Decking Bearers & Joists: Guide to Building a Solid Deck Sub-Frame

Decking bearers & joists are the foundational components of any deck, providing the structural integrity and stability needed to withstand decades of foot traffic and the harsh Australian climate. The secret to a long-lasting, beautiful deck isn’t just in the boards you choose—it’s in the unseen framework that supports it.

This guide provides an expert overview of the vital role these timbers play, detailing the right materials, critical sizing and spacing standards, and best practices for building a sub-frame that will last a lifetime.

The Backbone of Your Deck: What Are Bearers & Joists?

Hardwood Bearers

Bearers are the main horizontal timbers that run along the length of the deck, supported by posts. They are the primary load-bearing members, transferring the deck’s weight directly to the footings.

Hardwood Joists

Joists are the smaller timbers that run perpendicular to the bearers. They provide the direct support for the decking boards. Together, the bearers and joists form a rigid, durable sub-frame that prevents the deck from warping, sagging, or becoming unstable over time.

Choosing the Right Hardwood for the Job

While softwood can be used, hardwood bearers and joists offer superior strength, durability, and natural resistance to pests and decay. Choosing a Class 1 or Class 2 durability hardwood for your sub-frame ensures its longevity, often matching or exceeding the lifespan of the decking boards themselves.

Common Australian hardwoods used for bearers and joists include:

  • Blackbutt: A durable, straight-grained timber with a high F-rating (F17), making it exceptionally strong.

  • Ironbark: Extremely dense and hard, providing unparalleled strength and a Class 1 durability rating.

  • Spotted Gum: Known for its high strength and natural resistance to lyctid borers and termites.

Choosing a timber with a Durability Class of 1 or 2 is a crucial first step toward building a deck that will stand the test of time.

Decking Sub-Frame Sizing & Spacing

Getting the sizing and spacing right is the most critical part of deck construction. Australian Standards (AS 1684) provide specific guidelines to ensure structural safety. Below is a simplified guide for common applications.

Hardwood Decking Bearer & Joist Span Tables (Indicative for F17 Hardwood)

Hardwood Joist Span Table

This table outlines the recommended maximum span for hardwood joists based on their size and typical “on center” spacing.

Joist Size (mm) Maximum Span (m) Joist Spacing (mm)
150 x 50 2.4 450
150 x 75 3.0 450
200 x 50 3.0 450
200 x 75 3.6 450

Hardwood Bearer Span Table

This table provides the recommended maximum span for hardwood bearers, based on their size and the maximum joist span they will be supporting.

Bearer Size (mm) Maximum Span Between Posts (m) Maximum Joist Span (m)
150 x 50 (double) 2.4 2.4
200 x 50 (double) 3.0 3.0
250 x 50 (double) 3.6 3.6

Note: This is a simplified table. Always consult a qualified engineer and local building codes before construction.
The spans above assume a standard deck load.

The Build: Best Practices for a Lasting Deck

A well-constructed deck sub-frame requires attention to detail to prevent future issues like rot, warping, and premature decay.

  • Ground Clearance & Ventilation: Maintain a minimum of 75mm of ground clearance between the lowest timber and the soil. This prevents moisture absorption and allows for critical airflow, which is the primary defense against rot and decay.

  • Ground Preparation: The ground beneath the deck should be sloped away from the house to ensure water drains properly. Consider using a plastic membrane or geotextile fabric to suppress weeds and moisture.

  • Fasteners: Always use high-quality, hot-dip galvanized or stainless steel screws, bolts, and nails. This prevents rust, which can not only weaken the structure but also stain your deck boards.

  • Moisture Barriers: For extra protection, apply a self-adhering moisture barrier or joist tape to the top of the bearers and joists. This prevents moisture from seeping into the timber from the surface, protecting it from rot and extending its lifespan.

Visual Guide to Deck Construction

decking sub frame exploded view

Exploded View of a Deck Sub-Frame

Three-dimensional diagram showing the relationship between the posts, bearers, joists, and decking boards.

decking joist & bearer spacing

Joist & Bearer Spacing

A top-down view showing the recommended “on center” measurements for joists and bearers.

decking sub-frame

Properly Ventilated Sub-Frame

correctly built sub-frame with ample ground clearance, proper drainage, and space for air to circulate freely

Frequently Asked Questions (FAQ)

A: Yes, many premium hardwoods like Blackbutt, Spotted Gum, and Ironbark are graded for both structural and decking applications, making them a great choice for a cohesive and high-performance deck.

A: The standard “on center” spacing for decking joists is 450mm. This spacing provides adequate support for standard-sized decking boards and prevents them from bowing or warping.

A: Adequate ventilation prevents moisture buildup, which is the primary cause of timber rot, fungal growth, and decay. It allows the sub-frame to dry out after rain and protects the integrity of the timber.

A: While not mandatory, joist tape is a highly recommended best practice. It acts as a waterproof barrier on top of the joists, preventing water from being absorbed by the timber and significantly extending the lifespan of your deck’s sub-frame.

Glossary of Decking Terms

  • Bearer: The main horizontal structural member of a deck, supported by posts.

  • Joist: A timber that runs perpendicular to the bearers, providing direct support for the decking boards.

  • Sub-Frame: The entire structural framework of a deck, including the posts, bearers, and joists.

  • Span: The distance a timber can safely support between two points without excessive deflection.

  • On Center: The distance from the center of one timber to the center of the next timber.

  • Deflection: The degree to which a structural element is displaced under a load.