Fasteners, Joints and Adhesives

The way timber members are joined together has a profound impact on the appearance, strength and service life of a structure. From traditional hand-cut joints to advanced engineered connectors, the selection and detailing of joints should always reflect the load requirements, exposure conditions and movement characteristics of timber.

Traditional Timber Joints

Mortice and tenon, dovetail, lap joints and halving joints have been used for centuries. These joints rely on precise carpentry to transfer loads through interlocking timber shapes, often secured by timber pegs or glue.

• Best suited for:
  • Internal joinery, furniture and decorative framing where joint craftsmanship is visible and appreciated.
  • Locations protected from moisture cycling, reducing the risk of shrinkage-induced gaps.
• Limitations:
  • Labour-intensive and dependent on the skill of the fabricator.
  • Less adaptable to timber movement across large structural spans or exposed conditions without reinforcement.

Figure 12: Traditional scarf joint. Source: Heartwood Build

Modern Mechanical Connectors and Metal Fixings

Contemporary timber construction increasingly uses metal connectors, screws, bolts and proprietary systems, which bring consistency and simplify construction.

Lightweight framing connectors

• Joist hangers, truss boots and tie-down brackets provide quick, reliable assembly in house framing. Typically made from galvanised steel, with manufacturer-tested capacities.
• Used to resist gravity, uplift and lateral forces. Essential in NCC-compliant wall, floor and roof framing per AS 1684.

Figure 13: Joist Hanger. Source: Pryda

Engineered Timber Connectors

• Fin plates and self-drilling dowels: used in large truss nodes or mass timber where slots are cut into the timber, doubling shear planes and significantly increasing capacity.
• Dovetail and hook connectors: proprietary concealed systems that enable rapid on-site assembly without additional screwing (often removable for disassembly).
• Timber rivets and glued-in rods: offer high-load and stiff joints, often used in long-span engineered structures.

Figure 14: Engineered timber connector. Source: Construction Canada

Hybrid joints

Many mass timber connections combine mechanical fasteners and adhesives to optimise stiffness, transfer loads smoothly and reduce creep. For example, CLT panels may be joined with a mix of screws and glue lines.

Joinery Techniques for Internal Elements

For cabinetry, doors, windows and other internal features:

• Biscuits, dowels and small mechanical connectors are commonly used to maintain alignment and improve glue area.
• Often paired with decorative adhesives that remain clear or stainable.

These joints do not usually carry significant structural loads but must still accommodate seasonal timber movement.

Movement Considerations in Joints

Timber’s natural tendency to shrink and swell means joints need careful design:

• Slotted holes or oversized bolt holes (typically ~10% larger than the bolt diameter) allow for movement without inducing splitting stresses.
• In wide beams, designers may reduce stress concentrations by staggering fasteners, avoiding continuous plates across timber width, or reinforcing with inclined screws (as highlighted in TDG 52).

Tables in AS 1720.1 specify minimum end, edge and spacing distances to reduce splitting risk - these are especially critical for joints under repeated loading or in unseasoned hardwoods.

Aesthetic and Fire Considerations

• Concealed joints, such as dovetail or slotted fin plates, not only create clean visual lines but also enhance fire performance by embedding metal connectors within the timber mass, slowing heat transfer.
• In contrast, exposed metal brackets or bolts may detract from natural timber aesthetics but provide faster installation and straightforward inspection.

Practical guidance

Choosing the right joint type is about balancing:

• Load capacity - does it need to resist shear, tension, or moments?
• Durability - is the joint exposed to weather, or protected inside?
• Visual objectives - should the joint be highlighted as a feature or hidden entirely?
• Fabrication practicality - does it suit workshop prefabrication or require skilled carpentry on-site?
   

Adhesives play a vital role in timber construction, whether enhancing the load-carrying capacity of structural members or achieving crisp, clean finishes in decorative joinery. The right adhesive choice improves performance, prolongs life and helps manage timber’s inherent movement.

Internal vs External Adhesive Selection

Internal decorative applications

• PVA (polyvinyl acetate): the most common woodworking glue for dry, internal applications. Easy to use, clear drying, cost-effective.
• Aliphatic resins (yellow carpenter’s glue): stronger than basic PVA, offering slightly improved water resistance, often used in joinery and cabinetry.

Structural or moisture-exposed applications

• Polyurethane (PUR): foams slightly on curing, filling gaps, highly water resistant. Suitable for joinery that may see intermittent moisture.
• Resorcinol and phenolic adhesives: dark-coloured, fully structural and rated for long-term external exposure. Widely used in laminated timber beams (glulam) and marine joinery.
• Melamine-formaldehyde systems: also found in engineered wood products, requiring precise mixing and clamping but delivering long-term moisture durability.

Fun fact: Resorcinol is a key ingredient in phenolic resins, like one of the first common plastics, bake-o-lite.

Mass timber and engineered products

Factory-controlled adhesives: CLT, LVL and glulam rely on industrial adhesives that meet rigorous durability standards (per AS/NZS 1328, AS/NZS 4357 or EN standards).

Detailing for Adhesive Effectiveness

Surface preparation

Timber must be well fitted, dust-free, and within recommended moisture content ranges (often 10-15%). Too dry or too wet can weaken bonds. Furthermore, shrinkage of members can limit the time between surface planing of timber components and glueing, especially for very stiff hardwoods and face glueing of boards.

