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Reviving a Roofing Standard: The Return of a Canadian Mod-Bit Membrane Ideal

Adapted From The Article Published In Construction Canada, March 2016

For the longest time, the standard that governed modified-bitumen (mod-bit) sheets used in roofing was CGSB 37-GP-56M, titled Membrane, Modified, Bituminous, Prefabricated, and Reinforced for Roofing, published in 1980 and updated for the last time in 1985. The National Building Code of Canada (NBC), along with many provincial codes, referenced it in Parts 5 and 9.

The CGSB Roofing Committee met regularly throughout the 1990s to revise this standard. Numerous drafts were reviewed. Despite these efforts, consensus could not be reached. Following these unsuccessful attempts, the CGSB Roofing Committee went dormant around 1998. Several subsequent attempts to reactivate the committee failed, and 45 CGSB standards from the “roofing and waterproofing materials” series (i.e. No. 37) were withdrawn in early 2005 due to a lack of interest. The list included standards for roof cements, coatings, primers, and membranes, but Standard 37-GP-56 was not part of the list.

Over the following eight years, the CSA A123 Technical Committee attempted to obtain the responsibility and jurisdiction of the mod-bit roofing standard from CGSB. In February 2013, the CGSB officially announced the withdrawal of 37-GP-56. Within months, a new CSA task group was formed with the objective of developing a new standard for modified-bitumen roofing membranes. The task group’s work culminated with the publication of CSA A123.23, Product Specification for Polymer-Modified Bitumen Sheet, Prefabricated and Reinforced. It was released in August 2015, 30 years after the latest update of its predecessor.

A Standard with a Wide Scope

The new standard covers various types of polymer-modified bitumen sheets used in roofing. Its scope and classifications include products manufactured with styrene-butadiene-styrene (SBS) modified bitumen, as well as atactic polypropylene (APP) modified bitumen, using one of the three common reinforcement types encountered today in North America:

  • Glass fibre (Type A);

  • Polyester fibre (Type B); and

  • A combination of both (Type C).

Sheets could then be classified as follows:

  • Grade 1 (granule-surfaced sheets intended to be exposed);

  • Grade 2 (exposed sheets surfaced with any other surfacing, or unsurfaced); or

  • Grade 3 (sheets not intended to be exposed).

Several participants of the CSA task group insisted on developing an all-inclusive standard, out of consideration for the users of these products. Specification writers and contractors would then only have to deal with one single document that covers the same range of products as six different ASTM specifications. Realizing the similarities between the Canadian and U.S. markets, the task group also opted to require testing to existing ASTM test methods within A123.23. Manufacturers who qualify their products to ASTM specifications already have test reports useful for compliance with the new CSA standard, provided they meet the Canadian requirements of A123.23.

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When determining the minimum performance requirements in the standard, care was taken to ensure the performance levels offered by meeting the former 37-GP-56 were maintained. CSA A123.23 is therefore at least equally stringent as its predecessor in all aspects, even though some performance requirements were “modernized”.

Return of Strain Energy

The greatest portion of modernity brought by CSA A123.23 is the formal integration of the strain energy concept into a national standard of Canada. Efforts made in the 1980s by researchers of the National Research Council of Canada (NRC) to capture the mechanical performances of roofing membranes (i.e., load and elongation) using a single result were intended to be included in the revised CGSB standard, and strain energy was present in the later drafts. Many manufacturers of polymer-modified bitumen membranes opted to include such “strain energy values” in their data sheets over the last 10 years, recognizing that the former CGSB standard was outdated. Even though the 9th draft of the CGSB standard had not been approved and was not listed in any building code, it became a reference for those involved with mod-bit sheets in the Canadian roofing industry.

When applied to roofing membranes, strain energy can be defined as the energy a membrane specimen can absorb as a result of its deformation before it ruptures and fails. This measurement is helpful in comparing very strong roofing sheets that provide low elongation with other sheets that are not as strong, and that provide significantly higher elongation before they rupture. In the context of a laboratory evaluation, strain energy is calculated as the area under the load-deformation curve up to a point where the load-deformation curve drops below a value equal to five per cent of the peak load. Figure 1 illustrates how strain energy is obtained from a typical load-deformation curve.

