Roof Underlayment: Complete Guide to Sarking and Protection

Underlayment—known as sarking in Australia—is the secondary weather barrier beneath your roof covering. It improves water resistance, thermal performance and condensation control while keeping dust and debris out of the roof space.

Benefits

• Secondary water protection below tiles and metal
• Improved thermal performance (reflective foil sarking)
• Condensation management via vapour permeability
• Dust and ember protection in bushfire areas

Types

Reflective Foil Sarking

Common in Australia. Acts as a radiant barrier; emissivity ratings and installation air gaps influence performance. Single vs double-sided options.

Building Paper (Felt)

Traditional asphalt-saturated paper—limited use due to moisture sensitivity.

Synthetic Underlayment

Polymer-based with superior tear strength, water holdout and UV stability.

Breathable Membranes

Allow vapour to escape while resisting liquid water; ideal for condensation-prone assemblies.

Standards and Bushfire Requirements

Follow NCC and relevant standards. In BAL zones, use ember-resistant sarking and detail laps and penetrations to prevent ember ingress. Confirm fire ratings and compliance certificates.

Installation

• Timing: Installed over rafters/battens before roof covering
• Fixing: Stainless/galvanised staples or cap nails as specified
• Overlaps: As per manufacturer; tape/seal critical laps and penetrations
• Ventilation: Coordinate with intake/exhaust strategies

Costs

• Reflective foil: $4–$9/m² material; $12–$25/m² installed
• Synthetics: $6–$12/m² material; $18–$35/m² installed
• Self-adhered membranes: premium pricing

Common Issues

• Insufficient overlaps causing wind-driven rain ingress
• Damage during construction—walk boards recommended
• Condensation due to missing ventilation or wrong membrane type

Case Studies

Tile reroof, Northern Beaches: Double-sided foil sarking reduced summer ceiling temps and controlled wind-driven rain under tiles.

Conclusion

Select sarking type based on climate, roof covering and bushfire rating. Quality installation of overlaps and penetrations is crucial to performance.