Roof underlay is one of those building components most homeowners never see — but it plays a critical role in keeping your home dry, warm, and compliant with New Zealand regulations. Whether you're re-roofing, building new, or just curious after spotting a leak, understanding underlay will help you make smarter decisions about your roof.
What Is Roof Underlay?
Roof underlay (sometimes called sarking, roofing felt, or underlayment) is a water-resistant or breathable membrane installed directly over the roof structure — typically the rafters or purlins — and beneath the outer roofing material such as corrugated iron, long-run steel, or tiles. Think of it as a secondary line of defence: if wind-driven rain, condensation, or a failed fastener allows moisture past your cladding, the underlay catches it and directs it away before it can soak into your ceiling or framing.
Types of Roof Underlay Available in NZ
There are several types commonly used in New Zealand:
| Type | Description | Best For |
|---|---|---|
| Bitumen-saturated felt | Traditional black felt; low cost, limited lifespan | Older re-roofing jobs, low-pitch metal |
| Synthetic non-breathable | Polypropylene or polyethylene; durable and affordable | Ventilated roof spaces |
| Breathable (vapour-permeable) | Allows moisture vapour to escape while blocking liquid water | Insulated or unventilated roofs |
| Foil-faced underlay | Adds a reflective layer for thermal performance | Warm climates, energy efficiency upgrades |
For most modern NZ homes — especially those with insulation installed between or above the rafters — a breathable membrane is strongly recommended. It allows any trapped moisture vapour to escape, reducing the risk of condensation damage to your framing and insulation.
Does the NZ Building Code Require Underlay?
In short: usually yes. The MBIE Building Performance guidelines and the NZ Building Code Clause E2 (External Moisture) set out requirements for roofs to resist the penetration of water. For most pitched roofing systems, underlay is a required component to achieve compliance.
The specific requirements depend on:
- Roof pitch — lower-pitch roofs (under around 8°) have stricter requirements because water drains more slowly
- Roofing material — metal roofing, tiles, and shingles each have different underlay specifications
- Wind zone — NZ has defined wind zones under NZS 3604, and higher wind zones require more robust weathertightness detailing
- Whether the roof space is ventilated or not
BRANZ publishes detailed guidance on underlay selection and installation as part of their Build magazine and technical resources — well worth a read if you're planning a roofing project.
Why Is Underlay Especially Important in New Zealand?
NZ's climate varies enormously — from the wet, wind-lashed West Coast and Northland to the dry, UV-intense Central Otago summers. Across most of the country, roofs face:
- High rainfall and wind-driven rain, which can force moisture under laps and flashings
- Significant temperature swings that cause condensation inside roof spaces
- UV exposure that degrades roofing materials over time, making secondary protection more important
Without underlay, even a small gap or failed sealant can allow water to drip directly onto your ceiling lining or timber framing — leading to rot, mould, and costly repairs.
What Happens If You Don't Have Underlay?
Older NZ homes — particularly those built before the 1970s — were sometimes constructed without underlay, or with degraded felt that has since crumbled away. Signs that your underlay may be missing or failing include:
- Visible daylight through the roof space
- Staining or damp patches on ceiling linings after rain
- Mould or musty smells in the ceiling space
- Deteriorating timber in the roof framing
If you're re-roofing, this is the ideal time to install or upgrade your underlay. Skipping it to save money is a false economy — the cost of repairing water-damaged framing or linings far outweighs the cost of a quality membrane.
Choosing the Right Underlay
When selecting underlay, consider:
- Compatibility with your roofing material — some underlays are not suitable for use with certain metal profiles
- Vapour permeability — if your roof space is insulated or has limited ventilation, choose a breathable product
- Durability and warranty — look for products that meet NZ standards and come with a manufacturer's warranty
Your roofing contractor or a supplier like Bunnings or Mitre 10 can help you identify suitable products for your specific roof type and region.
Can You Install Roof Underlay Yourself?
While underlay installation itself is not restricted building work in the legal sense, working at height on a roof is genuinely dangerous. WorkSafe NZ reports that falls from height are one of the leading causes of serious injury and death in construction. Unless you have proper scaffolding, fall-arrest equipment, and experience working on roofs, this is a job best left to a professional.
Additionally, if your re-roofing project involves structural changes or affects weathertightness in a way that requires a building consent, the work may need to be carried out or supervised by a Licensed Building Practitioner (LBP).
When to Call a Professional
You should engage a qualified roofer if:
- You're re-roofing or replacing underlay across a significant area
- You've found water damage or rot in your roof framing
- Your roof pitch is low or the detailing is complex (valleys, skylights, penetrations)
- You're unsure whether your current underlay is compliant or still functional
- You need a building consent for the work
A licensed roofer will know which underlay products suit your roof type, install them correctly, and ensure the finished job meets the NZ Building Code. You can find qualified local roofers on tradedirectory.co.nz.