How to Build a Frame for Composite Decking in Australia?
The frame is the most important part of any deck. It carries the weight of the boards, the furniture, and everyone standing on it. If the frame is built correctly, the deck will last decades. If it is not, the boards above will sag, bounce, or worse.
Composite decking boards have specific framing requirements that differ from timber boards. This guide covers how to build a frame designed for composite decking in Australian conditions.
Table of Contents
Two Frame Types: Attached vs Freestanding
Before you start, decide which type of frame suits your project. This guide covers both types:
Attached (to the house)
One side of the frame is fixed to the house wall using a ledger board. This is the most common design for decks accessed through a back door or sliding door. It requires cutting into or fastening against the house wall, which means waterproofing the connection is critical.
Freestanding
The frame stands independently on its own footings without any connection to the house. This is simpler to build and avoids interfering with the house structure. It is required when the house wall is not suitable for a ledger (e.g., lightweight cladding without a solid frame behind it). Freestanding decks may also have different permit requirements. In some councils, they are easier to approve.
Materials List (for a 4.0 m × 3.6 m Deck)
| Item | Typical Size | Quantity | Approx. Cost (AUD) |
|---|---|---|---|
| Posts (H4 treated pine) | 90 × 90 mm or 100 × 100 mm | 6 | $20–$40 each |
| Bearers (MGP10 or MGP12 treated pine) | 140 × 45 mm (doubled) or 190 × 45 mm | 3 lengths @ 4.0 m | $15–$35 each |
| Joists (MGP10 treated pine, H3) | 140 × 45 mm or 190 × 45 mm | 10 lengths @ 3.6 m | $12–$25 each |
| Ledger board (attached deck only, H3) | 190 × 45 mm | 1 length @ 4.0 m | $20–$35 |
| Rim joists | 140 × 45 mm | 2 lengths @ 4.0 m | $12–$25 each |
| Concrete for footings | Pre-mix bags 20 kg | 12–18 bags | $8–$12 each |
| Post stirrups (galvanised) | Suit post size | 6 | $15–$25 each |
| Joist hangers (galvanised) | Suit 140 × 45 mm or 190 × 45 mm | 20 | $3–$5 each |
| Coach bolts (galvanised) | M12 × various lengths | 1 box | $30–$50 |
| Structural screws (galvanised) | 14 × 75 mm | 1 box (100) | $25–$40 |
| Joist flashing tape | 75 mm wide | 1 roll (20 m) | $30–$60 |
Estimated total materials cost: AUD $800–$1,500 for the subframe only (boards not included).
Use the EVODEK Decking Calculator to estimate board quantities once your frame dimensions are confirmed.
Step-by-Step: Building the Frame
Step 1: Check Permits and Services
In most Australian states, a deck requires a building permit if it exceeds 10 sqm or is more than 600 mm above ground. A 4.0 m × 3.6 m deck is approximately 14.4 sqm, so a permit is likely required.
Before digging, contact BYDA at 1100 to locate underground services.
Step 2: Set Out the Footings
Mark the footing positions using string lines and pegs. For a 4.0 m × 3.6 m deck with three bearer lines, you need six footings, two per bearer, spaced at the bearer span.
Dig footing holes to a minimum depth of 450 mm in stable soil. In reactive clay soils (common in Melbourne, Adelaide, and parts of Sydney), deeper footings (600 mm+) may be required. Footing holes should be approximately 350 × 350 mm square or 400 mm diameter.
Pour concrete into each hole. Set a galvanised post stirrup into the wet concrete, centred and level. Allow the concrete to cure for at least 48 hours before loading.
For attached decks: The house-side row of footings is replaced by the ledger board (see Step 3). You only need footings for the outer bearer line(s).
Step 3: Install the Ledger Board (Attached Decks Only)
The ledger board is the horizontal timber fixed to the house wall. It supports one side of every joist. This is the most critical connection in an attached deck. Ledger failure is the leading cause of deck collapses.
On brick veneer walls (most common in Australia):
- Mark the ledger position. The top of the ledger should sit so that the finished deck surface is 40–50 mm below the interior floor level. This prevents rainwater from running back into the house.
- Drill into the brick using a masonry bit. Use M12 × 100 mm galvanised sleeve anchors spaced at 450 mm centres.
- Apply a bead of exterior-grade sealant between the ledger and the wall to prevent moisture ingress.
On timber-framed walls with cladding:
- Remove the cladding where the ledger will sit. You need to bolt directly into the wall studs or rim joist behind the cladding.
- Install metal flashing (Z-flashing) above the ledger to direct water away from the wall.
- Bolt the ledger through the cladding into the structural frame using M12 coach bolts at 450 mm centres. Each bolt must pass through the ledger, through the wall frame, and be secured with a washer and nut on the inside.
Important: Never fix a ledger board to cladding alone. The ledger must connect to the structural frame of the house. If you are unsure what is behind the cladding, engage a licensed builder to assess.
Step 4: Install the Posts
Place posts into the stirrups. Check each post for plumb (vertical) using a spirit level on two adjacent faces. Bolt the post to the stirrup using the pre-drilled holes in the stirrup hardware.
Cut all posts to the same height. The top of each post should be level with the bottom of the bearer. Use a laser level or string line transferred from the ledger board (or from a datum point for freestanding decks) to mark the cut line.
Step 5: Install the Bearers
Bearers run horizontally on top of the posts. They support the joists.
