Underlays

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.

Applications

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

Synthetic Fire Retardant Underlays

Synthetic Fire Retardant self supporting underlays are designed for use under roofing and cladding.

The synthetic product consists of a micro –porous water-resistant film laminated between two layers of non woven spun bonded polyolefin where the following is required:

• Fire retardant
• Moisture permeability
• Air barrier
• Water barrier

Features and Benefits

The fire retardant properties of these underlays do not support combustion from Fire both during and after construction. Being synthetic they may be exposed to rainfall during installation without affecting future durability or performance.

Being Vapour permeable the underlay allows moisture to pass through minimising the build up of condensation within the roof space and allows it to escape to the outside.

Self supporting on spans up to 1200mm, its tough to tear, so offers best resistance when exposed in all wind zones including up to extra high. Can be left exposed to weather for 7 days.

Shrinkage is minimal so there is no restriction on maximum run length.

It’s not considered to be hazardous under the New Zealand Health Safety and Employment Act 1992.

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.

Mesh

Refer Section 2.4.5 Safety for information on Aus 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.

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 roofing 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 wind speeds 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

Netting

Netting should be run across purlins and be tensioned to remove unnecessary sag. Fastening to timber should be with either galvanised staples or 25mm clouts avoiding contact with the roofing, and to steel with flat head screws. Fixings should be at 150mm centres on end purlins in such a way that the netting cannot pull past the fixing. Edges of the netting should be tied together or twitched at 300mm centres and fixed to each purlin. Safety mesh should be installed to manufacturers recommendations.

Roofing Underlay

Horizontal Application: underlay is unrolled across the roof parallel with purlins and secured as necessary. Joins should be lapped by a minimum of 75mm and supported on netting if roof pitch is below 8 degrees for self supporting and have the side edges supported on purlins.

Vertical Application: underlay is unrolled vertically up the slope of the roof from guttering to ridge and secured to the purlins as necessary before laying the roof sheet and fixing down. Joins should be lapped by a minimum of 150mm.
Support such as netting or safety mesh must be used on pitches below 8 degrees, or when using self supporting underlays on purlin spacings greater than 1200 mm.

When used under roofing, all underlays must be supported on wire netting or strapping at 300mm maximum spacings.

Self sufficient underlays can be used on purlins spacing up to 1200mm without support.

Underlay should overlay into the gutter at least 20mm and not more than 50mm, and avoid lapping into the water flow.

Maximum single underlay sheet lengths shall be 10m for bituminous and fire retarded kraft papers. Longer runs are to be end lapped 150mm. Synthetic underlays have no limit on their run length.

In general it is recommended that prolonged exposure of the underlay to the weather is avoided by fixing the roofing over the same day. Always follow underlay manufacturers recommendations.

Wall Cladding Underlay

• Can be laid either horizontally on steel girts or vertically on timber studs.
  
• When run horizontally lap upper sheet over lower sheet a minimum of 75mm. Adequately secure to framing at 300mm centres. When installed in high wind areas fix through a reinforced tape such as Danband branded polypropylene tape.
• Run lengths to be no greater than 10m.
• End laps to be no less than 150mm over studs of vertical joints.
• The underlay should be pulled taut as possible.
• Best practice is to clad on the same day as installation provided the product is kept dry and undamaged.
• Maximum period that the underlays are exposed to the weather is contained in the underlay manufacturers literature.