Roofing

The roof slope for metal roofing (not including external covered ways) is to be 4° minimum to avoid drainage failure through:

• ponding in the trays and flow restrictions due to sagging of the roof deck purlins, that can be caused by roof loadings mainly during construction
• overflow at side laps during heavy rain when the trough capacity is exceeded at the base of lower pitched roof slopes

Suggested maximum purlin spacing is to suit roof sheeting and wire safety mesh to support insulation. Maximum spacing shall not exceed 2/3 of the manufacturer’s recommended spacing for the roof sheeting to be used.

Include support for blanket insulation and safety mesh as required  for safe working on roofs.

Purlin / batten fixing:

• Must be securely fixed to rafters/trusses and they in turn to columns or top plates strapped to walls, etc., so that all forces acting on the roof are transmitted through the structure to the foundations. 
• Special care is required in exposed positions or areas known to be subjected to high winds.
• Double fixing may be required and should always be strapped securely to the sub frame at maximum points of exposure - ridge, eaves and gable ends. 

Refer to DG / ROOF FIXING

The minimum size for hardwood battens to be 70mm wide x 35mm finished deep to ensure adequate holding for fastenings without splitting the timber. The minimum depth for softwood battens is 45mm finished.

Set out purlins/battens accurately to the desired pitch. 

• Maximum deviation permissible is 5mm over any 1200mm span below the decking line (line between eaves and ridge). 
• Maximum vertical tolerance 2mm at abutting ends of purlins.  Specifications should provide for the plane of the purlins to be checked in the presence of the Supervising Officer before any roof sheets are fixed.
• Maximum permissible deflection: Refer to DG / STRUCTURE /  DEFLECTIONS section for requirements

DG / STRUCTURE / STRUCTURAL DEFLECTIONS

Roofing in the coastal strip north of Coffs Harbour or other areas with similar wind conditions (Region B as per AS for wind loading) should be fixed in accordance with manufacturers recommendations for extreme conditions.

Roof guttering should be outside the line of the external walls so that any overflow will be outside the envelope of the external walls.  Concealed gutters behind parapet walls, or box gutters between roof slopes should be avoided.

All roofing must be of continuous sheets wherever possible, complying with the minimum pitch requirements noted in roofing.

This need not be a design restriction, as many opportunities are apparent in the visual expression of roof drainage and climatic control systems, pertinent to our environment.

Refer DG / STORMWATER

Refer DG / WATER / WATER TANKS

GUTTERS

• Detail eaves gutters to avoid blockage back-flows into buildings. Ensure outer gutter edge is lower than the inner edge.
• Valley gutters should be designed to BCA requirements and AS to suit  local environmental conditions
• Allow for expansion and contraction of all rainwater goods.
• When a higher roof drains to a lower roof, detail and locate spreaders to avoid concentration of water - especially avoid discharging water over ribs of pan and rib decks.
• Provide metal leaf guards to all gutters.

General

The performance of metal roofing and walling is affected by:

• Atmospheric exposure conditions
• Service conditions due to the nature of nearby activities taking place in the installation
• Contact with other materials or contact with water washing off other materials

Metals are vulnerable to attack from certain gases or particles found in the surrounding atmosphere including salts, industrial fallout and acids. 

Galvanic corrosion between metals occurs when moisture is present.

The manufacturers of metal roofing and rainwater accessories give advice on the suitability of their products to various exposure conditions and corrosive elements.  Follow a conservative Whole of Life approach in the selection of roofing materials.

In severe corrosive situations. It may be appropriate to consider applying protective coatings.  Steel decks can be coated with paint and/or resins (as can aluminium), PVC or a build up of materials including bitumen.

Electro-Chemical Reactions

• Avoid the use of galvanically incompatible materials in contact with each other or where moisture can carry charged particles of one metal onto a more active metal, particularly in the roof and roof penetration design. 
• If incompatible materials cannot be avoided, an appropriate material should separate them, e.g. bituminous, zinc or rubber paints, plastic sheet or tapes, or bituminous membrane.
• Where air-borne pollutants are a problem, detail to minimise the use of metal externally, e.g. don’t expose the steel roof structure.
• Rainwater must never run off from copper, brass or copper alloy onto aluminium, galvanised iron or Zincalume surfaces.
• Overflows from copper lined hot water tanks should not be discharged onto the above metals. 
• Lead must never be used with Zincalume (use zinc or soft aluminium) and where condensation on the underside of the roof sheeting is likely to occur steel purlins should be zinc coated.  Lead must not be used with aluminium.
• Aluminium:  Under normal conditions aluminium can be fixed directly to zinc coated steel members. In extreme conditions such as in industrial or coastal environments, the aluminium should be separated from the steel and suitable fasteners used.  Only aluminium flashings should be used.
• Contact with materials such as hardwood or chemically treated timber, lime, cement, concrete or mortar should always be avoided by using a bituminous separator strip or painting metal surfaces to be in contact with two coats of bituminous paint.

Roof design should take into consideration the lengths of roofing material that are available, the impact of potential expansion due to weather extremes and the maximum length of material that can be delivered on a vehicle.

It should be noted that the police department in the past have placed a restriction of 21.34m on the length of transported materials.   

Roof sheeting should be installed in single lengths.

Where longer spans are required, use site rolling if available or end lap the sections of sheets and increase the roof pitch to prevent leaking. 

Note that long sheets are heavy, needing special handling equipment, increased manpower, can be easily damaged and subject to thermal expansion problems. The cost of transport, crane hire and increased labour could mean cheaper roof installation, in two or more lengths.