What Is Roof Underlay and Why Does Your Metal Roof Need It?

Roof underlay is a thin, water-resistant membrane that sits between your metal roof sheets and the framing beneath. In Auckland, it serves one critical job: stopping condensation, wind-driven rain, and moisture from reaching your ceiling and timber structure. Without it, metal roofs in our climate trap warm, damp air against cold steel overnight, and that moisture has nowhere to go but down into your home.

At HDK Long Run Roofing, we’ve installed roof underlay on hundreds of Auckland properties over 10+ years. Every project is backed by a 5+ year workmanship guarantee and up to 50 year material warranty from our supplier partners, including Colorsteel and Eurostyle. As Licensed Building Practitioners, we ensure every underlay installation meets NZ Building Code clause E2 requirements for your specific wind zone and roof pitch.

This guide covers what we recommend, what we avoid, and how to match the right roof underlay to your Auckland property.

What Does Roof Underlay Actually Do?

Roof underlay forms a secondary weather and moisture-management layer beneath the external roof cladding. It does not replace correctly installed metal sheets, flashings or drainage. Instead, it helps manage condensation and incidental moisture that reaches the space beneath the roof sheets.

Its practical functions can include:

• helping direct incidental moisture towards the eaves
• reducing uncontrolled air movement through the roof assembly
• absorbing or managing droplets that form beneath cold metal
• supporting vapour movement when a vapour-permeable product is specified
• providing limited temporary weather protection during construction
• separating compatible roof components where the approved system requires it

The exact performance depends on the product classification and installation method. Reflective foil, absorbent synthetic membranes and bitumen-based papers do not behave in the same way.

A roofing team should select the membrane for the full roof system. Appearance, roll size or price alone does not provide enough information.

Why Is Roof Underlay Important Beneath Long Run Metal Roofing?

Metal roofing changes temperature quickly, so the layer beneath it plays an important role in moisture management. When warm, moisture-laden air reaches a cold metal surface, the air may cool below its dew point and form condensation.

Correctly specified roof underlay helps manage this moisture before it reaches insulation, ceiling linings, timber framing, steel framing or stored equipment. It can also help prevent small amounts of wind-driven moisture from travelling directly into the occupied space.

Underlay supports three practical outcomes:

Underlay cannot compensate for poor roof detailing. Valleys, ridges, penetrations, barges, aprons and flashings still require correct design and installation.

Is Roof Underlay Required Under New Zealand Building Requirements?

Underlay requirements depend on the building type, roof construction and selected New Zealand Building Code compliance pathway. For buildings that fall within its scope, E2/AS1 provides an Acceptable Solution for meeting parts of Clause E2 External Moisture.

The current E2/AS1 Fourth Edition took effect on 28 July 2025. It applies within defined scope limitations, including limits relating to building height, floor area, structural system and cladding type.

NZS 3604:2011 may apply to certain timber-framed buildings. NZS 2295:2006, including Amendment 1:2017, sets performance requirements for permeable roofing underlays. A CodeMark certificate, BRANZ Appraisal or other recognised evidence may also establish that a particular product is suitable for its stated use.

A compliant specification should consider:

• the membrane’s duty classification
• resistance to water penetration
• vapour permeance
• surface water absorbency
• tensile and tear strength
• compatibility with adjoining materials
• support spacing
• laps and fixings
• penetrations and repair methods
• permitted exposure before roof cladding installation

The complete roof build-up is important. Climate zone, insulation placement, ceiling sealing, ventilation and building use can all influence the correct approach.

A coastal home, a high-humidity commercial kitchen, and a dry storage building will not all require the same underlay performance.

What Is Self Supporting Roof Underlay?

Self-supporting roof underlay is a membrane tested and classified to support its own weight across an approved unsupported span. In New Zealand roofing guidance, self-supporting products may generally span up to 1,200 mm, but only where the product documentation and installation arrangement permit it.

This type of membrane is often useful beneath long run metal roofing because it can span across rafters or purlins. However, “self supporting” does not mean unlimited span capability.

It may be installed without continuous support in approved applications, although it still requires correct fixing, laps, tension and support at edges or junctions.

The phrase “self-supporting” does not mean that the material can cross any framing distance or be installed in any direction.

Before specifying self supporting roof underlay, check:

1. The maximum permitted unsupported span.
2. Whether it will be laid horizontally or vertically.
3. The roof pitch and metal profile.
4. The purlin or rafter centres.
5. The site wind zone or specific design pressure.
6. Whether the roof space is lined or unlined.
7. The lap, fastening and edge-support requirements.
8. The approved repair method.
9. The maximum exposure period before roofing is installed.

Horizontal installation beneath long run metal roofing may require the membrane edges or laps to be supported by purlins or another approved support system. The roofing contractor must therefore check both the product literature and the selected installation method.

Which Roof Underlay Suits Different New Zealand Properties?

The correct membrane depends on moisture levels, roof geometry, framing, occupancy and the consequences of failure. No single product automatically suits every residential, commercial or public building.

Residential Properties

Homes produce moisture through cooking, bathing, laundry and everyday occupancy. For lined residential buildings with profiled metal roofing, permeable and absorbent underlay is generally required under the recognised industry guidance.

