DG27

Roofing

This is a general description of requirements for metal roofing of all types.

Note particularly the minimum pitch requirements and guide to roof drainage. Also note requirement for standard coloured sheeting and problems with regard to flashing, capping, etc.

All roofing is to be designed and installed in accordance with the manufacturer’s recommendations, building regulations and associated Australian Standards.

Roof materials and finishes must be selected based on a “Whole of Life” approach, Refer to DG / FABRIC

GUIDE NOTE: Refer to Design Guide sections DG/ FABRIC, DG/ ROOFING, DG/ ROOF MOUNTED TURBO VENTS

Title

Compliance Level

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27.01

Minimum Pitch Requirements

Mandatory

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

27.02

Roof Design And Access For Maintenance

Mandatory

Provide access for the maintenance of roof surfaces, gutters and down pipes on buildings of 2 storeys or more, within the design of the building. The method of maintenance is to be identified for review during design and subsequent documentation, to ensure the provision and proper co-ordination of maintenance requirements.

Subject to site specific WHS considerations, portable ladders may be suitable for use on 1 storey buildings.

Safety measures for servicing of roof-top plant and equipment, including ladders, ladder fixing points, walkways and handrails around roof zones, must be indicated at design phase. The design is to meet the WHS safe access requirements.

27.03

Substructure-Purlins

Mandatory

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

27.04

Roof Fixing

TBC

Negative pressure (or suction), a critical effect of wind on roofs, increases as the roof slope decreases, and low pitch roofs can incur very severe negative pressure problems
Eaves projections are particularly vulnerable with both the suction effect on the upper surface and positive pressure below
Roof fixing: As per Manufacturer’s recommendations for the Site Wind category

27.05

North Coast Wind Loads

Mandatory

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.

27.06

Flashing And Accessories

Mandatory

All copings, ridging pieces and similar are to be securely fixed and adequately supported to hold their shape.
Detail complex penetrations of the roofing by pipes or ducts where unavoidable.
Upstand of apron flashings to be 150mm minimum.
Overflashings to lap 75mm.
Slope flashings away from the upstand. Project side flashings over two ribs and lap flashing joints in the direction away from the prevailing wind.

Provide for expansion and contraction at all flashings.

27.07

Roof Drainage

Mandatory

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.

DOWNPIPES

Downpipe size guide: 100 mm2 per downpipe based on AS.
Downpipe area of not less than half the area of the gutter and min 75mm dia. or 75mm deep for rectangular sections.
Acceptable materials are stainless steel, galvanised water pipe, cast iron, UPVC, aluminium, zincalume, and copper. Unless the building is heritage listed, it is advisable to choose a material with low maintenance and ultimate longevity.
Protection for light gauge material is required below 2100mm with a guard of minimum 1.6mm thickness, independently fixed or change the downpipe to heavy gauge material.
Fix downpipes hard against wall or column where possible. Where downpipes are supported by cleats off columns or walls, the gap between downpipe and column or wall must be sized to avoid creating the potential for entrapment.
Refer to DG / SAFETY (ACCIDENT AVOIDANCE ) & GENERAL SECURITY / DOWNPIPES ATTACHED TO COLUMNS
Fix Astragals of similar material at maximum 1800mm centres to downpipe as well as support.
Locate downpipes if possible away from main circulation, driveways etc. They are not to be recessed into brickwork, or fully enclosed.
Downpipes should not connect directly into stormwater pipes but have bottom splayed 30 DEG with lowest edge closest to the supporting column or wall and finishing above the pipe collar for cleaning access. (Except for charged downpipes to rainwater tanks.)
Refer to Standard Detail - Typical Downpipe Sump
Finish collars 100mm above ground and min. 100mm below floor level, and fitted with a grate which cannot be removed by unauthorised persons.
Protect collars from mechanical damage and avoid possible splashing or flooding surrounding areas, eg by providing 300mm diameter dish, min 50mm deep, around grate.

27.08

Condensation, Heat Transfer And Noise

Negotiable

Condensation is caused by comparatively warm humid air coming in contact with the colder surface of the metal decking. Unless controlled, condensation can damage ceilings and insulation, and set up conditions conducive to corrosion.

A vapour barrier will prevent warm moist air coming in contact with the cold metal surface thereby preventing condensation. In areas where substantial moist warm air is generated, the vapour barrier will need to be sealed at all joints, wall junctions and ceiling penetrations.

Insulation is included in building walls, floors and ceilings to:

Reduce heat transfer through the building envelope
Reduce sound transfer through the building envelope

There are varying forms of insulation which will perform differently, but generally:

Thermal insulation is measured according to its Thermal resistance (R Value)
Acoustic insulation is measured according to its sound resistance

One layer of double-sided foil laid under bulk insulation such as polyester or mineral wool with a 1.5 R-value, is to be pressed against the underside of the roof sheeting by means of wire stretched tightly over the purlins. This provides an initial level of thermal insulation and can reduce sound levels from rain and hail by damping vibration.

Additional bulk insulation is to be placed above the ceiling in order to reduce heat loss or gain in the spaces below and meet BCA insulation requirements.

Refer BCA Section J – ENERGY EFFICIENCY- Table J1.3 ROOFS & CEILINGS

Refer SG / THERMAL INSULATION AND PLIABLE MEMBRANES

Foil should be double-sided, reinforced, anti-glare and fire retardant type.

The vapour barrier must always be on the warm side of any insulation. For air-conditioned buildings in hot areas (particularly hot humid climates) the vapour barrier is placed on the outside of the structure.

27.09

Corrosion

Mandatory

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.

27.10

Thermal Movement

Mandatory

Many metals will expand and contract in extreme heat and cold conditions. Metal roof decks, flashings, ridge cappings and similar are to be designed to allow for thermal expansion.

27.11

Length Of Roof Sections

Negotiable

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.

27.12

Coloured Roof Sheeting

Mandatory

Select colours from the manufacturer's standard colour range. This should enable the use of matching colour flashings and cappings as well as reducing the ordering lead time.

The colour selected will have an impact on the thermal performance of the roofing. Light colours will reflect more of the sun’s heat and darker colours absorb more of the sun’s heat, which will be transferred into the roof structure.

Unless prevented by glare issues to surrounding development, light colours must be selected to reduce the thermal load from solar heating and contribute to heat island effect mitigation.

The product selected must meet the following three-year Solar Reflectance Index ( SRI) requirements :

For roof pitch < 15°, minimum SRI of 64

For roof pitch > 15°, minimum SRI of 34

Where a three-year SRI is not available, the following requirements must be met:

For roof pitch < 15°, minimum SRI of 82

For roof pitch > 15°, minimum SRI of 39

When selecting roofing for an extension to an existing building the colour of the roofing should match the existing roofing.