Roof batten and underlay inspection for residential builds in Australia

What it is

The roof batten inspection is a pre-cover-off hold point checking the sarking or underlay, the batten gauge, the batten fixings and the anti-ponding board before tiles, metal sheet or shingles are laid. For tile roofs the controlling standard is AS 2050 Installation of roof tiles, applied alongside AS 4200.1 for the sarking material and AS 4200.2 for its installation. For metal sheet roofs the controlling standard is AS 1562.1.

The inspection sits between the frame inspection (which signed off rafters and trusses) and the lock-up inspection (which signs off external cladding). Once the tiles or sheets are down, the underlay and the batten fixings are concealed.

What the inspector checks

Sarking and underlay

The sarking is checked for product class, fall direction, lap and fixing. For tile roofs in BAL-rated bushfire zones, the sarking must comply with AS 4200.1 with a flammability index appropriate to the zone. Horizontal laps must be at least 150 mm and run downhill. Vertical laps run over a rafter or truss. The sarking must drape between rafters at a 10 mm to 15 mm sag so any water that gets past the tiles runs to the gutter rather than ponding at the lap.

Batten gauge

The batten gauge is the centre-to-centre spacing between battens, set by the tile manufacturer's installation manual based on the tile profile and roof pitch. For a standard concrete tile at 22 degree pitch the gauge is typically around 330 mm to 345 mm. The inspector measures gauge at multiple points across the roof. Inconsistent gauge produces uneven tile overlaps and water entry points.

Batten size and fixing

Timber battens for tile roofs must meet AS 1684.2, AS 1684.3 or AS 1720.1, with 38 x 38 mm or 50 x 38 mm structural timber the common sizes. Metal battens, usually 40 mm high in 0.55 mm or 0.75 mm hi-tensile steel, are checked for product specification against the engineer's design. Self-embedding screws of 8 gauge to 18 gauge must penetrate not less than 15 mm into the batten.

Anti-ponding board

AS 2050 requires an anti-ponding device at the eaves on tile roof pitches below 20 degrees. The board fixes along the top of the fascia and runs back up the rafter, finishing approximately 50 mm below the first batten. It lifts the underlay clear of the gutter so any water that crosses the lap sheds into the gutter rather than ponding behind the fascia and rotting the rafter ends.

Sheet roof preparation

For metal sheet roofs the inspector checks safety mesh installation if specified, sarking, batten spacing to suit the sheet profile and span tables, and the fixing schedule. Self-drilling screws with EPDM washers are confirmed against the manufacturer's specification, and the fixing pattern is checked against wind classification.

Penetrations

Vent pipes, flue penetrations and skylight upstands are checked for flashing under the sarking on the uphill side and over the sarking on the downhill side, with all sides sealed against the membrane.

Common defects flagged on site

• Sarking lapped uphill, allowing water to run under the lap.
• Sarking pulled tight between rafters with no sag, ponding water at the batten line.
• Anti-ponding board missing on a pitched tile roof below 20 degrees.
• Batten gauge inconsistent across the roof.
• Batten fixing screws too short to penetrate the rafter or truss top chord by the required minimum.
• Sarking torn during batten installation and not repaired.
• Flue penetration flashed on top of the sarking only.

A TradeLens scan flags missing sarking product compliance certificates, missing BAL-rated sarking documentation in bushfire zones, and missing tile manufacturer batten gauge data.

Sign-off documentation

• Building surveyor or principal certifier batten inspection record, dated and signed.
• Sarking product certificate confirming AS 4200.1 compliance and BAL rating where applicable.
• Tile manufacturer installation guide with the batten gauge for the roof pitch documented.
• Engineer's batten fixing schedule for cyclonic regions (Region C and D wind classifications).
• Photographic record of sarking lap direction, anti-ponding board, batten gauge and penetration flashings.

In NSW the batten and sarking are typically inspected as part of the broader frame or pre-line stage. In Victoria the registered building surveyor inspects under Mandatory Inspection requirements. In Queensland the private building certifier may include it as part of the roof framing inspection. The sarking and anti-ponding board cannot be inspected once the tiles or sheets are down, so document everything before the roof cover goes on.