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Roof Truss Calculator (Australia)

Free Australian roof truss calculator. Size prefab trusses to AS 1684 and AS 4440. Member lengths, count and delivered cost in AUD across NSW, VIC, QLD, WA.

Roof Truss Calculator

Estimate truss member lengths, the number of trusses needed for your building, total lumber, and a delivered prefab cost.

Top chord (each)
5.2
m per side
Bottom chord
9
m
Web members total
5.58
m per truss
Lumber per truss
24.97
m
Truss count
21
incl. end pair
Total lumber
524
m (linear)
Peak height
2.7
m above plate
Per truss
A$270
prefab, delivered
Total truss cost
A$5,670
excludes crane / install

Slope factor used: 1.155. Lumber LF is the linear sum of all chords and webs per truss, useful for comparing site-built vs prefab. Add 5–10% for cuts and connectors.

Roof truss calculator — Australian builds

This tool sizes prefabricated timber roof trusses for Australian residential and light-commercial roofs to AS 1684.2-2021 and AS 4440-2004. Set your span (m), pitch (degrees) and truss centres to get:

  • Top chord (rafter) length per side
  • Bottom chord length
  • Web member total by truss type — fink, mono-pitch, attic, scissor, queen
  • Truss count based on building length and centres
  • Total carcassing run in linear metres
  • Per-truss and total delivered cost in AUD

Pitch is in degrees throughout, in line with AS 1684 framing drawings. The calculator defaults to a typical project-home build (9 m × 12 m, 30°, 600 mm OC).

How the math works

Australian prefab trusses are triangulated MGP10 or MGP12 radiata-pine assemblies pressed with Multinail or Galintel metal connector plates. The geometry is fully determined by span, pitch and depth.

Top chord. For a duo-pitch fink, each top chord runs from the wall plate to the apex:

slopeFactor = √(1 + tan²θ) = 1 / cos θ
topChord    = (span / 2) × slopeFactor

For a 9 m span at 30°, that’s 4.50 ÷ cos 30° = 5.20 m per side. The yard orders 6.0 m MGP10 90×35 carcassing and trims on the docking saw.

Bottom chord. Equal to the span plus a 50 mm bearing allowance each end. One-piece bottom chords up to 9 m are standard; over 9 m you’ll see a finger-joint or a plated splice over an internal bearing point.

Webs. Internal diagonals depend on type — fink ≈ 0.62 × span, mono-pitch ≈ 0.30 × span, scissor ≈ 0.70 × span, attic ≈ 1.10 × span.

Truss count. Count = ceil(building length ÷ centres) + 1. A 12 m long house at 600 mm centres needs 21 trusses.

Truss types — when to specify which

TypeBest forSpan limitCost vs fink
Fink (W-truss)Project homes, single-storey11 m1.00×
Mono-pitchLean-to additions, carports8 m0.80×
Howe / PrattIndustrial sheds, agricultural18 m1.10×
ScissorCathedral / raked-ceiling living9 m1.30×
Attic / room-in-roofLiveable loft conversions9 m1.85×
Cyclonic (Region C/D)North QLD, north WA11 m1.40×
Hip endHipped roofs (last 2–3 sets)9 m1.30×

Cyclonic versions of fink trusses use the same geometry but heavier MGP12 / LVL chords, larger nail plates and additional uplift connectors per FTMA Technote 7.

Australian pricing — 2026 reference data

MGP10 KD radiata pine has averaged $1,150/m³ for 90×35 sections through Q1 2026, with prices steady against the 2024 settlement. Translated to delivered prefab fink trusses on a 60 km radius, FTMA-member yards in metropolitan NSW, VIC and QLD are quoting A$26–A$36 per metre of span for residential 22°–45° pitches in non-cyclonic regions.

Worked example (the calculator’s reference test): a 9 m span × 12 m long single-storey project home at 30°, 600 mm OC, Wind Region A2:

  • 21 fink trusses at A$30/m × 9 m span = A$270 each → A$5,670 truss order
  • Delivery: included on orders above 15 trusses within 60 km of the yard
  • Crane and 3-person crew on set day: A$850–A$1,300
  • Total prefab roof structure: A$6,500–A$7,000

By comparison, a cut roof using site-cut MGP10 rafters runs A$2,800 in carcassing + A$4,200 in chippy labour (3 carpenters × 5 days at A$280/day) = A$7,000. Material cost is similar; trusses save you a week on programme and fit project-builder schedules.

NCC and Australian code

  • AS 1684.2-2021 — Residential timber-framed construction — Non-cyclonic areas. Default reference for project-home trusses across Wind Regions A0–B.
  • AS 1684.3-2010 — Residential timber-framed construction — Cyclonic areas. Used for designs north of the 26th parallel and in Region C/D zones.
  • AS 1720.1-2010 + Amendment 5 — Timber structures — Design methods. The structural code the truss manufacturer’s engineer designs to.
  • AS 4440-2004 — Installation of nailplated timber roof trusses. Sets installation, bracing and connector requirements; FTMA’s installation guide is the practical interpretation.
  • AS 1170.2-2021 — Structural design actions — Wind actions. Wind classifications and design speeds; drives uplift hold-downs.
  • AS 3959-2018 — Construction of buildings in bushfire-prone areas. BAL-rated detailing on eaves and gables.
  • NCC Volume Two, Part 3.4 — Performance solutions and deemed-to-satisfy provisions for residential framing.

