Soffit

The soffit is the horizontal underside of the projecting eaves of a roof, running from the external wall plate out to the fascia. On most Australian residential builds the soffit is a finished lining (HardieFlex, fibre cement sheet, FC Eaveline, or timber boards) fixed to the underside of the rafters or to a separate eaves bearer.

The role on a finished house. Three jobs at once:

1. Visual cap to the eaves: closes the view of rafter tails, sarking, and the underside of the roof so the eaves read as a clean horizontal line under the gutter.
2. Roof-space ventilation: most builds use perforated soffit panels (FC Eaveline V) or proprietary soffit vents to let air enter the roof space at the eaves and exhaust at the ridge. This is part of the path to satisfying the roof-ventilation provisions of the NCC Housing Provisions.
3. Weather barrier: keeps wind-driven rain, wasps, possums, and embers out of the roof space. In bushfire-prone areas, the soffit lining is a critical part of the BAL-rated assembly.

Common soffit materials:

• Fibre cement sheet (HardieFlex, Cemintel) is the residential default. Typical thicknesses 4.5 mm or 6 mm in 1200 mm sheets. Paint finish.
• FC Eaveline (perforated variant) for ventilated soffits without separate vents.
• Timber boards (T&G hardwood, plywood, cedar) on premium or heritage builds.
• Aluminium or vinyl prefab planks for re-clads and budget jobs.

Common defects and pre-handover checks:

• Sag between fixing points, especially on long spans without adequate batten support.
• Visible joins where butt joints have not been set and stopped or jointed with H-strip.
• Cracking at fixing points from over-driven screws or nails into FC sheet.
• Paint failure on soffits painted in direct south-facing rain shadow where moisture cycles slowly dry out the substrate.
• Missing ventilation where the soffit was lined with solid sheet only and no separate vents were installed.

Bushfire (BAL). In bushfire-prone areas, the soffit lining is one of the elements that the assembly must rate to. For BAL-12.5 and BAL-19, fibre cement soffit is generally compliant. For BAL-29 and above, the sealed assembly behind the soffit (no gaps to roof space) becomes the load-bearing detail; check the specific manufacturer system documentation.

For builders. Two practical points:

1. Confirm ventilation before lining. If the sarking, insulation, and ventilation strategy don’t add up to NCC-compliant roof-space airflow, the soffit becomes a defect when the certifier checks at OC.
2. Match colour and grain on timber soffits across orientations. Sun fade differs by exposure; a recently-finished cedar soffit on the north side will look different from the same product on the south within 18 months.

Also known as: eaves lining, soffit lining, underside of eaves.

Category: Practical / framing / roofing.

Related

• Brick veneer cladding
• Cement sheet
• Chippy (carpenter)

See also

• Sarking

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Last updated: 2026-05-14. Verified: 2026-05-14.