As1684.2 vs As1684.3

The Differences Between AS1684.2 and AS1684.3: A Focus on Roofing

The Australian Standard AS1684 covers residential timber-framed construction, providing re-roofing approval guidelines for the design and construction of timber-framed buildings. AS1684 is divided into different parts, with AS1684.2 and AS1684.3 addressing specific conditions for building in non-cyclonic and cyclonic regions, respectively. In this essay, we will explore the key differences between these two sections, focusing particularly on the roofing aspects of each standard. This comparison will highlight how each part caters to different environmental conditions, the structural differences in roofing requirements, and the general implications for builders, designers, and contractors working under these two standards.

Understanding AS1684: An Overview

AS1684 is a key standard in Australia for timber-framed residential buildings, offering design and construction guidelines that ensure safety, stability, and durability. The standard is divided into several parts:

• AS1684.1: General provisions for timber-framed construction
• AS1684.2: Residential timber-framed construction in non-cyclonic regions
• AS1684.3: Residential timber-framed construction in cyclonic regions
• AS1684.4: Simplified provisions for small buildings

While AS1684.1 sets out general provisions for timber framing, the focus of AS1684.2 and AS1684.3 is on providing detailed guidance for the construction of timber-framed buildings in specific environmental conditions: AS1684.2 applies to non-cyclonic areas, and AS1684.3 addresses regions subject to cyclonic conditions.

AS1684.2: Residential Timber Framing in Non-Cyclonic Regions

AS1684.2 is specifically designed for residential timber-framed construction in areas that are not subjected to extreme wind forces, such as those found in regions that experience typical Australian weather conditions but are not exposed to cyclones. This part of the standard is applied in many urban and rural areas, including parts of New South Wales, Victoria, South Australia, and Tasmania.

In AS1684.2, the design of roofing structures is simpler because the roofing system is not required to withstand the same extreme forces as in cyclonic areas. The standard allows for more flexibility in the choice of materials, sizes of timber framing members, and the overall design, provided that they meet basic safety and performance criteria.

Roofing Design Under AS1684.2

In AS1684.2, the roofing system is designed with a focus on resistance to standard wind loads and other common environmental conditions. The primary considerations for roofing in non-cyclonic areas include:

• Timber Selection and Sizes: AS1684.2 specifies timber species, grades, and sizes for rafters, purlins, and other structural components of the roof. For non-cyclonic regions, the timber members do not need to be as large or as heavily reinforced as in cyclonic areas, given the lower risk of high wind loads.
• Roof Pitch and Design: AS1684.2 provides guidelines on the minimum and maximum roof pitch to ensure that the roof is adequately drained and can withstand expected loads, including those caused by snow and rain, but it does not require the same level of structural reinforcement as is required in cyclonic zones.
• Wind Loading: The wind loads in non-cyclonic areas are generally lower and are calculated using regional wind classification (as per AS/NZS 1170.2). The roofing structure in these regions, while still needing to comply with appropriate safety standards, is generally less robust than in cyclonic zones.

AS1684.3: Residential Timber Framing in Cyclonic Regions

AS1684.3 applies to residential timber-framed construction in areas that are prone to severe wind events, including cyclones and tropical storms. These regions, including parts of northern Queensland, the Northern Territory, and northern Western Australia, experience wind speeds that can exceed 200 km/h during a cyclone, which significantly impacts the design and construction of roofs.

Given the extreme weather conditions in cyclonic areas, AS1684.3 includes more stringent requirements for the structural integrity of roofing systems. The goal is to ensure that roofing systems can withstand the high wind forces associated with cyclones, reducing the risk of roof failure during severe weather events.

