Roof underlay: the breather membrane that keeps steel roofs dry

Roof underlay is the breather membrane that drapes over the rafters or purlins below the roofing. It has to be AS/NZS 4200.1 compliant and, like wall wrap, vapour-permeable so the roof space can dry — get the laps, overhang or UV exposure wrong and you invite leaks and condensation damage you won’t see until it’s in the ceiling.

What roof underlay actually does

It does two jobs at once, and the second one catches a lot of people out:

• Catches any wind-driven rain or snow that gets under the roofing and drains it out to the gutter.
• Manages condensation when warm, moist house air hits the cold underside of a steel roof on a clear night.

Condensation is the quiet killer

Un-lined or poorly-ventilated steel roofs “rain” on the inside as overnight condensation drips off the underside. A correct breather underlay — draped to allow an air gap, and lapped to shed water to the outer face — absorbs and re-evaporates that moisture instead of letting it fall on the insulation or ceiling. Skylight and low-pitch details are where it most often goes wrong.

Installation specifics

The details that keep water moving to the outer face and out to the gutter:

• Overhang the fascia by 20–25mm at the eaves so it drips water clear.
• Vertical laps a minimum of 150mm wide; horizontal laps a minimum of 150mm (so water sheds to the outer face).
• Wall underlay horizontal laps a minimum of 75mm (water to the outer face).
• Self-supporting synthetic underlay is typically OK on spans up to 1200mm.
• Non self-supporting underlay needs galvanised wire or mesh support (Bayonet, etc.).
• Cavity battens at more than 450mm centres need wall underlay restraint to stop it bridging the cavity.

UV exposure and cover

Underlay degrades in sunlight, so cover it before it clocks up too much exposure:

• Maximum UV exposure is 30 days for roof underlay and 90–120 days for wall underlay — this varies by product, so check the Appraisal.
• Same-day cladding cover is recommended for roof underlay.

Extra High wind zone

Sheathing is now mandatory in the Extra High wind zone as a weathertightness backup.