Key considerations include:

• insulation placement
• ceiling airtightness
• bathroom and kitchen extraction
• roof-space ventilation where specified
• Auckland’s local exposure conditions
• the household’s heating and moisture patterns

During reroofing, the existing membrane should be inspected rather than automatically retained. Replacing worn roof cladding while leaving brittle or damaged underlay in place may shorten the effective life of the renewed assembly.

Commercial Properties

Commercial roofs may cover warehouses, offices, workshops or retail spaces. Each use creates different internal conditions.

The specification should consider:

• internal humidity levels
• purlin spacing
• roof access requirements
• service penetrations
• fire and compliance requirements
• acoustic expectations
• future solar installation
• equipment or stock below

Condensation above sensitive stock, electrical equipment or occupied workspaces may cause a greater operational risk than condensation in an open, unlined shed.

Body Corporate Properties

Multi-unit and body corporate roofing work can affect individual units as well as common property. The scope should clearly state whether the existing membrane will be retained, repaired or replaced.

Project records should include:

• manufacturer and product code
• installation date
• technical documentation
• appraisal or certification details
• warranty information
• photographs taken before cladding installation
• details around shared vents and penetrations

Clear documentation helps future maintenance planning and reduces uncertainty.

Public Properties

Schools, community facilities and government buildings often require long service life, clear compliance records and reliable moisture management.

The membrane should support:

• the consented design
• documented product compliance
• practical future maintenance
• appropriate moisture management
• compatibility with the full roof system

A cheaper membrane may provide little value if premature failure disrupts classes, public services or community activities.

How Does Thermakraft Roof Underlay Fit Into a Specification?

Thermakraft roof underlay refers to several products rather than one universal membrane. Products within the range differ in self-support capability, fire performance, strength, vapour properties and permitted applications.

When assessing Thermakraft roof underlay, confirm:

• the full product name and code
• whether it is self-supporting or requires support
• approved roof-cladding types
• applicable wind-zone limitations
• fire-retardant classification, where required
• unsupported span limits
• lap and fastening requirements
• framing compatibility
• exposure limitations
• repair procedures

For example, Thermakraft Covertek 401 and Covertek 407 have been appraised for specified self-supporting roof applications. Covertek 403 is appraised as a non-self-supporting membrane and requires suitable support.

The brand name alone does not confirm suitability. The exact product must match the building, roof design and current technical documentation.

How Should Roof Underlay Be Installed?

Roof underlay should be installed in accordance with the consented design, applicable compliance pathway and current manufacturer instructions. Correct installation maintains drainage paths and helps the membrane perform as intended.

Step 1: Check the specification

Confirm product code, roof pitch, framing centres and wind exposure.

Step 2: Prepare the framing

Remove sharp edges, debris and anything that could damage the membrane.

Step 3: Set the direction

Install in the correct orientation based on roof design and lap requirements.

Step 4: Apply correct tension

Keep the membrane firm without overstretching or creating sagging pockets.

Step 5: Form laps correctly

Ensure laps shed water towards the eaves.

Step 6: Secure the membrane

Use approved fixings and ensure edges are protected from wind uplift.

Step 7: Detail penetrations

Seal around pipes, vents and openings using compatible methods.

Step 8: Repair damage

Fix any tears or punctures before roof cladding is installed.

Step 9: Respect exposure limits

Do not leave the membrane exposed beyond its rated period.

Step 10: Keep records

Retain product data, photos and installation details for future reference.

What Roof Underlay Installation Mistakes Reduce Performance?

Incorrect orientation, unsupported laps, excessive sag and unrepaired damage are common causes of reduced underlay performance.

Other issues include:

• using a product outside its approved application
• exceeding the tested unsupported span
• blocking drainage near the eaves
• using incompatible tapes or sealants
• leaving edges unsecured in windy conditions
• creating untidy openings around penetrations
• exceeding the permitted exposure period
• retaining degraded material during reroofing
• failing to follow the consented roof detail

Even self-supporting roof underlay must remain within its documented limits. A Thermakraft roof underlay product must be installed according to the technical requirements for that exact product.

How Can You Choose the Right Roof Underlay?

Selection should be based on real site conditions rather than appearance or cost alone.

When Should Roof Underlay Be Replaced During Reroofing?

Replacement is recommended when the membrane is:

• aged or brittle
• torn or damaged
• poorly supported
• incorrectly lapped
• contaminated or degraded
• incompatible with new roofing

Once new metal roofing is installed, the underlay is no longer accessible, so inspection before reroofing is essential. A proper roof underlay NZ reroofing plan should document the condition of the existing membrane and confirm whether replacement is required.

Does Roof Underlay Provide Thermal Insulation?

Roof underlay is not a replacement for bulk thermal insulation. Depending on its classification, it may assist with water, air, vapour or radiant-heat management, but insulation performs a different role.

A reflective membrane may contribute to heat management when installed with the correct air gap, but it does not replace insulation.

How Can HDK Long Run Roofing Help With Roof Underlay?

For a roof that manages hidden moisture as carefully as it sheds rain, speak with HDK Long Run Roofing. The team can assess your roof underlay, explain suitable roof underlay options in NZ, check whether self supporting roof underlay is appropriate for your framing, and review the correct thermakraft roof underlay product. You get a clearer specification, better installation outcomes and a long run metal roof designed for New Zealand conditions.