Screw & Bolt Guide: Types, Sizes, Materials & Load Ratings for Common Fixings

Summary

Screws and bolts are among the most frequently used fasteners on any building site, yet they are also among the most frequently misspecified. Using the wrong screw in the wrong substrate — or the wrong material grade in a corrosive environment — can lead to structural failure or, at minimum, expensive remedial work.

UK tradespeople need to understand three dimensions of fixing selection: mechanical performance (thread form, tensile strength, shear resistance), material compatibility (corrosion resistance in wet, treated timber, or masonry environments), and compliance (CE/UKCA-marked structural screws, load-rated anchors for masonry). Many screws sold at builders' merchants are suitable only for light joinery work — not structural connections.

A common misconception is that "stainless" means A4. There are three grades: A2 (304 stainless — suitable for most external uses), A4 (316 stainless — required in coastal areas, marine environments, or where in contact with treated softwood). Understanding the difference can prevent premature failure of deck fixings, fascia boards, and structural connections.

Key Facts

• Woodscrew thread form — partial thread (shank + thread) for joining two pieces; full-thread screws for securing single pieces to substrate
• Torx/TX drive — now standard over Pozi/Phillips for construction screws; reduces cam-out, allows higher torque
• C4 corrosion class — minimum requirement for fixings in treated timber (CCA, ACQ, copper-based preservatives) per BS EN 335
• A2 stainless — 304 grade; suitable for most external applications, general joinery, and non-aggressive environments
• A4 stainless — 316 grade; required in coastal/marine environments or direct contact with copper-treated timber
• Hot-dip galvanised (HDG) — minimum 45µm zinc coating; used for structural connectors (joist hangers, post bases) per BS EN ISO 1461
• Property class 8.8 — structural bolt minimum for load-bearing connections; yield strength 640 MPa, tensile 800 MPa
• M10 coach bolt — typical for decking post-to-joist connections; 90–120mm length common
• Rawlbolt (expanding anchor) — used for heavy fixings into concrete/masonry; torque-controlled (M8 = 25 Nm typical)
• Chemical anchor — resin anchor for cracked/hollow masonry, rebar starter bars; 24-hour cure before loading at 20°C
• Sleeve anchor — single-stage expansion anchor for solid concrete/masonry; pull-out values must be verified for application
• Self-drilling screws — tek screws for fixing steel purlins/sheeting; drill point rated by steel thickness (No.3 = 1–3mm, No.5 = 4–6mm)
• Concrete screw (HEX-CS) — Hilti HUS-H / Fischer ULTRACUT type; no anchor required, direct drive into 6mm pre-drilled hole
• UKCA/CE marking — structural anchors must carry UKCA or European Technical Assessment (ETA) with load tables
• Grade 5.6 vs 8.8 — many generic bolts sold at hardware stores are 5.6 grade (non-structural); check head markings

Quick Reference Table

Detailed Guidance

Thread Forms: Full vs Partial Thread

Most construction screws are either full-threaded or partial-threaded. Full-thread screws grip the entire shank depth — useful when fixing a single material to a substrate (e.g., cladding to battens). Partial-thread (twin-thread) screws have a smooth shank near the head, which pulls two components tightly together when the thread bites in the receiving material only. This is the correct choice for through-fixing timber joints.

Single-thread screws (coarse pitch) drive faster and are better in softwood. Fine-pitch screws resist vibration loosening and suit hardwood or dense substrates.

Structural Timber Screws

For structural timber connections — joist hangers, rafter connections, timber frame — ordinary BZP screws are insufficient. Structural screws must carry a UKCA mark and European Technical Assessment (ETA) with published load tables (characteristic withdrawal loads, lateral resistance). Products like the Spax-S, Fischer PowerFull III, and Rothoblaas Timberfix carry ETAs and are acceptable to structural engineers and building control.

Key principle: the ETA load value for a structural screw is typically 40–60% of the headline manufacturer figure — engineers use the characteristic (5th percentile) value. Never specify structural screws based on marketing literature alone; use the ETA document.

Corrosion Classes for Treated Timber

Modern preservative-treated timber (CCA, LOSP, boron, copper-based treatments) is highly aggressive to standard zinc-plated steel. The relevant standard is BS EN 335 and the TRADA guidance on fixing compatibility.

