How to frame a timber deck substructure

What is involved in framing a timber deck substructure?

🪚 Framing a Timber Deck Substructure

A properly framed deck substructure is the backbone of a durable, compliant and straight-finished deck. For Australian residential work, the critical requirements are accurate set-out, correct footing and stump installation, appropriate bearer and joist sizing, and compliant fixing methods in line with AS 1684 Residential Timber-Framed Construction and relevant local council or engineer requirements.

This guide covers the standard process for setting out and constructing a timber deck subframe for typical residential applications, including ground-level and raised decks. It is written for trade professionals who need a practical, site-ready sequence using common Australian materials, hardware and framing methods.

What skill level is required to frame a timber deck substructure?

What tools are essential for framing a timber deck substructure?

What materials are needed for a timber deck substructure?

🪵 Materials

Safety Requirements

⚠️ Safety

Step-by-Step Guide

Step 1: Establish the deck set-out and datum

1️⃣ Set out the footprint

Mark the overall deck size from approved plans, boundary offsets and finished floor relationships. Establish profile boards clear of the excavation zone and run string lines to define the external faces of the deck.

Transfer a consistent datum using a rotary laser level or dumpy level. From that datum, determine the finished deck height, bearer underside levels, joist top levels and footing depths.

Check the deck for square by comparing diagonals. For larger decks, break the frame into bays and confirm each section remains square before excavation begins.

💡 A reliable set-out at this stage prevents cumulative errors when lining up posts, bearer centrelines and joist spacing.

Step 2: Confirm spans, member sizes and structural layout

2️⃣ Check the framing design

Before cutting timber, confirm bearer spans, joist spans, cantilevers and post spacings against the approved design, span tables and AS 1684 Residential Timber-Framed Construction. If the deck is engineered, follow the engineering documentation ahead of generic span tables.

Determine:

Mark all footing centres from string lines. Typical framing members may include 140x45 or 190x45 bearers and 90x45 or 140x45 joists, but final selection must match span, species, treatment class and imposed loads.

💡 Do not assume standard spans for hardwood and treated pine are interchangeable; species and stress grade matter.

Step 3: Excavate and pour footings

3️⃣ Install footings to correct depth and size

Excavate footings to the required diameter and depth based on engineering, soil classification and local authority requirements. Ensure founding conditions are adequate and free of loose material.

Place concrete footings and install:

Recheck all footing positions from the string lines before concrete sets. Maintain consistent centreline accuracy, especially where multiple bearer rows are involved.

Where the deck adjoins a dwelling, ensure finished excavation and footing placement do not compromise adjacent foundations, drainage or damp-proofing.

💡 On sloping sites, set all footing heights from laser, not by eye. Minor footing height variation quickly translates into major bearer packing issues.

Step 4: Install and brace posts or stumps

4️⃣ Stand posts and set heights

Install the posts/stumps plumb and temporarily brace them in both directions. Common post sizes include 90x90, 100x100 and 125x125, depending on load and height.

Mark cut heights from your established datum, allowing for:

Trim posts accurately and treat all cut ends with a preservative resealer suitable for treated timber. If using stirrups or brackets, ensure fixings are the correct type and corrosion resistance for the exposure environment, particularly in coastal areas.

Check alignment along each bearer line before final tightening or fixing.

Step 5: Fix bearers to posts

5️⃣ Install bearers straight and level

Fix bearers to the posts using the nominated structural connection, typically M12 hot-dip galvanised bolts with washers, or proprietary structural brackets. Built-up bearers should be fixed in accordance with manufacturer or engineering details.

Set the tops of bearers to a consistent level, unless a designed fall is specified. Confirm straightness with a string line and pack only with approved structural packers where allowed.

Stagger joints in built-up or joined bearers away from adjacent connection points unless specifically detailed otherwise. Ensure adequate bearing at supports and maintain edge distances for bolts.

For wider decks, verify parallelism between bearer lines so joist spans remain consistent across the frame.

💡 If a bearer has a natural crown, install it with the crown up to reduce future sag.

Step 6: Install joists and rim members

6️⃣ Lay out joists at correct centres

Mark joist positions on bearers using the nominated spacing, commonly 450 mm or 600 mm centres, depending on decking type, board direction and load requirements. Composite or narrow decking boards often require tighter centres than standard timber decking.

Install joists with crowns up and keep the top edges flush. Fix joists to bearers using direct skew nailing, structural screws, joist hangers or proprietary framing connectors as specified.

Install rim joists/boundary joists to lock the frame together and maintain joist spacing. Check the overall frame for square again by measuring diagonals once the perimeter is fixed.

Where joists are lapped over internal bearers, maintain full bearing and consistent lap lengths. Avoid introducing twist into the frame by forcing bowed members into place without restraint.

Step 7: Noggings, blocking and lateral restraint

7️⃣ Stiffen the subframe

Install noggings or solid blocking between joists where required to control twist, distribute load and improve frame rigidity. Spacing and arrangement should suit joist depth and overall deck geometry.

For elevated decks, install diagonal bracing or other lateral restraint measures to meet engineering and serviceability requirements. This may include:

Confirm that the subframe is rigid with no visible sway or roll before decking commences.

💡 Elevated decks often fail in serviceability before ultimate strength. Lateral movement complaints usually come back to inadequate bracing, not undersized joists.

Step 8: Check fixings, corrosion protection and treatment

8️⃣ Finalise all structural connections

Inspect every structural fixing before proceeding:

In external applications, use hot-dip galvanised hardware as a minimum, and upgrade to stainless steel in severe coastal or poolside environments. Recoat all field cuts, notches and drill penetrations in treated timber.

Ensure any interface with masonry, steel or existing building fabric includes appropriate separation or flashing where required.

Step 9: Final level, line and square check

9️⃣ QA the completed substructure

Before decking boards are laid, complete a final quality assurance check:

Check drainage under the deck and ensure no timber is in unintended ground contact unless specifically rated for it. Confirm termite management provisions remain intact and visible where required.

A well-finished subframe should allow decking installation without requiring corrective packing, excessive planing or forced board alignment.

Common Mistakes to Avoid

Misread offsets, unsquared profiles or poor datum transfer create ongoing framing issues that are difficult to correct later.

Using 90x45, 140x45 or 190x45 members outside allowable spans leads to bounce, sag and non-compliance.

Inaccurate footing locations or inconsistent stump heights result in excessive packing and out-of-level bearers.

Standard zinc-plated fixings are not suitable for external structural framing, treated pine contact or coastal environments.

Even when vertical loads are covered, lack of lateral restraint can produce frame movement and serviceability failures.

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