For a 4.0 m × 3.6 m deck, bearers run along the 4.0 m dimension (the long side). A single 190 × 45 mm MGP12 bearer, or a doubled 140 × 45 mm bearer, is suitable for typical residential spans up to 2.0 m between posts.
Fix bearers to posts using galvanised post-beam connectors or M12 coach bolts. Ensure the bearers are level along their length and level with each other. Check diagonals to confirm the frame is square.
For specific span and spacing tables, refer to AS 1684.2 (Residential Timber-Framed Construction) or ask your building certifier.
Step 6: Install the Joists
Joists run perpendicular to the bearers. They directly support the deck boards.
For composite decking, the maximum joist spacing is 450 mm centres for a standard straight-lay pattern. For diagonal board layouts (45°), reduce spacing to 300 mm centres. These are general guidelines; always check the specific product’s installation manual.
Fix joists to bearers (and to the ledger board on attached decks) using galvanised joist hangers. Drive structural screws through every pre-punched hole in the hanger; do not skip any. Each joist should sit level and flush with the top of the bearer.
Step 7: Add Rim Joists and Blocking
Fix rim joists across the open ends of the joist array. Use two structural screws per joist-to-rim connection. The rim joists close off the frame and provide a fixing surface for fascia boards.
Add blocking (noggings) at the midpoint of each joist span. Cut offcuts to fit between each pair of joists. Stagger them slightly (20–30 mm offset) so you can screw through the joist face into the block end from both sides. Blocking prevents joist twist and adds rigidity.
Step 8: Apply Joist Tape
Before laying any deck boards, apply self-adhesive joist flashing tape to the top of every joist. This waterproof membrane prevents moisture from sitting on the joist surface and accelerating rot.
Run the tape along the full length of each joist, pressing it firmly to ensure adhesion. Also, tape the top of the ledger board and rim joists.
This step costs under AUD $60 in materials and can add years to your subframe’s lifespan, especially in high-rainfall areas like QLD and coastal NSW.
Why the Frame Matters More for Composite Boards
Composite boards behave differently from timber boards on the subframe.
- Thermal expansion. Composite boards expand and contract more than timber with temperature changes. Your frame must allow for this. Leave 6–8 mm gaps between boards (hidden clip systems set this automatically) and 10 mm gaps at each board end where it meets the rim joist or fascia.
- No structural contribution. Timber deck boards add some lateral rigidity to the frame. Composite boards do not — they are surface cladding only. This means the frame itself must be fully rigid. Blocking, joist hangers, and proper bearer-to-post connections are not optional.
- Closer joist spacing. Composite boards have more flex than hardwood of the same thickness. The tighter 450 mm joist spacing (vs 600 mm for some hardwoods) compensates for this. Do not stretch joist spacing beyond the manufacturer’s recommendation.
EVODEK composite decking boards have a density of 1.3 g/cm³, denser than most competing composites. This higher density reduces flex, but the 450 mm joist spacing rule still applies. Order free samples to assess the board weight and feel before finalising your frame design.
Summary
The frame is the foundation of every composite deck. It must be level, square, and built to handle the specific behaviour of composite boards, tighter joist spacing, expansion gaps, and full structural rigidity without relying on the boards for stiffness.
Use H3/H4 treated pine or galvanised steel. Fix connections with galvanised joist hangers, coach bolts, and structural screws. Apply joist tape to every horizontal surface. And always follow the board manufacturer’s specified joist spacing.
Frequently Asked Questions
Can I use a steel frame instead of timber?
Yes. Steel frames (typically galvanised or aluminium) are more durable than timber and will not rot or attract termites. They cost more — roughly 40–60% more than treated pine for the same deck. Steel frames work well with composite boards. Ensure the steel sections are rated for the span and load, and use appropriate fixings (tek screws or bolts, not timber-grade screws).
What timber grade should I use for the frame?
Use MGP10 or MGP12 graded treated pine. Posts in contact with or near the ground should be H4-rated. Joists and bearers above ground should be at least H3-rated. These ratings indicate the timber’s resistance to rot and insect attack. Avoid untreated timber for any part of the subframe.
Do I need a building permit for the frame?
In most Australian states, yes. If the deck exceeds 10 sqm or is more than 600 mm above ground. The frame is the structural element that councils inspect. Submit your framing plan (including footing details, member sizes, and span tables) with your permit application.
How do I know if my joist spacing is correct?
Check the decking manufacturer’s installation guide. For EVODEK composite boards, the maximum joist spacing is 450 mm centres for straight-lay and 300 mm for diagonal patterns. If you plan to place heavy items on the deck (spa, large planter, outdoor kitchen), consider reducing spacing to 300 mm in those zones.
Can I build the frame on deck blocks instead of concrete footings?
Deck blocks (surface-level concrete pads) are suitable for low-level freestanding decks under 300 mm above ground on flat, stable soil. For elevated decks, attached decks, or any deck on sloping or reactive soil, concrete footings with post stirrups are required. Your local council may specify a minimum footing depth for your soil classification.
Should I apply joist tape to a new frame?
Yes. Joist tape is inexpensive and significantly extends the life of the timber subframe. It is especially important in humid and high-rainfall regions. Apply it to the top of every joist, bearer, ledger board, and rim joist before installing deck boards.
How long does a treated pine frame last under composite boards?
A properly built H3/H4 treated pine frame with joist tape typically lasts 25–30 years, matching the lifespan of the composite boards above it. Without joist tape, the frame may degrade faster than the boards, requiring early replacement.
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