈ 4.8 mm; No.12 ≈ 5.5 mm; No.14 ≈ 6.3 mm. These are not exact equivalents — a No.8 traditional screw has a tapered shank ranging from approximately 4.2 mm at the head to 2.5 mm at the tip, while a modern 4.0 mm wood screw has a consistent 4.0 mm shank diameter. When replacing traditional gauge screws with modern metric equivalents, specify the metric size closest to the maximum (head-end) diameter of the traditional screw.
Head Types and Drive Systems
Dimensional Reference, Gauge Conversion
& Pilot Hole Sizing
Clearance Hole · Countersink · Softwood vs Hardwood
| Gauge (No.) | Approx. Metric (mm) | Pilot Hole — Softwood (mm) | Pilot Hole — Hardwood (mm) | Clearance Hole (mm) | Countersink (mm) |
|---|---|---|---|---|---|
| No.4 | 3.0 | 1.5 | 2.0 | 3.5 | 7 |
| No.6 | 3.5 | 1.5 | 2.5 | 4.0 | 8 |
| No.8 | 4.0–4.2 | 2.0 | 3.0 | 4.5 | 9 |
| No.10 | 4.8–5.0 | 2.5 | 3.5 | 5.5 | 10 |
| No.12 | 5.5 | 3.0 | 4.0 | 6.0 | 12 |
| No.14 | 6.0–6.3 | 3.5 | 4.5 | 7.0 | 14 |
Pilot hole diameter for softwood = approximately 0.5×screw shank diameter. For hardwood (oak, ash, beech, iroko, teak) = 0.7×shank diameter. Clearance hole in the top piece = screw shank diameter + 0.5 mm to allow the top piece to slide freely along the shank as the thread draws the two pieces together. Countersink diameter = screw head diameter + 0.5 mm. In hardwoods, always drill the pilot hole to the full depth of engagement — failure to pilot drill adequately causes the brass or even steel screw to shear when driving.
| Size | Drive | Common Lengths (mm) | Typical Application |
|---|---|---|---|
| No.4 / 3.0 mm | Slotted / PZ1 | 12, 16, 20, 25 | Cabinet backs (3–6 mm ply/MDF), small hardware, hinges on light boxes |
| No.6 / 3.5 mm | Slotted / PZ2 | 16, 20, 25, 30, 38 | Small cabinet hinges, light cladding, thin batten fixing, small hardware |
| No.8 / 4.0 mm | PZ2 / TX20 | 25, 30, 38, 50, 60, 75 | General joinery, butt hinges, small brackets, 18–22 mm softwood/hardwood |
| No.10 / 5.0 mm | PZ2 / TX25 | 38, 50, 60, 75, 100 | Heavy hinges, joist hangers, cladding to battens, structural joinery |
| No.12 / 5.5 mm | PZ3 / TX25 | 50, 75, 100, 125 | Heavy structural fixings, post connectors, decking to joist, cladding rails |
| No.14 / 6.0 mm | PZ3 / TX30 | 75, 100, 125, 150 | Heavy structural timber, pergola beams, heavy gate hardware, coach rail fixing |
Materials, Coatings
& Outdoor / Treated Timber Selection
ACQ / CCA Treated Timber · Marine · Outdoor
| Material / Coating | Indoor Dry | Outdoor / Sheltered | Full Outdoor Exposure | Treated Timber (ACQ/CCA) | Marine / Coastal |
|---|---|---|---|---|---|
| BZP (Bright Zinc Plate) | Excellent | Sheltered only | Not suitable | Not suitable | Not suitable |
| Hot-Dip Galvanised (HDG) | Yes | Excellent | Excellent | Suitable | Limited |
| Stainless A2 (SS 304) | Yes | Excellent | Excellent | Suitable | Moderate |
| Stainless A4 (SS 316) | Yes | Excellent | Excellent | Excellent | Excellent |
| Brass | Excellent | Sheltered only | Not suitable | Not suitable | Not suitable |
| Silicon Bronze | Yes | Excellent | Excellent | Suitable | Excellent (traditional) |
Modern timber preservatives — ACQ (Alkaline Copper Quaternary), Tanalith E, Celcure and similar copper-based systems — contain copper compounds that are electrochemically corrosive to zinc and galvanised coatings. BZP screws in ACQ-treated outdoor timber will corrode through within 12–24 months in damp conditions. Standard galvanised (zinc-dipped) screws may fail within 3–5 years. For all treated timber applications in exposed conditions, stainless steel A4 (SS 316) is the recommended minimum. Hot-dip galvanised is acceptable for dry-stored treated timber or where the timber will be painted and protected. Never use BZP screws with ACQ/CCA/Tanalith-treated timber in external applications.
Brass Wood Screws — Special Considerations
Brass is the traditional premium material for wood screws in high-quality joinery, cabinet making and antique restoration. Key engineering considerations:
- Strength — brass (CuZn37) has approximately 40% of the tensile strength of steel. Never over-torque. Use appropriate drive bit size to avoid slipping and head damage. Hand-drive only for most applications — never use power drivers on brass screws.
