2.0 Design Considerations

This section contains general information you need to keep in mind when designing buildings with Gerard Roofs.

The numbered sections (2.1; 2.2; etc.) correspond to the equivalent parts of the Gerard Roofs Specification Manual. All details and diagrams can be downloaded and added to your plans.

2.1 Scope

This specification covers the supply and fixing of:

  • Materials
  • General information
  • Limitations of use
  • Service of life
  • Workmanship - pre-roofing requirements
  • Timber battens.
  • Roofing underlay
  • Valleys
  • Gerard Roofs - pressed metal tiles/panels
  • Trim, standard, barrel and angle
  • Wall flashings
  • Fixings/fasteners
  • Incompatible materials
  • Extra high wind zone variations
  • Solar panels
2.2 Building Code

The following standard specifications should be adhered to in both materials and workmanship.

NZS 2295 Roofing underlays
NZS 4217 Standard for pressed metal tile roofs
NZS 3604:2011 Code of practice for light timber framed buildings, not requiring specific design
AS/NZS 4680 Hot-dip galvanized (zinc) coatings on fabricated ferrous articles - galvanising of nails
E2/AS1 New Zealand Building Code clause E2 External moisture
AS1397:2011 AS 1397-2011 continuous hot-dip metallic coated steel sheet and strip - coatings of zinc and zinc alloyed with aluminium and magnesium.
AS/NZS 1530 pt. 3:1999. Fire standard

NZ Building Code sections B1,B2,E2,G12

2.3 Serviceable Life

Durability (B2)

Without recoating, the Gerard Roofs products will meet the provisions of NZBC B2.3(C) for 15 years, when used and maintained.

Structure (B1)

When installed according to the manufacturers recommendations the Gerard Roofs products will allow provisions of NZBC B1.3.1, B1.3.2 and B1.3.4 to be met for the relevant provisions of B1.3.3.

External moisture (E2)

When correctly used and installed, the Gerard products meet the requirements of the NZ Building Code E2/AS1

Live loads

During installation and subsequent construction process or maintenance, walking load must be positioned to the front of the panel where the panel is supported by a batten.

Wind zones

On new roofs when fixed to battens sized and installed according to NZS 3604:2011 Gerard Roofs systems are suitable for use in all NZS 3604:2011 building wind zones up to and including extra high.

Spread of fire (C3)

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Water supplies (G12)

Rainwater collected from the Gerard Roofs systems will meet the provisions of NZBC G12.3.1, providing the surface coating is maintained as directed. It is recommended that a potable water collection system is not installed until 3 good rainfalls have washed the new roof surface free of building debris and surface coating surfactants.


The Gerard Roofs products are suitable for use where
buildings are designed for a 1kPa snow loading.

2.4 Other information sources

NZ MRM Code of Practice

2.5 Alternatives

No alternatives or substitutions permitted.

2.6 Materials

2.6.1 Tile and tile accessory materials

The roofing panels and accessories are made by Gerard Roofs.

The base material is aluminium-zinc coated steel - 0.39 mm, AZ150 of AS1397:2011.

Panels may be finished in either a granulated textured or satin paint finish.


An actual product sample should be sighted to confirm colour choice before material orders are placed/accepted.

2.6.2 Installation materials

  1. Roofing underlay shall be either a bitumen impregnated Kraft or synthetic underlay. Kraft underlays are covered by NZS2295 while synthetic roofing underlays are approved by Acceptable Solution documentation from the suppliers.
  2. Rough sawn radiata pine type meeting of suitable dimensions (table 02) with H1.2 protection.
  3. Ridge, hip, gable and wall flashings shall be supplied as standard products from Gerard Roofs.
  4. Valleys may be used as supplied pre-finished in 2.4 metre lengths by Gerard Roofs or made to order out of pre-painted 0.55 mm G300 AZ150 aluminium coated steel in a matching colour from a sheet metal fabricator. If made to order, valleys should have similar dimensions to those supplied by Gerard Roofs.
  5. Valleys must be painted to the same colour as the roof panel if not pre-finished.
  6. Hip end caps to be installed where necessary to stop ingress of moisture or bird-life.
  7. All tile nails shall be 50 mm galvanised and coated to meet AS/NZS 4680 specification (Zn 390 g/m²). Galvanised nails are used for ridge hip caps 30 mm, all other trim or flashings 50 mm.

2.7 General Information
ProfileMinimum Pitch
Heritage 12°
Classic 12°
Stratos 15°
Milano 12°
Alpine 15°
Minimum Roof Pitches For Gerard Roof Profiles

For full specifications - refer to individual panel profile data on the Products & specs page for dimensions, coverage and weights.

2.8 Limitations on use

2.8.1 Corrosive environments

The Gerard system must not be used in high alkaline environments (such as intensive animal shelters or other buildings with high ammonia concentrations); contact with fresh concrete or plaster must be avoided. The roofing must not be used in contact with permanently damp materials, nor on or near industrial buildings in which corrosive processes occur, nor over swimming pools.

2.8.2 Dissimilar metals/materials

When two different metals are in contact and moisture is present, one metal is relatively protected while the other suffers accelerated corrosion. A similar effect often occurs with water flowing over different metals.

This form of corrosion is commonly found:

  • Where water is discharged from copper or brass systems over a galvanised or aluminium-zinc coated steel roof.
  • Where unpainted lead flashings are applied directly to aluminium-zinc coated steel products.
  • Where fasteners are incompatible with the roofing material.
  • Any electrical contact between dissimilar metals should be avoided, e.g. wet leaves making contact between copper gutters and aluminium-zinc coated steel roof tiles.

Note: Special consideration should be given when replacing existing roofs as some roof claddings use unsuitable materials for flashing such as lead.