Structural Fasteners: Joist Hangers, Framing Anchors, Post Bases and ETA Certification

Summary

Structural fasteners include every metal component that transfers load between structural elements — joist hangers, framing ties, hurricane straps, post bases, structural bolts, structural screws, and anchor systems into concrete or masonry. Selecting the wrong fastener type, using the wrong nails in a joist hanger, or omitting required fixings are among the most common and consequential errors in timber frame and structural carpentry.

The European Technical Assessment (ETA) system provides third-party verification of structural fastener performance. For any fastener that is doing structural work — transferring load, connecting primary structural members, or anchoring to concrete — the product data sheet should reference an ETA, UKCA, or national technical approval document. Products without these approvals may not have been tested to the loads stated on the packaging.

Corrosion resistance is the other critical dimension. A joist hanger installed inside a timber-frame house in a dry, ventilated cavity may be adequately protected by Z275 galvanising. The same hanger in an outdoor pergola, a timber deck, or in contact with preservative-treated timber requires significantly better corrosion protection — typically A2 or A4 stainless steel.

Key Facts

• Joist hanger standard — BS EN 845-1 for masonry anchors; ETA for structural timber-to-timber connections
• Joist hanger nails — 3.75mm × 30mm hot-dipped galvanised round wire nails; fill ALL nail holes; using fewer nails dramatically reduces the rated load
• Internal galvanised coating — Z275 (275g/m² zinc coating) minimum for interior use; Z350 or hot-dip galvanised for moderately exposed conditions
• External or treated timber — A2 stainless steel minimum; A4 stainless steel for marine/coastal environments
• ETA (European Technical Assessment) — required for structural screws, anchors, and connections to confirm they meet the stated load capacities; look for ETA-XXXXX reference on packaging
• Structural screws — must have an ETA for use in load-rated connections; common approved products: SPAX (ETA-12/0114), Rothoblaas Torx Power (ETA-assessed), GRK R4 (ICC/ETAs)
• Structural screw thread — typically full-threaded for timber-to-timber; partial thread (smooth shank) for drawing boards together
• Bolt grade 8.8 — standard structural bolt (proof load 640 N/mm², tensile strength 800 N/mm²); galvanised or stainless for exposed use
• A2-70 stainless — 70 = minimum tensile strength 700 N/mm²; suitable for most external structural bolting
• A4-80 stainless — 80 = minimum tensile strength 800 N/mm²; marine and chemical environments
• Coach screw vs through bolt — coach screws (lag screws) are threaded fasteners driven with a spanner into timber; through bolts (M10, M12) pass fully through and take a nut; through bolts provide higher, more reliable tensile capacity
• Nail schedule for stud walls — BS 5268: 3.1mm × 90mm round wire nails for stud-to-plate connections; nail schedules vary by loading; check structural engineer's specification
• ETA-assessed anchor systems — Hilti HIT-RE 500 V4 (epoxy resin, high load) for dense concrete; Hilti HST3 (wedge anchor) for predrilled concrete; Hilti TB 3 (screw anchor) for solid dense concrete; different products for aerated concrete (Thermalite/Celcon — use designated aerated-block anchors)
• Glulam connectors — standard joist hangers are not appropriate for glulam or engineered timber without specific product assessment; use Sherpa connectors, Bulldog ring connectors, or manufacturer-specified hardware

Quick Reference Table

Detailed Guidance

Joist Hangers: Installation Requirements

Joist hangers are the most commonly installed structural connector in domestic timber frame and masonry construction. They support joist ends at walls and beams, transferring vertical and lateral loads into the supporting structure.

Nail requirement: The rated load for a joist hanger is only achievable when all nail holes are filled with the specified nail size. Manufacturers typically specify 3.75mm × 30mm hot-dipped galvanised round wire nails (also called joist hanger nails or clout nails). Using fewer nails — even half the holes — reduces the rated load by more than half in most products due to the non-linear load distribution.

A common installation mistake is filling only the "main" nail holes and leaving the smaller or angled holes empty. Fill every hole. If the nail hole is too small for the 3.75mm nail, do not drill out — the specified nail must be the right size; obtain the correct nail or the correct hanger.

Sizing joist hangers: Hangers are specified by joist width (the dimension perpendicular to the wall face) and joist depth. A 47 × 195mm joist requires a hanger specified for 47 × 195mm timber. Hangers are typically available in 25mm depth increments. Using an undersized hanger bearing on only part of the joist depth is incorrect and reduces load capacity.

Hanger orientation: Most joist hangers are designed for vertical load only — this is adequate for floor joists. For hangers in wind-uplift or lateral-load situations (roof connections), hurricane ties or top-flange hangers with moment resistance may be required. Check the structural engineer's specification.

Masonry-type hangers: Where a joist hanger is built into a masonry wall, it is an "anchor joist hanger" (type ABA or similar). The back plate is embedded in the mortar joint during brickwork construction. These must be specified for the correct load and must meet BS EN 845-1 for masonry ancillaries.

Framing Anchors and Hurricane Straps

Framing anchors (L-brackets, angle ties) connect studs to plates in timber frame construction. Hurricane straps (also called roof ties or rafter ties) connect roof rafters or trussed rafters to the wall plate.