• Service class 1 (dry, internal) — BZP acceptable
• Service class 2 (occasionally wet, covered external) — minimum A2 stainless or HDG
• Service class 3 (fully exposed external) — A4 stainless or hot-dip galvanised to BS EN ISO 1461
• Treated timber (copper-based) — C4 corrosion class minimum; A4 stainless or HDG

Using BZP screws in treated decking boards is one of the most common errors on domestic jobs — rust staining and structural failure within 3–5 years is typical.

Masonry Anchors: Torque and Load Values

Masonry anchors must be selected based on:

1. Substrate type (solid concrete, solid brick, aerated concrete/Thermalite, hollow blockwork)
2. Load type (tension, shear, or combined)
3. Edge and spacing distances

Expanding anchors (sleeve anchors, wedge anchors, Rawlbolts) are only suitable for solid masonry with compressive strength above 30 N/mm². In Thermalite, hollow blockwork, or historic soft brick, chemical (resin) anchors must be used.

Critical: torque to the manufacturer's specified value using a calibrated torque wrench. Under-torque = no expansion; over-torque = pull-out failure. Common values: M8 sleeve anchor = 20–25 Nm; M10 = 40–50 Nm; M12 = 60–80 Nm.

Bolt Property Classes

Bolts are graded by property class stamped on the head (e.g., 4.6, 8.8, 10.9):

• The first digit × 100 = minimum tensile strength (MPa)
• The product of both digits × 10 = yield strength (MPa)

So 8.8 = 800 MPa tensile, 640 MPa yield. For structural timber connections requiring bolts, 8.8 is the standard minimum. Grade 4.6 bolts (common bright zinc plated bolts from hardware stores) have only 400 MPa tensile strength and should not be used for load-bearing connections.

Screw Drive Types

Frequently Asked Questions

Can I use ordinary screws in tanalised (treated) timber?

No. Standard bright zinc-plated screws will corrode rapidly in copper-based treated timber — often showing failure within 2–3 years outdoors. Use A4 stainless steel screws or fixings rated C4 minimum. This applies to decking, fence posts, pergolas, and any external treated softwood.

Do I need UKCA-marked structural screws for stud walls?

For standard non-load-bearing stud walls, ordinary construction screws are acceptable. However, if you are making structural connections (rafter to wall plate, joist to rim board, shear connections in a timber frame), you should use ETA-rated structural screws with published load values. Your structural engineer will specify the fixing if one is involved.

What size rawlbolt for fixing a TV bracket or heavy shelf?

For moderate loads (up to 100 kg) into solid brick or concrete, M8 sleeve anchors or M8 wedge anchors are typically sufficient. Space anchors at least 150mm from edges and 100mm from each other. For heavier loads or critical fixings, use M10 and verify the substrate. Never use expanding anchors into Thermalite or hollow block — use chemical resin anchors instead.

What's the difference between a coach bolt and a coach screw?

A coach bolt (cup square bolt) has a round domed head with a square shank that locks into timber when tightened — requires a nut and washer on the reverse. A coach screw (lag screw) has a hexagonal head and threaded shank that drives directly into timber without a nut — tighten with a spanner. Coach bolts are used where both faces are accessible; coach screws where only one face is accessible.

How deep should masonry fixings go?

Minimum embedment depth is typically 6× the bolt diameter for sleeve anchors and chemical anchors into solid concrete/brick. For M8: minimum 48mm embedment; M10: 60mm; M12: 72mm. Always refer to the anchor's ETA document for the published effective embedment depth (hef) and corresponding load values.

Regulations & Standards

• BS EN 1995 (Eurocode 5) — Design of timber structures; includes fastener design rules

• BS EN 14592 — Timber fasteners specification; structural timber screws

• BS EN ISO 1461 — Hot-dip galvanised coating requirements (45µm minimum for structural items)

• BS EN 335 — Durability of wood; hazard class and service class definitions

• ETAG 001 / ETA — European Technical Assessments for anchors; superseded by EAD 330232 (concrete anchors) [verify current UK references post-Brexit]

• BS 4190 — ISO metric black hexagon bolts; property class markings

• TRADA — Fasteners and Connectors for Timber — Timber Research and Development Association guidance

• British Stainless Steel Association — Corrosion of Fasteners — Stainless steel grade selection guide

• Hilti Technical Documentation — Load tables and ETA documents for masonry anchors

• Fischer UK — Fixing Technology — Anchor selection guides and ETA references

• SPAX UK — Structural Screws — ETA load tables for structural timber screws

• timber sizes — Timber size and grade reference including C16/C24

• stud walls — Stud wall construction and fixing requirements

• structural steel — Steel beam connections and bolting requirements

• working at height — Safe access when installing overhead fixings