CRCA Policy Statement on Wind Loads and Responsibility for Commercial Roof Design

Roofs: More Than Just Waterproofing

In November 2023, the Canadian Roofing Contractors Association (CRCA) released a pivotal document: “Test Standard CSA A123.21 – Application Policy“. This policy guide addresses both roofing contractors and the design community.

You can find this informative document at: Test Standard CSA 123.21 – Application Policy

This document builds on the CRCA policy, exploring:

1. Wind loads and roof design intricacies
2. CSA A123.21’s role in wind uplift testing
3. Design responsibilities for commercial roofing systems

Wind Loads and Roof Design

The National Building Code of Canada (NBCC) classifies wind loads as structural loads, giving them prominence in Division B, Part 4 (Structural Design) and Division C, Part 2. This classification underscores the critical nature of wind in building design. Key elements include:

1. Design Wind Pressure (q): This is calculated using local climate data, building height, exposure, and the surrounding landscape.
2. Pressure Coefficients (Cp): Different zones (corner, perimeter, field) experience different uplift forces, and these coefficients help quantify those variations.
3. Load Combinations: Designers consider how it interacts with dead loads, live loads, snow loads, and seismic forces to create a resilient structure.
4. Documentation: Wind load calculations must be clearly presented in structural drawings and included in permit applications.

The NBCC sets a clear standard: the design wind uplift loads (calculated through Part 4) must be less than the wind uplift resistance demonstrated through testing that complies with CSA A123.21.

CSA A123.21: Setting the Standard for Wind-Resistant Roofs

The CSA A123.21 Standard test method for the dynamic wind uplift resistance of membrane-roofing systems is a test method that simulates real-world wind conditions.

1. What it does: The test mimics actual wind conditions using dynamic load cycles.
2. What it covers: This standard applies to a wide range of membrane roof systems—adhered, mechanically attached, and partially attached. It applies to conventionally insulated and uninsulated systems.
3. What it doesn’t cover: Protected or ballasted systems and liquid-applied membranes
4. Who’s responsible: Manufacturers test their membrane roofing systems according to CSA A123.21.
5. The results: Detailed lab reports break down the roofing system components, and detail how each piece should be installed.
6. Why it matters: This standard is referenced in Sentence 5.2.2.2.(4) of Division B in the National Building Code of Canada is a building code requirement.

Design Responsibilities

The National Building Code of Canada (NBC) Division C, Article 2.2.1.2. “Structural Design” mandates that a skilled professional engineer or architect must handle designs following Part 4 of Division B. Furthermore, Article 2.2.4.3. “Information Required on Structural Drawings” stipulates that structural drawings and related documents submitted with building applications must include all effects and loads, except dead loads, used in designing structural members.

Since Parts 4 and 5 of Division B clearly define the roof assembly as a structural building member, determining roof wind loads must be carried out by a qualified professional before submitting a project to the Authority Having Jurisdiction (AHJ) for a building permit. This crucial task cannot be delegated to the roofing contractor.

Design Responsibilities in Roofing Projects
Design Authority/Architect/Engineer

The Design Authority or Architect plays a pivotal role in ensuring roofing projects meet all necessary standards.

Their responsibilities include:

1. Retaining qualified professionals for roof design and field review
2. Calculate wind loads in strict accordance with NBCC Part 4 requirements
3. Verifying that roof designs comply with National Building Code of Canada (NBCC) Part 5 requirements for Environmental Separation
4. Clearly document these wind load calculations on structural drawings and/or specifications/

Roofing Contractor

Roofing Contractors’ responsibilities include:

1. Installing the specified roof system as per the approved design
2. Following manufacturer recommendations during installation

Important Note: Roofing contractors are not responsible for wind load calculations. This task lies outside their scope of work.

Conclusion: The Structural Significance of Roofs

Roofs are integral structural members of buildings. Both the National Building Code of Canada (NBC) and CSA A123.21 standard underscore this critical fact. While design authorities may delegate the selection of an appropriate roofing system to roofing contractors, the responsibility for wind load calculations and overall roof design is the responsibility of qualified design professionals.

1. Design professionals’ responsibilities:

• Calculate wind loads on roofs
• Clearly communicate this information in specifications and drawings
• Design the roof to withstand these calculated loads

2. Roofing contractors’ role:

• May be assigned the task of selecting a suitable roof system given the wind load information calculated by the design professional (Note: certain provincial laws may prohibit delegating the choice of a roofing system to the roofing contractor such as in Quebec where it must be done by an architect in Quebec)
• Install the specified system according to design and manufacturer recommendations

3. Key takeaways:

• Wind load calculations are not the roofing contractor’s responsibility
• Structural integrity of the roof is the design professional’s responsibility

This division of responsibilities ensures that roofs are designed and installed to meet both structural requirements and performance standards.
By adhering to these principles, we safeguard the integrity of our buildings and the safety of their occupants.

CRCA – Test Standard CSA A123.21 Application Policy, November 2023
RCABC Bulletin – Wind Loads and Responsibility for Design, July 2024
QMRA – Bulletin #13, January 8, 2025