Trussed rafter ties (hurricane straps): In UK domestic construction, trussed roof rafters must be strapped to the wall plate to resist wind uplift. Building Regulations require this; the standard is typically one strap per truss unless the structural engineer specifies otherwise. Standard straps are Z-shaped, nailed through the truss into the top of the inner leaf blockwork.

Installation: Straps must be nailed with the manufacturer's specified nails through all nail holes. Do not bend straps sharply at tight angles — the metal work-hardens at the bend and loses ductility. Use factory-shaped straps for non-standard angles.

Corrosion protection: Roof void straps (internal, dry, ventilated) may use Z275 galvanised. If straps are within 600mm of external air (cold roof with gaps at eaves), consider Z350 or use stainless.

Post Bases

Post bases support vertical timber posts above ground, separating the end grain of the post from direct contact with concrete or ground — which would cause rot. They also provide moment resistance (resistance to overturning) and lateral restraint.

Types:

• Spike post base (ground anchor): A galvanised steel spike driven or concreted into the ground, with an adjustable plate accepting the post. Good for concrete or compacted ground. Not suitable for decking where the spike would penetrate a substrate without adequate purchase.
• Plate post base (bolt-down): A flat plate bolted to a concrete pad or patio slab with resin or masonry anchors. Better load distribution for larger posts. Requires correctly specified anchor bolts.
• Concealed post base: Slots into a pre-drilled hole in the base of the post; barely visible when installed. Lower lateral load capacity than surface-mounted types.

Post base selection: For structural posts (carrying a load — pergola beams, carport, gate posts under load), use a post base with a declared ETA load capacity that meets the calculated load. For decorative posts (fence posts under normal wind load), standard Z275 galvanised spike bases are typically adequate.

Corrosion note: External post bases in contact with treated timber (UC4 preservative treatment) must be A2 stainless steel or hot-dip galvanised to at least Z350. The preservative chemicals in modern pressure-treated timber are corrosive to standard Z275 galvanising.

Structural Screws and ETA Certification

Standard wood screws (zinc-plated, cross-head) are not structural fasteners. They have no assessed tensile or shear capacity and should not be used in load-bearing connections.

What makes a screw "structural": A structural screw has:

1. An ETA confirming assessed load capacity (shear and axial/withdrawal)
2. Thread geometry designed for timber engagement (typically a coarse, deep thread for softwood, with a reamer tip to reduce splitting)
3. Material specification (typically C1022 steel or equivalent) and hardening process

Common UK structural screw brands:

• SPAX — Widely used; ETA-12/0114 covers most of the SPAX range for timber; 4CUT point reduces splitting
• Rothoblaas — Italian brand; ETAs for most products; good for CLT and engineered timber applications
• Buildex / ITW — Range includes structural hex-head screws with ETAs

Screw grade for treated timber: For treated (tanalised/pressure-treated) softwood in external applications, use screws rated at least C4 corrosion class (equivalent to A2 stainless or class 4 hot-dip galvanised). Standard zinc-plated structural screws are rated C2–C3 and will corrode in treated timber within a few years.

Typical structural screw applications:

• Timber-to-timber connections in post-and-beam construction
• Joist-to-beam connections (alternative to joist hangers in some configurations)
• Stair stringer to trimmer joists
• Ledger board connections to rim joists

Bolt Grades and Selection

For bolted structural connections, bolt grade determines strength:

Grade 8.8 (standard structural grade):

• Proof load: 640 N/mm²
• Ultimate tensile strength: 800 N/mm²
• Used for: beam-to-column connections, structural timber with bolt holes
• Available hot-dip galvanised (HDG) for external use

A2-70 Stainless Steel:

• 70 = minimum ultimate tensile strength 700 N/mm²
• External structural applications, balconies, external timber frames
• Not suitable for marine environments or where chloride exposure is significant

A4-80 Stainless Steel:

• 80 = minimum ultimate tensile strength 800 N/mm²
• Marine, coastal, chemical environments
• Higher cost; specified when A2 would corrode

Coach screws (lag screws): Large hexagonal-headed screws driven into timber using a socket or spanner. Available M8, M10, M12, M16 for structural applications. Used where through-bolting is impractical (e.g., ledger board to existing structure). Coach screws in structural applications should have load data from the manufacturer or a structural engineer's specification.