Minimum requirements for strain energy have been integrated in the tables of CSA A123.23 for Type B and C sheets. The values were based on those found in the well-known 9th draft revision of the former CGSB document. As a courtesy to users of the standard who are unfamiliar with strain energy, CSA A123.23 requires manufacturers to disclose (without imposing any target value) peak load, elongation at peak load, and ultimate elongation results, in addition to meeting the strain energy requirements. However, this will not represent additional testing expense—all these properties are obtained during the same test procedure.

Load and Elongation GraphicLoad and Elongation Graphic

Figure 1 – This chart shows the typical load-deformation curve obtained for a polyester-fibre-reinforced SBS-modified bitumen sheet at 23 °C (73 °F). Since peak load for this specimen is equal to 428.1N (96.2 lbf), strain energy is computed as the area under the curve up to where it drops below 21.4 N (4.8 lbf), which is 5% of peak load. In this case, this happens at 59.5% elongation, which is considered the ultimate elongation. The area under the curve to this point (the strain energy) is 7.3 kN/m (41 lbf/in).

On the other hand, the strain energy of Type A sheets (reinforced with glass fibres) was less studied over the years. Therefore, limited data is available to establish a minimum value. For Type A sheets, minimum values of peak load, elongation at peak load and ultimate elongation are imposed in CSA A123.23. Manufacturers will also be required to report strain energy numbers for their sheets (without having to meet a minimum number). This exercise will generate data for the next revision of the CSA standard, which will most likely be modified so that all types of polymer‑modified bitumen sheet materials have minimum strain energy values to meet.

Testing for the Long-Term

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Calling durability of construction materials important to industry stakeholders would be an understatement. Canadian roofs need to perform adequately from the day they are installed until they reach their expected end of life. CSA A123.23 recognizes this fact, and roofing sheets will be tested for the aforementioned load-strain tests in both directions (longitudinal and transverse), at two test temperatures (23 and -18 °C [73 and 0 °F]), both before and after a 90-day heat conditioning.

After several years of inactivity, Canada’s roofing industry has finally caught up with the 21st century with respect to the standard covering mod-bit roofing membranes. Published in early 2022, the 2020 edition of the National Building Code of Canada introduces a reference to CSA A123.23 in both Parts 5 and 9. At the same time, the reference to CGSB 37-GP-56 has been removed from Part 5, making the CSA standard the only recognized document to demonstrate compliance of mod-bit sheets used for roofing in Canada. The reference to CGSB 37-GP-56 in Part 9 of the Code has been retained, but only because it is used by the insulated concrete form (ICF) industry as a basis for the evaluation of below-grade waterproofing membranes.

As the provinces and territories gradually adopt the 2020 model codes, choosing mod-bit sheets that comply with the requirements of CSA A123.23 will become mandatory. Several manufacturers have already started publishing results on their data sheets according to the CSA standard. Specification writers should familiarize themselves with CSA A123.23 and use it in their projects.

Technical Bulletin ImageTechnical Bulletin Image

Download our technical bulletin “The Importance of Cover Boards on Roof Durability” to learn how to integrate approved practices and materials into your roofing designs!

Conclusion

After several years of inactivity, Canada’s roofing industry has finally caught up with the 21st century with respect to the standard covering mod-bit roofing membranes. Published in early 2022, the 2020 edition of the National Building Code of Canada introduces a reference to CSA A123.23 in both Parts 5 and 9. At the same time, the reference to CGSB 37-GP-56 has been removed from Part 5, making the CSA standard the only recognized document to demonstrate compliance of mod-bit sheets used for roofing in Canada. The reference to CGSB 37-GP-56 in Part 9 of the Code has been retained, but only because it is used by the insulated concrete form (ICF) industry as a basis for the evaluation of below-grade waterproofing membranes.

As the provinces and territories gradually adopt the 2020 model codes, choosing mod-bit sheets that comply with the requirements of CSA A123.23 will become mandatory. Several manufacturers have already started publishing results on their data sheets according to the CSA standard. Specification writers should familiarize themselves with CSA A123.23 and use it in their projects.

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SOPREMA Products Keep Up With CSA A123.23

Did you know that all of SOPREMA’s modified-bitumen roofing membranes meet the requirements of the CSA A123.23 Standard?

Here is an overview: