Understanding the Roof Non-fragility Testing Process

Roof safety is critical for both construction professionals and building owners. Fragile roofs pose a significant hazard, leading to accidents that could result in severe injuries or fatalities. Recognising this, the Advisory Committee for Roofsafety (ACR) created the widely used “Red Book,” which outlines a testing standard to determine the non-fragility of roof assemblies.

Alongside it, The Rooflight Association provides further guidance, such as their NTD03.1 and NTD11.1 documents and Quickguide 01, focusing on non-fragility, durability and the differentiation between non-fragile and walk-on rooflights.

By exploring these principles and processes, Filon here will help you understand how these standards improve safety across the roofing industry.

Why Should I Test Roof Non-fragility?

Roof fragility refers to the inability of a roof to withstand the forces it may encounter without failing. Most roofs are designed to shield the interior from weather conditions, not to bear frequent foot traffic or accidental falls. However, routine maintenance or emergency access may necessitate walking on a roof.

Testing for non-fragility ensures that roofs can resist human impact without causing injury or allowing falls through weak areas. According to the ACR and The Rooflight Association, proper testing not only assesses initial strength but also accounts for long-term durability under real-world conditions.

The Principles of Roof Integrity Testing

The ACR’s testing process is built on the following core principles, ensuring reliable and, frankly, realistic evaluations of roof integrity:

Simulating Human Impact Loads

Traditional static load tests fail to account for the instantaneous forces created by a person stumbling or falling on a roof. The ACR test replicates these stresses using dynamic methods, a critical step in gauging real-world performance.

Focusing on Vulnerable Areas

Testing targets the most fragile points of a roof, such as unsupported edges, overlaps, and weak intersections. This ensures that the entire assembly is robust.

How Roof Non-fragility Is Tested

Step 1: Setting Up the Test

Testing begins with constructing a rigid rig to simulate actual roof assembly conditions. The rig’s stiffness matches that of a roof’s firmest points, like areas near rafters. Following the ACR’s standards, samples are assembled to ensure a realistic representation of how the roof will be constructed.

Step 2: The Sandbag Drop Test

At the heart of non-fragility testing is the sandbag drop test. Designed to simulate the force of a falling person, it employs a cylindrical sandbag weighing 45 kilograms. The sandbag is dropped from a height of 1.2 metres onto various points on the roof sample, chosen to reflect worst-case scenarios or others which may be identified by the competent person. These points include:

• The centre of the sheet
• The edges of the sheet
• Close to the purlins
• On the crowns and troughs when testing profiled sheets.

To pass, the roof must prevent the sandbag from breaking through and retain it for at least five minutes. Any evidence of weakening, such as tearing or deformation, is monitored closely.

Step 3: Evaluating Results

Roof assemblies are categorised into three classifications following the tests:

• Class C roofs can withstand a single drop but may fail subsequent tests.
• Class B roofs can withstand two drops at the same locations.
• Class A roofs withstand 2 drops and incur no damage that could accelerate degradation of the roof system, but isn’t typically possible for profile sheeted roofs.

Importantly, an assembly’s weakest element determines its overall classification. If a weak accessory is incorporated, even a robust roof could be downgraded.

Highlighting GRP Rooflights in Non-fragility Testing

Glass-reinforced polyester (GRP) rooflights offer several advantages, including durability and energy efficiency. According to NTD03.1 by The Rooflight Association, GRP rooflights are highly resistant to environmental degradation and, when manufactured to industry standards and specified with the appropriate sheet weights, can retain their non-fragility classification for over 25 years. However, as emphasised in NTD11.1, it’s critical to distinguish between non-fragile rooflights and walk-on rooflights.

Non-Fragile vs Walk-On Rooflights

The term “non-fragile” means a rooflight can safely resist one or more impact tests, as outlined by the ACR, without allowing penetration. Non-fragile GRP rooflights are designed for infrequent access, such as maintenance or inspection, but should never be walked on. Walk-on rooflights, on the other hand, are engineered to support deliberate foot traffic and comply with floor-loading requirements (Class 0 under CWCT standards). GRP rooflights do not fall under this category and must be treated accordingly to ensure user safety.

Quickguide 01 provides further clarification, highlighting the importance of proper installation and clear labelling. For instance, installers must emphasise that non-fragile rooflights are not designed for continuous or intentional foot traffic, even if they pass ACR tests.

Special Safety Considerations

For buildings with extensive rooflight use, additional non-fragility testing is necessary to account for unique risks. For example, glass rooflights are susceptible to shattering and must meet extra criteria developed by the Centre for Window and Cladding Technology (CWCT). These include hard body impact tests to ensure shards of broken glass do not endanger people beneath.

Recommendations for Specifiers and Designers

When specifying or designing roof assemblies, consider the following key points:

• Ensure all components match or exceed the required classification for long-term non-fragility.
• Specify GRP rooflights with enhanced protective layers, like those classified as CE30E in NTD03.1, for added durability.
• Do not use GRP rooflights in areas where walk-on capacity is required.

Maintenance for Long-Term Safety

Regular inspections are crucial for maintaining non-fragility. NTD03.1 recommends cleaning GRP rooflights every two to three years to remove dirt and debris, as this helps preserve their light transmission properties and structural integrity. Equally important is inspecting fixings, seals and surrounding materials for signs of deterioration.

Why Rigorous Testing and Standards Matter

The ACR’s non-fragility testing process, combined with guidance from The Rooflight Association, saves lives by ensuring that roofs meet clear safety benchmarks. These tests provide confidence to manufacturers, designers, and contractors, supporting safer practices industry-wide.

The distinction between non-fragile and walk-on rooflights, as detailed in NTD11.1, also ensures clarity for specifiers and end-users. By adhering to these standards and choosing the correct rooflight type for each application, professionals can significantly reduce risk while maximising building performance.

Safety in roofing is non-negotiable. The non-fragility testing process not only identifies weak points but also encourages better material choices, robust design, and ongoing maintenance. By following the ACR Red Book and additional guidelines such as NTD03.1 and NTD11.1, we can build safer, more durable roofing systems that protect everyone who relies on them. If you’re involved in specifying, installing or maintaining roofs, taking these standards into account is not just recommended, it’s essential.