Few things sting quite like being stuck up on an aluminium plank at 3:30 on a Friday arvo, realising you're half a stick short on pre-primed fascia board because nobody factored in the rafter overhangs. When you order materials based purely on external brick dimensions, you instantly leave yourself in the red. Standard 450mm or 600mm eaves push the true roof perimeter well past the slab line, adding anywhere from four to six extra lineal metres on a modest single-storey build.
Then there’s the soffit side of the coin. Grab the wrong width of fibre cement, and you’ll spend your whole morning burning through diamond scoring blades, making awkward longitudinal rip cuts and wasting good sheets that don't span rafter centres. Whether you’re cutting out water-logged timber before gutter fit-off or pricing a new build off structural plans, this calculator takes the guesswork out. It works out lineal boards, HardieFlex sheets, PVC jointers, and hardware counts strictly against AS 1684.2 framing tolerances and NCC 2022 condensation rules.
Roof Perimeter Anatomy: Fascia, Soffits & Eaves Explained
On an Aussie framing site, these components work together to seal the roofline and hang stormwater guttering:
- Rafter Tails (Truss Overhangs): Overhang past the wall plate (typically 450mm or 600mm; up to 900mm on solar-passive designs).
- Fascia Board: Structural timber or Colorbond face board fixed across rafter ends. Carries gutter brackets (handling 15kg/m in heavy downpours) and features an internal rebate to capture the outer eave sheet edge.
- Soffit (Eave Lining): Horizontal underbelly panel (usually 4.5mm fibre cement like HardieFlex) bridging the wall to the fascia. Seals against rain, possums, and bushfire embers.
- Barge Boards: Angled fascia boards running up the sloping rake of gable and skillion roofs.
- Sprockets & Bearers: 70×35mm framing noggings fixed between the wall and rafter tail to prevent eave sag on overhangs over 450mm.
- PVC Jointers (H-Moulds) & Quad: H-moulds connect sheet ends with a 2mm thermal gap; 19×19mm quad bead seals the brickwork join.
Materials Compared: Timber, Colorbond & HardieFlex
Material choice dictates your framing prep, speed of installation, and long-term upkeep:
| Material | Common Sizes | Pros | Trade Notes |
|---|---|---|---|
| H3 LOSP Primed Pine Fascia | 180×18mm, 230×19mm (5.4m, 6.0m) | Solid, easy to mitre, accepts standard gutter screws anywhere. | Needs repainting every 7–10 years. Prone to rot if gutters overflow. |
| Colorbond® Steel Fascia | 0.42 BMT steel (6.0m, 7.2m, 8.0m) | Termite-proof, zero rot, never needs paint, rapid clip install. | Can dent under ladders; cut with hand snips only (friction saws burn zinc coating). |
| HardieFlex™ Eave Sheets | 4.5mm & 6.0mm (2400×450, 2400×600) | Non-combustible (BAL rated), rot-proof, score-and-snap cutting. | Brittle; carry on edge and support 3m sheets overhead with two people. |
| VJ Timber / HardieGroove | 9.0mm FC or 86×12mm treated pine VJ | Premium architectural finish for Hampton and Queenslander styles. | Higher cost ($45–$75/m²); requires extra framing noggings for secret nailing. |
How to Calculate Fascia Metres & Sheet Counts
Don't measure off the slab or brick line—overhangs extend the roof perimeter significantly. On a standard 4-sided hip roof:
Outer Length = Building Length + (2 × Eave Overhang)
Outer Width = Building Width + (2 × Eave Overhang)
Total Fascia Run (Lm) = 2 × (Outer Length + Outer Width)
For gable roofs, add the two horizontal gutter eaves to the sloping barge rakes: Rake = (Half Width + Overhang) ÷ cos(Pitch Angle).
Worked Example (18m × 10m Hip Roof, 450mm Eaves):
- Outer Perimeter: 2 × (18.9m + 10.9m) = 59.6 lineal metres.
- Timber Fascia (+10% waste): 59.6m × 1.10 = 65.56 Lm → 13 stock boards of 5.4m H3 Pine (70.2 Lm).
- HardieFlex 2400×450mm Sheets (+10% waste): (59.6m ÷ 2.4m) × 1.10 = 28 sheets.
- PVC H-Mould Jointers: 1 per sheet join = 28 pieces.
- Wall Quad Bead (19×19mm): 2 × (18m + 10m) = 56 lineal metres.
AS 1684 Site Rules: Scarf Joins & Fixing
- Stringline Tails First: Check truss tails with a line from corner to corner. Plumb-trim any long tails before installing fascia boards.
- 45-Degree Scarf Joints: Join timber fascia lengths at a 45-degree bevel, centered directly over a rafter tail. Never butt square or splice in mid-air.
- Fasteners: Minimum two 65mm × 3.15mm hot-dipped galvanized bullet head nails (or 10G × 65mm exterior screws) per rafter tail. Use Grade 316 stainless within 1km of the coast.
- 2mm Jointer Gap: Keep a 2mm thermal expansion gap inside PVC H-moulds so fibre cement sheets don't buckle in 40°C heat.
NCC 2022 Ventilation & Bushfire (BAL) Compliance
Condensation (NCC Part 10.8): Homes in Climate Zones 4 to 8 (Sydney, Melbourne, Adelaide, Hobart, Canberra) require roof cavity cross-ventilation equal to 1/300th of the ceiling area. Use pre-vented eave sheets or continuous strip vents behind the fascia.
Bushfire (AS 3959): In BAL-12.5 through BAL-29, eaves must be lined with non-combustible material (minimum 4.5mm or 6.0mm fibre cement). All vents must be screened with corrosion-resistant stainless steel or bronze wire mesh (maximum 2mm aperture). BAL-40 and BAL-FZ require full non-combustible systems with zero exposed timber.
2026 Material Costs & Trade Pricing
| Work Item / Material | Trade Rate ($AUD) | Standard 60m Perimeter Job |
|---|---|---|
| H3 Primed Pine Fascia (180×18mm) | $22 – $28 / Lm | $1,440 – $1,800 |
| Colorbond Steel Fascia | $16 – $24 / Lm | $1,050 – $1,550 |
| HardieFlex 4.5mm Sheets (2400×450) | $20 – $25 / sheet | $560 – $700 |
| PVC H-Moulds & Quad Trims | Lump sum | $250 – $380 |
| Galvanized Fasteners & Sealant | Lump sum | $120 – $180 |
| Carpenter Fixing Labour | $45 – $70 / Lm | $2,700 – $4,200 |
| Rotted Timber Strip-Out (Renovations) | $15 – $25 / Lm | $900 – $1,500 |
| Total Turnkey Scope (New Build, Ex GST) | $85 – $130 / Lm | $5,100 – $7,800 |