Clamping and cure time

Even the strongest adhesive is ineffective without adequate pressure during cure. Follow manufacturer’s guidelines - often requiring firm clamping for 20-60 minutes, and full cure over 24 hours.

Managing movement

Adhesives lock components together and may become stressed if the timber moves significantly due to humidity changes. Where possible:

• Use mechanical fasteners alongside adhesives to share load and resist creep.
• In wide members, detail joints to accommodate shrinkage (for instance, using floating tenons in joinery).

Adhesive Bonded Structural Connections

WS TDG 52 (Section 5.3) highlights innovations such as:

• Epoxy dowelled connections: metal rods set in oversized holes with injected epoxy. Common in mass timber nodes for transferring heavy loads.
• TS3 (Timber Structures 3.0) adhesive butt joints: bonding end grains of large panels using two-part polyurethane systems, enabling broad flat panels without mechanical fasteners.

These approaches demand high quality control, careful alignment, and often lab-tested systems to validate performance.

Typical Applications Overview

Design and Specification tips

• Always match adhesive class to service class (internal dry vs external wet) under AS/NZS 5068 or relevant glue-line classifications.
• Don’t rely on general wood glues for structural or external loads unless they are specifically rated.
• For architectural features, consider clear-drying adhesives to maintain visual quality.
• Where movement is a concern, use adhesives to augment, not replace, mechanical fasteners.

Timber is a living material that continues to respond to its environment. Its seasonal swelling and shrinking, combined with the demands of live and static loads, means careful detailing is essential to ensure connections remain strong and the structure performs as intended.

Accounting for Timber Movement

Moisture-driven movement: Timber expands across the grain as it absorbs moisture and contracts as it dries. Even a 1% change in moisture content can translate to measurable dimensional changes in wide members.

• Allow for slip:
  • Oversized bolt holes (typically 10% larger than the bolt diameter) or slotted holes let the timber move without inducing splitting stresses.
  • Where fin plates or slotted steel connectors are used, ensure slots do not overly restrain timber - consider internal slots that permit some movement.
• Split prevention
  • Pre-boring holes for nails and screws reduces the risk of splitting.
  • AS 1720.1 recommends 80% of nail diameter for pre-bored holes in unseasoned or high-density timbers.
  • Stagger fasteners along the grain rather than aligning in rows to minimise wedge effects that can initiate cracks.

Managing Load Paths and Connection Types

Bearing and shear

• Bolted joints rely on timber bearing against the bolt. Loads parallel to grain have much higher capacity than loads across the grain.
• For maximum capacity, arrange loads along the grain direction, or design according to Hankinson’s formula where loads are applied at an angle.

Withdrawal and tension

• Screws and nails loaded in withdrawal (pulled out) depend heavily on the timber’s grain orientation and joint group.
• For example, nails straight into end grain achieve only about 25% of their withdrawal capacity versus side grain. Skewed installation (angled entry) improves this markedly.

Reinforcement where needed

• Fully threaded screws or inclined screws can be used to reinforce areas prone to perpendicular-to-grain tension, such as around notches, holes or support ends.

Combining Fasteners and Adhesives

Using adhesives in conjunction with mechanical fasteners spreads loads more evenly and reduces stress concentrations. This is especially valuable in:

• Wide panels and engineered elements, where glue lines combined with screws prevent localised overstressing.
• Decorative joinery, where adhesives take primary load and small fasteners simply hold components in position during curing.

Detailing for Corrosion Protection

Avoid moisture traps

• Ensure no flat or upward-facing surfaces around joints where water can pond.
• Provide gaps or drain holes at supports - for example, a 10 mm gap under beams seated on masonry reduces end-grain wicking.

Match materials

• Avoid mixing stainless steel screws into galvanised plates without isolating washers to prevent galvanic corrosion.
• In treated timbers with copper-based preservatives, galvanised fixings may corrode faster. Use stainless or specially coated systems.

Fire Detailing

AS 1720.4 (referenced in TDG 52) notes:

• Embedment method: burying metal fasteners within the timber provides charring cover that maintains load capacity during a fire.
• Fire covers: for exposed plates, fire-resistant wraps or boards keep metal below critical temperatures longer.

Practical tips at a glance

• Use pre-bored holes and stagger fasteners to reduce splits.
• Oversize holes for bolts and use washers large enough to prevent crushing.
• Design slots in plates or elongated holes in timber to accommodate shrinkage and swelling.
• Keep water away from connections through detailing that encourages drainage.
• Where possible, embed connectors for improved fire performance.

Conclusion

The thoughtful specification and detailing of fasteners, joints and adhesives is fundamental to timber’s performance, appearance and longevity. Whether you’re working on a finely crafted interior fitout or a robust outdoor structure, taking time to match connection methods to the timber species, expected loads and environmental conditions pays dividends over the life of the project. By:

• Selecting corrosion-resistant fasteners suited to the exposure class.
• Detailing joints to accommodate timber’s natural movement.
• Pairing mechanical fixings with appropriate adhesives.
• Considering aesthetic outcomes alongside structural needs.

you’ll ensure your timber designs stand the test of time - structurally sound, visually appealing and built to endure Australia’s diverse conditions.