Roofing Design Under AS1684.3

The roofing requirements in AS1684.3 are significantly more complex and detailed due to the higher wind loads and more demanding environmental conditions. Key aspects of roofing design under AS1684.3 include:

• Stronger Timber Members: In cyclonic regions, the timber members used for roofing, including rafters, purlins, and trusses, must be of higher grades and sizes than those used in non-cyclonic areas. This ensures the roof structure can resist the additional stresses placed upon it by extreme winds.
• Increased Wind Load Resistance: AS1684.3 requires that the roof be designed to resist higher wind loads, including gusts typical of cyclonic conditions. These loads are determined by calculating the region’s wind classification, which considers factors such as geographical location and expected wind speeds. For cyclonic areas, the wind classification may require additional structural elements to ensure the roof can remain intact during high winds.
• Roof Bracing and Tie Downs: AS1684.3 places a greater emphasis on the need for bracing and tie-down systems to resist wind uplift and lateral loads. The roofing system must be securely anchored to the rest of the structure, particularly in areas like the eaves and ridges, to prevent the roof from lifting off during a cyclone.
• Roof Cladding and Materials: In cyclonic areas, materials that are more resilient to wind and impact damage are typically required. Roof cladding must meet specific standards for wind resistance, and materials like metal or reinforced tiles are often preferred over lighter options that could be torn off by high winds.
• Connection Detailing: AS1684.3 provides detailed instructions on how roofing components should be connected to the rest of the building, ensuring that the roof structure is securely tied into the frame of the house. This includes stronger connections for ridge beams, eaves, and rafters, using more durable fixings like stronger nails, bolts, and straps that can resist the intense forces of a cyclone.

Key Differences Between AS1684.2 and AS1684.3: A Roofing Perspective

The fundamental difference between AS1684.2 and AS1684.3 lies in the environmental conditions each standard addresses. While AS1684.2 provides guidelines for roofing systems in areas exposed to regular but moderate wind conditions, AS1684.3 focuses on the vastly more challenging demands placed on roofing systems in regions affected by cyclonic winds.

Here are the primary differences:

1. Wind Load Calculations:
   • AS1684.2 considers standard wind loads, typically ranging from 44 to 54 m/s (158 to 194 km/h), suitable for regions not subject to extreme wind conditions.
   • AS1684.3 applies higher wind load calculations due to the presence of cyclonic winds, often in the range of 55 to 70 m/s (198 to 252 km/h), reflecting the much higher wind speeds in these areas.
2. Timber Sizes and Grades:
   • In AS1684.2, the timber sizes and grades are based on the assumption that the roof will only be exposed to moderate wind conditions. The required timber sections are smaller and less heavily reinforced.
   • AS1684.3 calls for larger, stronger timber members to withstand the higher stresses caused by cyclonic winds. This may involve using higher-grade timber or reinforcing materials to meet the more stringent requirements.
3. Roofing Connections and Reinforcements:
   • AS1684.2 provides basic requirements for roofing connections, which are typically sufficient for non-cyclonic regions. The connection methods are designed to handle moderate wind loads and typical environmental conditions.
   • In AS1684.3, the standard specifies more robust connection systems, including the use of straps, bolts, and additional tie-downs to ensure the roofing system can resist the higher wind forces typical of cyclonic conditions.
4. Roof Design Flexibility:
   • AS1684.2 allows for greater flexibility in roof design, such as a wider range of acceptable roof pitches and materials.
   • AS1684.3 has more prescriptive design requirements, including the use of specific materials and design approaches to ensure the roof can resist both the pressure of high winds and the impact of flying debris.
5. Cladding and Materials:
   • AS1684.2 allows a broader range of roofing materials, including lighter options such as asphalt shingles or corrugated iron.
   • AS1684.3 typically mandates more resilient cladding materials that can withstand impacts from debris and resist being torn off in high winds. Metal roofing or reinforced tile options are often required.

Conclusion

The primary difference between AS1684.2 and AS1684.3 lies in the different environmental conditions they address, with AS1684.2 applying to non-cyclonic regions and AS1684.3 being specifically designed for cyclonic regions. These differences result in significant variations in the roofing design requirements, including timber sizes, connection methods, cladding materials, and wind load calculations. The increased wind forces and potential for severe damage in cyclonic areas necessitate more stringent requirements in AS1684.3, ensuring that roofing systems in these regions are adequately designed to withstand extreme weather conditions.

Understanding these differences is essential for designers, builders, and contractors working in different parts of Australia. Compliance with the appropriate section of AS1684 is crucial to ensuring the safety and stability of residential timber-framed buildings, particularly in regions exposed to the devastating effects of cyclonic winds.