Buying tips

  1. Get three FTMA quotes — pricing across FTMA members varies 10–18% on the same MBF feedstock. Membership directory at ftma.com.au.
  2. Specify wind region and BAL — Region C uplift and BAL-29 fire detailing change the truss design significantly. Send the BAL assessment with your enquiry.
  3. Energy heel for ceiling insulation — a 200 mm heel lets you maintain R5.0 ceiling insulation continuity over the wall plate with the 25 mm airway NCC wants for a cold pitched roof.
  4. Order eaves cut to soffit profile — saves a day of carpentry. Tell the yard your eave overhang and soffit thickness.
  5. Schedule the crane in advance — 9 m trusses at 600 mm OC on a project home need a 14 t crane truck on site for half a day, A$700–A$1,200 in metro areas.

Pair with these calculators

Outputs update as you change inputs. Print the result and pass it to an FTMA Australia member for a comparable quote.

Frequently asked questions

How many trusses do I need for a 12 m long house at 600 mm centres?
Number of trusses = ceil(building length ÷ spacing) + 1. For a 12 m long house at 600 mm centres that's ceil(12 ÷ 0.6) + 1 = 21 trusses, with one truss at each gable end. AS 1684.2 and the FTMA Australia (Frame & Truss Manufacturers Association) installation guide both work to this count, with hip-end trusses substituted for the last 2–3 sets if the roof is hipped rather than gabled.
What is the most common roof truss in Australian homes?
The fink truss (W-truss) is the dominant design for Australian residential roofs, used in around 85% of project homes. It's manufactured by FTMA-member yards from MGP10 or MGP12 stress-graded radiata pine, pressed with 1.0 mm Galintel or Multinail nail plates and certified to AS 1720 and AS 4440. For BAL-rated bushfire zones (BAL-29 and above), some manufacturers shift to fire-protected steel-truss alternatives or apply fire retardant impregnation to the timber.
How much do roof trusses cost in Australia in 2026?
Delivered fink trusses from FTMA-member manufacturers run $26–$36 per metre of span in 2026 — so a 9 m span fink is roughly $235–$325 per truss. A 21-truss order on a 9 m × 12 m project home runs $5,000–$6,800. Add 50%–80% for attic trusses, 25%–35% for scissor, and a separate $700–$1,200 line for crane hire on the day. MGP10 carcassing has been steady at $1,150/m³ through Q1 2026 — get live quotes from three FTMA members before committing.
Do trusses need engineering certification in Australia?
Yes. Under the National Construction Code (NCC) Volume Two and AS 1684.2-2021, every truss must be designed by a competent person — typically the truss manufacturer's in-house engineer — to AS 1720.1 (timber structures) and AS 4440 (timber-framed light commercial). The certifier wants to see the structural design pack and the manufacturer's stamp before signing off the framing inspection. FTMA Australia membership is a good proxy for plant audit and design competency, but isn't a substitute for the project-specific engineering sheet.
What spacing is standard for Australian roof trusses?
600 mm centres is the project-home standard, set so the standard 1200 × 2400 mm structural plywood or OSB sheet aligns at every other truss. 450 mm centres are used in cyclonic regions (Region C and D under AS 1170.2 — north of the 26th parallel) where wind uplift dominates. Commercial sheds with steel framing run 900–1200 mm centres. NCC and AS 1684 always size the roof sarking and battens to the truss centres — check Table 8 of AS 1684.2.
How does cyclonic wind classification change truss design?
Region C (Townsville to Carnarvon) and Region D (top of WA, north of Broome to Wyndham) under AS 1170.2 require designs to ultimate wind speeds of 69 m/s and 88 m/s respectively. That increases the truss-to-plate hold-down hardware (typically a Type C or Type D cyclone tie under FTMA's Technote 7), increases the metal connector plate sizes, and may upgrade the timber from MGP10 to MGP12 or LVL. A Region C truss on a 9 m span typically runs 30–50% more than a Region A1 (Brisbane, Sydney) equivalent.
Can I use roof trusses on a BAL-rated bushfire site?
Yes, with detail changes. AS 3959-2018 sets the construction requirements per BAL rating. BAL-12.5 and BAL-19 allow standard timber trusses; BAL-29 requires non-combustible decking under the truss tails or a sarking sealing the eave; BAL-40 and BAL-FZ require fire-retardant impregnated timber, steel sub-frames, or the entire eave detail boxed-in non-combustible. FTMA Technote 6 lists conformant truss / eave assemblies. Get the BAL rating from the bushfire attack assessment before ordering — it's a major cost driver for sites in WA, VIC and NSW fire-prone areas.

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