Low pitch roofs, particularly skillion roofs, are widely used in contemporary residential and commercial architecture. While architecturally desirable, these designs present challenges relating to condensation risk, limited cavity depth and restricted passive ventilation.

The Metal Roofing Manufacturers Association of New Zealand Code of Practice requires roof designs to allow free passage of air from eaves to ridge and calls for specific ventilation strategies in flat, sarked and skillion roofs. It also notes that long spans greater than 12m and shallow pitches below 12° should be designed to enable passive ventilation within the roof assembly.

COLORSTEEL Dridex provides a solution designed to address these challenges. Manufactured using COLORSTEEL with an absorbent fleece bonded to the underside of the roofing sheet, Dridex replaces traditional roofing underlay while improving condensation control and roof cavity ventilation.

Traditional underlays can act as an air barrier within roof assemblies, particularly in skillion roofs where ventilation pathways are constrained. Without a separate underlay, air can move more freely through the roof cavity. Each rib or crest effectively becomes a passive ventilation channel supporting airflow from eave to ridge.

This airflow can reduce the need for proprietary ventilation systems such as ventilated battens or additional cavity ventilation components. Independent testing by BRANZ demonstrated that replacing traditional underlay with Dridex increased the calculated effective free vent area by approximately 133% and nearly doubled airflow rates through the roof envelope.

Condensation management is achieved through the fleece layer bonded to the underside of the steel sheet. When the roof cavity reaches dew point, the fleece absorbs condensation and retains it before releasing moisture back into the cavity as ambient conditions improve. Testing in accordance with AS/NZS 4201.6 recorded an absorbency of approximately 349 g/m², nearly twice the requirement for roofing underlays specified in NZS 2295.

Dridex can be installed down to the minimum pitch recommended for the roofing profile manufacturer’s system. Trapezoidal profiles can be used at pitches of approximately 3 - 4°, while corrugate profiles are typically installed from around 8°, subject to manufacturer specifications.

The system holds CodeMark certification verifying compliance with the New Zealand Building Code, a distinction not available among other fleece-backed roofing systems in the New Zealand market. It also carries Sensitive Choice approval and is supported by Environmental Product Declarations (EPDs) and Eco Choice accreditation for COLORSTEEL products.

For architects designing low pitch roof assemblies, Dridex offers a technically validated solution supporting improved ventilation, condensation management and roof performance.

References

1. Metal Roofing Manufacturers Association of New Zealand. Code of Practice for Metal Roofing and Wall Cladding.
2. BRANZ. Test Reports DC2950 and DC10841.
3. Standards New Zealand. AS/NZS 4201.6 Testing of Materials Used in Roofing Underlays.
4. Standards New Zealand. NZS 2295: Pliable Building Membranes and Underlays.
5. New Zealand Steel. COLORSTEEL Dridex Design Guide.
6. Asthma and Respiratory Foundation NZ. Sensitive Choice Programme.