- Pilot drilling is mandatory in hardwood — always drive a steel screw of the same gauge first to cut the thread path, then remove and replace with the brass screw. Attempting to drive brass screws directly into hardwood (oak, beech, ash, iroko) without this technique results in the head shearing off at the shank in a high proportion of cases.
- Corrosion — brass is resistant to mild atmospheric corrosion but not suitable for consistently wet or salt-water exposure. Use silicon bronze for boatbuilding and marine applications where the premium appearance of non-ferrous screws is required alongside high corrosion resistance.
Applications by Substrate, Installation Notes
& Coach Screw Specifications
Coach Screw · Lag Bolt · Post-Beam · Decking · Marine
Application Guide by Substrate and Use
No.8 × 1.5" (4.0×38 mm) and No.8 × 2" (4.0×50 mm) BZP countersunk wood screws are the standard for internal joinery in softwood — fixing door linings, skirting boards, architraves, window boards, stair components and general site carpentry. Modern parallel-shank chipboard/wood screws have largely replaced traditional tapered-shank wood screws for these applications due to their faster driving with power tools and better holding performance. Pre-drill in all cross-grain connections within 50 mm of timber ends to prevent splitting.
All hardwood connections require pilot drilling. Use stainless A2 (SS 304) or A4 (SS 316) for oak and teak — the tannins in these timbers react with zinc and iron to produce black staining around the screw hole. This staining does not affect the structural performance but is visually objectionable in high-quality joinery. Brass screws (with steel pre-threading) for exposed decorative fixings. Lubricate all hardwood screws with beeswax, soap or purpose-made screw lubricant before driving to reduce driving torque and prevent head shearing.
Stainless A2 (SS 304) or A4 (SS 316) countersunk screws in No.8 or No.10 gauge are standard for softwood cladding (featheredge, shiplap, T&G boarding) to treated softwood battens. Countersink flush — overdriving causes the thin lower edge of featheredge boards to split. Fix through the face of each board into the batten with one screw per batten crossing. For open-jointed hardwood cladding (Ipe, Siberian Larch, Cedar), use SS A4 screws pre-drilled and countersunk with a combined bit — the predrilling is essential to prevent splitting of the high-density hardwood boards at their lower edges.
Coach screws (also called lag bolts or lag screws) are M6–M20 hex-headed wood fasteners for structural timber connections. Key installation rules:
- Always pilot drill the full length at approximately 0.7× shank diameter; driving without a pilot hole splits the timber and reduces holding strength
- Drill a shank clearance hole (= shank diameter) through the attached member (the piece being connected) so the thread only engages in the main member (the piece being connected to)
- Lubricate threads with beeswax or soap — dry driving risks shearing the hex head off the shank at high torque
- Hot-dip galvanised (HDG) is standard for outdoor structural coach screws; SS A4 for treated timber and marine environments; never BZP for external structural applications
- Minimum edge distance: 3.5× coach screw diameter from edge of timber; 7× diameter from end of timber
Quick Selection Reference
| Application | Size | Coating | Drive | Pilot? |
|---|---|---|---|---|
| Internal softwood joinery (skirting, linings) | No.8 × 38–50 mm | BZP | PZ2 | Near ends only |
| Internal hardwood joinery | No.8 × 38–50 mm | SS A2 / Brass | PZ2 / Slotted | Always — full depth |
| External cladding (softwood) | No.8–No.10 × 38–60 mm | SS A2 | PZ2 / TX20 | Near edges/ends |
| External cladding (hardwood) | No.8–No.10 × 38–60 mm | SS A4 | TX20 / TX25 | Always |
| Softwood decking to treated joist | No.10 × 65–80 mm | SS A4 / HDG | TX25 | Near ends |
| Hardwood decking (Ipe, Balau) | No.10–No.12 × 65–80 mm | SS A4 | TX25 | Always |
| Post-beam structural (coach screw) | M10–M12 × 100–150 mm | HDG / SS A4 | Hex/socket | Always — 0.7×dia |
| ACQ treated timber (any outdoor) | Any size | SS A4 minimum | Any | Per substrate |
RR Hydraulics supplies wood screws in traditional gauge (No.4–No.14) and metric (3.0–6.0 mm diameter, 12–150 mm length), all head types (countersunk, round head, raised countersunk, decking/bugle head), drive types (slotted, PZ1/PZ2/PZ3, TX20/TX25/TX30), materials (BZP steel, hot-dip galvanised, stainless A2, stainless A4, brass, silicon bronze) and coach screws (M6–M20, 25–200 mm, HDG/SS A4). Available in standard retail packs, trade boxes and bulk sacks. Contact sales@rrhydraulics.com for project and bulk pricing — response within 24 hours.