Dimond Underlay


Suitable for residential

Suitable for light commercial

Suitable for commercial

Underlays is manufactured in Whangarei, Auckland, Hamilton, Rotorua, New Plymouth, Wellington, Christchurch and Dunedin.

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The roof is the first line of defence against the elements, and the underlay is the second.  Underlays play an important role in managing moisture.  Dimond offer a range of underlays to suit your roof build up.

Bitumen Coated Building Paper

Black bitumen saturated building paper is a breather type kraft building paper intended for use within the roofs and wall cladding of a building where the following are required:

  • moisture permeability
  • reduced air flow
  • absorbency
  • non fire retardant

Features and Benefits

Black building paper reduces wind entry into the cavity, thereby assisting the performance of insulating materials and provides a temporary protection against wind, dust, rain, and other weathering elements until the external cladding is applied. Black building paper is not designed for use in extreme weathering conditions. Black building paper is permeable to water vapour thereby allowing any excess water vapour, which might otherwise condense in the structure, to escape. Black building paper is not considered to be hazardous under the New Zealand Health Safety and Employment Act 1992.


Dimond Black Building Paper is available in three grades for different applications –



Bitumac 710

As an extra light duty wall wrap (W1), fixed onto framing at 450mm centres. Framing at centres greater than 450mm require 710 to be installed with polypropylene tape at 300mm centres horizontally over the framing once the wrap has been installed. Recommended in medium wind zones.

Thermakraft 213

As a heavy weight building paper used as a wall wrap or roofing underlay (R1) supported on netting. Recommended in very high wind areas. 

Bitumac 750

As a heavy duty roofing underlay (R2), self supported up to a purlin spacing of 1200mm, otherwise supported on mesh or strapping at 300mm centres. Recommended in very high wind areas.

Duty grade of underlay in accordance with NZS4200.1.

10m maximum run lengths are recommended to minimise shrinkage.

Surewrap Roof Underlay

Sisilation® Surewrap® Roof underlay manufactured by Tasman Insulation is a synthetic self support fire retardant, vapour permeable underlay designed for use under profiled metal roofs.

Features and Benefits

·         Self-supporting underlay – can be installed without support for spans up to 1200mm

·         Fire Retardant – will not support combustion

·         Synthetic – may be exposed to rainfall during instillation without affecting future durability or performance

·         Vapour permeable – allowing vapour to pass through the membrane minimising the build-up of condensation

·         No shrinkage – allowing vapour to pass through the membrane minimising the build-up of condensation

·         No Shrinkage – allowing no maximum run length on installation

·         BRANZ appraised – meets New Zealand Building Code requirements.

Surewrap Wall Underlay

Sisilation® Surewrap®  Wall  underlay manufactured by Tasman Insulation is flexible, fire retardant, absorbent and breathable wall underlay. It can be used on timber or steel framed buildings with absorbent or non-absorbent wall cladding either direct fixed or over drained cavity. IT can also be used as an air barrier to non lined walls or over a rigid air barrier as a flexible wall underlay.

Features and Benefits

·         Fire retardant – will not support combustion

·         BRANZ appraised – meets New Zealand Building Code requirements

·         Suitable for absorbent and non-absorbent wall claddings

·         Excellent UV resistance – can be left unclad for up to 42 days

·         Light colour – allows working on the interior and makes fixing easier

Galvanised Wire Mesh

Galvanised Wire Mesh or Netting is used to support roofing underlay. Mesh is recommended in an appropriate grade to qualify for use as Safety Mesh.

Bayonet Galvanised Wire Netting

Bayonet Galvanised Wire Netting consists of a range of hexagonal netting made from imported standard galvanised mild steel wire which conforms to NZS/AS 1650:1989. The wire tensile strength is between 380 and 550 MPa and its galvanising weight is between 24 and 31 g/m2.
The product range consists of two sizes of wire netting for use as building paper support.

Mesh Size (mm) Wire Diameter (mm) Roll Width (mm) Roll Length (mm) Roll Area (m²)
75 1.0 2000 50 100
50 1.0 2000 50 100
- - 900 50 45

Handling and Storage

Bayonet Galvanised Wire Netting must be handled with care to prevent damage to the netting.
The rolls must be stored on end, under cover and protected from moisture. They must not be double stacked or used to support other materials. Bayonet Galvanised Wire Netting rolls should not be stored on concrete floors for long periods, particularly where moisture is present, as this can result in an accelerated corrosion of the galvanising.

Mesh Size Selection

Bayonet Galvanised 50mm and 75mm Wire Netting is suitable for supporting building paper or roofing underlay when the wire netting is supported by timber or steel roof or wall framing. 50mm mesh size must be used where the Wind Zone is very high, or where heavy weight underlay is to be used, e.g. in all non residential roofing systems.

The 50mm mesh size must be used for buildings not covered by NZS 3604, and is recommended when:

  • the paper is to be exposed for more than two days and up to one week.
  • there is a high risk that ongoing construction work may cause damage.
  • the building is in a high rainfall region (>1400mm/year)

Netting used on spans greater than 2m, or in windspeeds greater than 50 m/s, is outside the limitations published in BRANZ Appraisal No 318A (1997).

Serviceable Life

Bayonet Galvanised Wire Netting, when used to support building paper or roof underlay in roof spaces or wall cavities, will have a serviceable life in excess of 50 years when:

  • enclosed with the roof space or wall cavity
  • subject to a dry interior environment with relative humidity not greater than 90%
  • not exposed to a corrosive environment
  • not exposed to the atmosphere for more than one week before cladding is installed
  • air extraction or dehumidifying devices are not vented into the roof space or wall cavity
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