COP v3.0:roof-ventilation; underlay

10.7 Underlay 

Condensation that forms on the cold under-surface of the roof system must be contained until it can evaporate. This is normally achieved by using an absorbent roofing underlay.

It is a common misconception that the function of a roof underlay is to act as a drainage plane to channel water condensing on the underside of the metal roof to the gutter. Most condensation forms on the underside of the underlay, because while roofing underlays are permeable they still form a substantial vapour check, and as they are in contact with the roof they are at a similar temperature.

Underlay is also affected by holes from roofing fasteners.





10.7.1 Types Of Underlay 

An underlay under metal roof cladding on residential buildings must be Vapour permeable and absorbent.  It may be one of two types:

  • Kraft paper-based, permeable, both bitumen-impregnated or non-bituminous.  These are generally absorbent.
  • Synthetic, permeable complying with the requirements in 10.7.1A Properties of Permeable Roofing Underlays. These may be absorbent or non-absorbent.

Permeable underlays should comply with NZS 2295, Amendment 1:2017 as shown in 10.7.1A Properties of Permeable Roofing Underlays
Minimum requirements for underlays for the metal roofing industry are as follows:

  • NZS 2295 classifies underlays by their use in roofs or walls.
  • Those that are suitable for metal roof and wall cladding are Roofs R1 and R3 (heavy) and R2 and R4 (self-supporting), Walls W2 (heavy) and W4 (synthetic)

10.7.1A Properties of Permeable Roofing Underlays

Classification R1R3R2R4
Grade Heavy weightHeavy weightSelf supportSelf support
Type KraftSyntheticKraftSynthetic
Application  Residential or light commercial buildings  
Absorbencyg/m²≥ 150≥ 150≥ 150≥ 150
Water Vapour ResistanceMN s/g≤ 7≤ 0.5≤ 7≤ 0.5
Water resistancemm head≥ 100≥ 100≥ 100≥ 100
Tensile Strength MDKN/m≥ 9≥ 3≥ 11≥ 3
Tensile Strength CDKN/m≥ 4.5≥ 2≥ 6≥ 2.5
Edge Tear Resistance MDN≥ 40≥ 100≥ 70≥ 150
Edge Tear Resistance CDN≥ 35≥ 80≥ 55≥ 130
Based on Table B1 of NZS 2295 Amendment 1:2017.
  • Self-supporting (S/S) is defined as strong enough to support its own weight up to a 1200mm span.
  • pH between 5.5 and 80.
  • Kraft based underlays shall have shrinkage less than 0.5% and maximum run-length of 10 m.
  • Synthetic underlays may have any length of run.
  • Any underlay is regarded as Fire retardant if it has a FI (Flammability Index) of 5 or less when tested to AS/NZS 1530 Part 2.


10.7.2 Underlay Usage 

Absorbent vapour permeable underlays must be used under profiled metal roofs and direct-fixed metal cladding on lined buildings, non-absorbent permeable underlays can be used on a drained wall cavity.

10.7.3 Underlay Durability 

With normal maintenance, metal cladding can last 50 years or more and the “hard-to-replace” elements beneath it should have the same durability. That also applies to support materials, such as netting, safety mesh or plastic strapping. Netting and mesh are available with plastic coating for use in severe environments.

10.7.4 Installing Underlay Direction of Lay 

Horizontally laid underlay must be supported, while vertically laid underlay can be self-supported if purlin spacings allow.  Roof and wall underlay can generally be laid either horizontally or vertically.  However, on pitches of less than 10°, E2/AS1 requires underlay to be laid horizontally on support. Horizontal underlay must be laid starting at the lowest point of the roof, running over the bottom purlin. Subsequent layers must be lapped over the lower layer to ensure water is shed to the outside face of the underlay.

Underlay for roof cladding must have a minimum side and end lap of 150 mm.

The lower end of underlay may terminate on the outside of the fascia line, or overshoot it by a small margin. Underlay should not overhang the fascia to the extent that it can wick up moisture from the spouting. In most situations, the overhang should not exceed 20 mm.

Flue penetrations must have a minimum distance of 50 mm from the outer liner to any underlay or flammable material.

Rips smaller than 75 mm on walls or roofs can be repaired by using a compatible flashing tape, but roof underlay damage greater than this requires a new piece of underlay captured by the cladding fastenings. Underlay Support 

Self- support underlay can be used unsupported on spans up to 1200 mm at pitches of 10° or more.  At roof pitches less than 10°, any underlay should be supported. Underlay support may be safety mesh, hexagonal galvanised wire netting, or alternative support such as builders strapping tape.

Safety mesh used as a fall arrest must be designed and installed to comply with Safety Mesh Standard AS/NZS 4389:2015.

Corroded galvanised safety mesh and wire netting can be damaging to any metal roofing and especially to pre-painted aluminium. Pre-painted aluminium cladding must be protected from contact with potentially corroding steel including netting, staples or fasteners.

10.7.5 Non-Residential Buildings 

There is no requirement for underlay under the roof of an unlined building, underlay is normally used in internal situations.

The main reason why aluminium foil underlay is installed in commercial and industrial buildings under roof cladding is to maintain surface reflectance, limit heat emission, and enhance lighting conditions. These may be non-permeable, or permeable.

Permeable options normally use the same underlays as those for residential applications. Non-permeable is often reflective foil, either double-sided or white-faced.

Foil can be double-sided aluminium or one side may be white polythene faced.  Some types may have an anti-glare coating on one side. White polythene faced foils are often used in public buildings with an exposed roof structure to enhance the appearance from below.

They may also be lightweight, standard weight or heavyweight (light, medium or heavy duty under AS/NZS 4200.1) and may or may not be fire-retardant (having a Flammability Index of 5 or less when tested to AS/NZS 1530.2).

Although foil itself is not permeable, a foil underlay will not function as a vapour barrier unless all joints, penetrations, and intersections are fully sealed.
Non-permeable underlays should comply with AS/NZS 4200.1:2017.


10.7.5A Properties of foil (non-permeable) roof and wall underlays

Classification F1F2F3F4
Grade/Duty - Industrial buildings Extra lightLightMedium heavyExtra Heavy
Self Support
Type Double SidedDouble SidedDouble SidedDouble Sided
Application WallsWall or roofRoofRoof
Water Vapour ResistanceMN s/g> 450> 450> 450> 450
Water resistancemm head≥ 100≥ 100≥ 100≥ 100
Tensile Strength MDKN/m≥ 6≥ 7.5≥ 9.5≥ 12.5
Tensile Strength CDKN/m≥ 3.5≥ 4.5≥ 6≥ 7.5
Edge Tear Resistance MDN≥ 30≥ 35≥ 65≥ 80
Edge Tear Resistance CDN≥ 30≥ 35≥ 65≥ 80
Based on  NZS/AS 4200.1:2017
  • An underlay is regarded as Fire Retardant if it has a FI (Flammability Index) of 5 or less when tested to AS/NZS 1530 Part 2.
  • Self-supporting (S/S) is defined as strong enough to support its own weight up to a 1200mm span.
  • pH between 5.5 and 8.0




10.7.6 Special Requirements 

Buildings of the following types using metal cladding should have a sealed vapour barrier and are outside the scope of this Code of Practice. They are subject to specific design.

  • Swimming pools.
  • Buildings containing liquids stored in open containers.
  • Buildings where water is used in manufacturing, cleaning or storage processes.
  • Ice rinks, cold stores and freezers.
  • Buildings where unvented gas heating is used .

A vapour barrier incorrectly located on the cold side of a structure or poorly installed can result in a rapid build-up of cavity or interstitial condensation and result in high costs for rectifying defects and damage.

Cold stores, freezers or buildings where the temperature is near or below 0° all require a fully sealed vapour barrier located on the warmer outer side of the insulation but are subject to specific design. See 15.5 Insulated Panels.