Decking Calculator

Ordering timber decking boards strictly by square metres (m²) is one of the quickest ways to end up short on-site. Australian timber yards (like Bowens, NHS, and Bunnings Trade) sell decking boards by lineal metres (lm), which accounts for the actual board cover width plus the required expansion gap between runs.

How to Calculate Decking Lineal Metres (lm) in Australia

Whether you're a chippy generating a material take-off for a residential build or a homeowner planning a weekend deck build, getting the lineal metre calculation right ensures you order the exact quantity of timber without paying for excessive offcuts.

Step 1: Calculate Effective Board Coverage Width

Every decking board requires a ventilation and drainage gap between runs. The effective coverage width is the physical width of the timber board plus the recommended gap spacing:

Effective Cover Width (m) = [Board Width (mm) + Gap Spacing (mm)] ÷ 1000

For example, a standard 90mm Merbau board installed with a 5mm expansion gap has an effective coverage width of (90 + 5) ÷ 1000 = 0.095m.

Step 2: Convert Deck Area (m²) to Lineal Metres (lm)

Divide your total deck surface area (Deck Length × Deck Width in m²) by the effective cover width:

Net Lineal Metres = Total Deck Area (m²) ÷ Effective Cover Width (m)

For a 6.0m × 4.0m deck (24.0 m² total surface area): 24.0 ÷ 0.095 = 252.63 lineal metres.

Step 3: Add Wastage Allowance (10% to 15%)

Timber is a natural product with natural end checks, knots, and slight bows that need trimming on-site. Furthermore, butt joints must align cleanly over subfloor joists:

• Standard Rectangular Deck: Add 10% wastage (252.63 × 1.10 = 277.9 lineal metres).
• Complex Deck Layouts (Diagonal or Picture-Frame Border): Add 15% wastage to cover extra angle cuts and double joist trimming.

Standard Australian Decking Board Profiles & Species

Australian timber merchants supply several standardized board profiles. Choosing between narrow 86mm/90mm boards or wide 135mm/140mm boards changes both the visual aesthetic and the total lineal metres required.

Comparing Merbau, Treated Pine, and Composite Decking

Each decking timber species and composite material brings specific performance characteristics under Australian climate conditions:

• Merbau (Class 2 Hardwood): The benchmark decking timber in Australia. Naturally bushfire resistant up to BAL 29 under AS 3959. Highly durable against decay, but bleeds natural tannins during initial rain cycles requiring a thorough wash with oxalic acid cleaner before staining.
• Treated Pine (H3 Radiata): Economical, lightweight softwood treated with CCA or ACQ to resist rot and termites. Ideal for painted finish or budget residential decks. Requires annual sealing.
• Spotted Gum & Blackbutt (Class 1 Hardwood): Native Australian premium hardwoods offering exceptional density and high bushfire resistance (BAL 29 & BAL 40). Extremely durable against heavy foot traffic.
• Composite Decking (Trex, ModWood, Ekodeck): Engineered wood fibre and plastic polymer. Eliminates sanding, oiling, and splinters, but requires tighter subfloor joist spacing (400mm max) to prevent thermal sag.

Deck Fastener & Subfloor Framing Rules (AS 1684.2)

Subfloor framing integrity directly impacts the lifespan of your deck. Framing layout must strictly comply with AS 1684.2 (Residential Timber-Framed Construction):

Joist Spacing Guidelines

• 450mm Joist Centres (Standard): Suitable for standard 19mm and 22mm timber boards (Merbau, Spotted Gum, Treated Pine).
• 400mm Joist Centres (Composite / Commercial): Mandatory for most composite decking products to avoid board flex under direct summer heat.
• 600mm Joist Centres (Heavy Timber Only): Permissible only when using thick 32mm structural hardwood boards.

Decking Screw Take-Off Formula & Pre-Drilling

Every timber board crossing over a joist requires 2 screws. To calculate total screws needed:

Total Screws = [Total Board Runs × Joist Count × 2] × 1.10 (Waste)

Use 10G × 50mm stainless steel screws (Grade 304 inland or Grade 316 for salt air within 1km of the coast) for 19mm boards into timber joists. For dense Australian hardwoods, always pre-drill with a combination countersink bit within 50mm of board ends to avoid splitting.

AS 1684 Ground Clearance & Subfloor Ventilation Rules

Poor subfloor ventilation traps ground moisture, causing timber decking to warp, cup, and rot prematurely. The National Construction Code (NCC) and AS 1684 enforce strict clearance minimums:

• Clearance Over Soil: Maintain a minimum of 150mm clear air space between the underside of joists/bearers and bare soil.
• Clearance Over Concrete: Maintain at least 75mm clearance over sloped concrete pads fitted with adequate surface drainage.
• Cross-Ventilation: Provide continuous perimeter subfloor vents equal to 6000 mm² per lineal metre of wall.

Before installing subfloor framing, cross-check your bearer spans, joist sizes, and timber stress grades (MGP10 / F7 / LVL) using our Timber Span Calculator and Wall Framing Stud Calculator.

Worked Example: Estimating a 6m × 4m Merbau Deck

Let's walk through a full material take-off for a typical suburban back deck:

Trade Timber Ordering Tips for Chippies & Builders

When submitting your material list to your trade timber merchant or calculating project costs, keep these tips in mind:

• Order Set Lengths where possible: Random length packs (RL) often contain short 1.8m–2.4m offcut pieces. Requesting set lengths (e.g. 4.8m or 5.7m) reduces butt joints on large decks.
• Factor in Subfloor Hardware: Don't forget post stirrups, M12 concrete anchors, joist hangers, and triple-grip framing ties.
• Quote Accurately: Check your labor charge-out rate using our Carpenter Charge-Out Rate Calculator and verify business tax deductions with our Sole Trader Tax Calculator.
• Complementary Structure Calculators: If adding handrails, stairs, or roof cover to your deck project, use our Baluster Spacing Calculator, Stair & Stringer Calculator, and Roof Pitch Rafter Calculator.