Anchor Systems by Substrate

Selecting the wrong anchor for the substrate is a frequent cause of fixing failure. Key substrate types and appropriate products:

Dense concrete (C20 or better):

• Mechanical expansion anchor: Hilti HST3 (torque-controlled), Fischer FAZ II — reliable, quick to install, tested in cracked and uncracked concrete
• Chemical resin anchor: Hilti HIT-RE 500 V4, Fischer FIS V 360 — high-load applications, edge distance less than mechanical, rebar installation

Solid brick / dense concrete block:

• Universal anchor: Fischer UX plug with screw, Hilti HUD-1 — standard masonry fixing
• Resin anchor: For high loads or close to edge/crown

Aerated concrete block (Thermalite, Celcon, Hebel):

• Standard mechanical anchors perform poorly in aerated block — the block crushes
• Use dedicated aerated block fixings: Hilti HIT-HY 170 with sleeve, Fischer FIS H injection system, or spring toggles
• Load values for aerated block are significantly lower than dense concrete; always check the manufacturer's load table for the specific block density

Timber frame walls:

• Use frame anchors designed to transfer load through the boards to the structural members
• Plasterboard fixings (Toggler, Rawlplug spring toggle) are not structural; locate the stud

Load tables: Every anchor system has manufacturer-published load tables showing characteristic tensile and shear values for specific substrate strengths, embedment depths, and edge distances. For any structural anchor, the installed load must not exceed the characteristic value divided by the appropriate partial safety factor (typically γ = 1.4–2.0 depending on the standard).

Nail Schedules for Stud Walls

BS 5268 (structural use of timber, superseded but still referenced) and Eurocode 5 (EN 1995) both provide nail schedules for timber frame connections. For standard 50 × 100mm stud-to-sole plate connections in domestic timber frame:

• 3.1mm × 90mm round wire nails — minimum for stud-to-plate connections (BS 5268)
• Skew-nailed (toe-nailed) — two nails each side of stud into plate, driven at 30° angle
• End-nailed — for non-structural connections; through the plate into the end grain

For structural sheathing connections (plywood or OSB to studs in a structural shear panel), the nail schedule is critical — typically 3.1mm × 65mm at 150mm centres at panel edges and 300mm elsewhere, but this is specified by the structural engineer for each project.

Glulam and Engineered Timber Connections

Glulam beams and engineered timber products (LVL, CLT) require connectors specifically assessed for engineered timber. Standard joist hangers may not be rated for the loads or beam depths involved.

Appropriate products:

• Sherpa connectors — hidden connectors for glulam beam-to-column connections; clean appearance; ETA-assessed
• Bulldog ring connectors (toothed plate connectors) — increase bolt load capacity in glulam connections; used in pairs
• SPAX-T timber connectors — for CLT panel connections
• Custom fabricated brackets — for large glulam structures; designed and specified by the structural engineer

Frequently Asked Questions

Do I need an ETA for every structural connection?

No — an ETA is required for the fastener product when that product is being used in a load-rated structural application. For general timber framing (stud walls, non-primary structural connections), appropriate nails or screws per BS 8000-5 (workmanship in timber framing) are sufficient. An ETA becomes critical when the connection is load-rated (joist hangers, post bases, anchors into concrete, structural screws in calculated connections).

Can I use stainless steel nails in a galvanised joist hanger?

Generally yes — stainless nails in a galvanised hanger is not a corrosion-incompatibility issue. The reverse (galvanised nails in a stainless hanger) is slightly less ideal but acceptable for internal use. Mixing incompatible metals (e.g., copper nails in a galvanised hanger) can cause bimetallic corrosion. Follow the manufacturer's specification for nail material.

Why can't I use ordinary wood screws instead of joist hanger nails?

Joist hangers are rated based on tested nail configurations. The rated load assumes specific nail diameter, length, and the shear capacity of that nail in the timber. A wood screw does not have the same shear capacity profile as a round-wire nail of equivalent diameter. Using screws in joist hanger nail holes without explicit manufacturer approval invalidates the load rating.

What is the difference between C4 and C5 corrosion class for fasteners?

The EN ISO 12944 (and EN 13501 for connectors) corrosion classification:

• C1–C3: Internal, low to moderate humidity; standard galvanised fasteners adequate
• C4: Industrial or coastal environments with moderate saltwater exposure; minimum A2 stainless or hot-dip galvanised Z600+
• C5: Very aggressive marine or industrial; A4 stainless or specialist coatings required

For UK residential external timber construction (garden structures, decking, balconies), C3–C4 is the typical classification. Use A2 stainless as a minimum.

Regulations & Standards

• BS EN 845-1 — Specification for ancillary components for masonry; joist hangers built into masonry

• Eurocode 5 (BS EN 1995-1-1) — Design of timber structures; nail and screw schedules, connection design

• BS 5268-2 — Structural use of timber (withdrawn but referenced); nail sizes and schedules

• BS EN 14592 — Timber structures; dowel-type fasteners; requirements

• BS EN 1337 — Structural bearings (for bearing pad connections)

• ETA system — European Technical Assessment under Construction Products Regulation; confirms assessed load values for structural connectors

• Simpson Strong-Tie UK — Technical Guidance — Comprehensive joist hanger and connector load tables

• Hilti UK — Anchor Design — Anchor selector and ETA documentation for anchor systems

• TRADA — Timber Connections — Timber Research and Development Association guidance

• BS EN 845-1 — Masonry ancillaries specification

• Fischer UK — Fixings Selection — Anchor and fixing technical data

• NHBC Standards Chapter 6.4 — Timber framing and structural fastener requirements

• joist installation — Joist installation and hanger specification

• screw bolt guide — General screw and bolt selection reference

• structural calculations — When structural calculations are required

• timber sizes — Structural timber section sizes and grades