Code and Roof Design

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Code and Roof Design



Division E - General Information

Roof Design: From Code to specifciations

This is the third part of a multi-part series on the Building Code and roofing:

Part 1: Understanding the BC Building Code
Part 2: The Building Code and wind
Part 3: Design Responsibility: from Code to Specification

You can find these articles in both the printed and digital editions of Roofing BC, the trade magazine published by the RCABC. You can also watch the video presentation, “Blown Away: Code Requirements for Membrane Roofs”, that addresses several articles in this series.

The content on this page has been adapted from the original published article in Roofing BC (Fall 2021).

Introduction

In the second article of this series on the British Columbia Building Code, we examined why wind matters, located Code requirements for membrane roofs and discovered that roof design is as much about structural design as it is about the building enclosure. In this third article, we address the fundamental question of design responsibility.

Who's in charge here?

Looking back at the previous article, we know that Part 5 in Division B contains new language (in 2018) about roof design that provides two pathways by which a roof system design can conform to the Code. What exactly does it all mean for the design process? More importantly, who is responsible for the design?

The question of responsibility has not been adequately answered in past years. Some designers provide detailed specifications that include Specified Wind Load values, but examples are rare. Many hold that the roof design can be assigned to the roofing contractor. Assigned or Delegated Design to constructors is a common practice1. But is the assignment of structural design permissible under the Code?

When we refer to the Design Authority (a term used in the Roofing Practices Manual), we mean anyone who takes responsibility for the design of a roof system. The term designer used in the Code imparts the same meaning. Regardless of new construction or replacement work2, the Design Authority could be any of the following:

  • Architect
  • Engineer
  • Roofing Consultant
  • Roof Manufacturer / Supplier
  • Roofing Contractor
  • Building Owner

So, what makes any one of these people potentially responsible for the design? The answer to that question, as we will discover, is in the Code.

Article 5.2.2.2. in Division B refers directly to Article 4.1.7.1., Specified Wind Load. Recalling that the term Specified Wind Load is mentioned in Sentence (2) of Article 5.2.2.2., we see in Part 5 a deliberate connection between the two Parts: the roof must successfully separate the outside environment from the interior conditioned space (Part 5 – Environmental Separation), and to do that it has to remain where it is built (Part 4 – Structural Design). Or, to put it another way, the roof must maintain a water-tight state (Part 5) by capably resisting the upward forces exerted by wind (Part 4). Designing the roof, then, begins with the overall structural design of the building (Part 4).

Keeping in mind this keystone relationship between Parts 4 and 5 of Division B, we turn to Division C to understand who takes ownership of a design that is both “structural” and “building enclosure” in nature.

The Building Code: Division C, Administrative Provisions

Division C of the Code contains the administrative requirements. While Division C is relatively small and lacks the “how to” appeal of Division B, it’s nevertheless packed full of critical language about responsibility for design. So, take a deep breath. We’re diving deep.

In the Code, the term “designer” is defined as “the person responsible for the design” (Division A, Article 1.4.1.2. Defined Terms). Part 2, Article 2.2.1.2. Structural Design, states that “for design work carried out in accordance with Part 4 of Division B, the designer shall be a registered professional skilled in the work concerned (See Note A-2.2.1.2.(1).).”3 Notice that this sentence pertains to the work of Division B, Part 4 Structural Design. The Notes referenced in this Code Article state that “[it is] the assumption that structural design will be carried out by a registered professional who is qualified to perform such design. Sentence 2.2.1.2.(1) is not intended to imply that a registered professional may not also be required in the application of requirements in other Parts of the British Columbia Building Code.”4

Article 2.2.1.2. and the corresponding Notes do not state whether the registered professional must be the “coordinating registered professional” or even a registered professional who is part of the design team for the building. We will circle back to this issue later in this article.

The term “registered professional” is also a defined term (Division A, Article 1.4.1.2. Defined Terms):

“Registered professional means
  • a person who is registered or licensed to practise as an architect under the Architects Act, or
  • a person who is registered or licensed to practise as a professional engineer under the Engineers and Geoscientists Act.”

In the Notes to Division C, Part 2, Note A-2.2.7. Professional Design and Review states the following (red text is by the writer for emphasis):

It is typical that the registered professional responsible for the design is also responsible for the field review. There are instances where this is not the case and having a different registered professional doing the field reviews is unavoidable. Schedule C-B requires that the registered professional who provides the field review provide assurance that the building as finally constructed is in substantial conformance with the Code. In the event that another registered professional is to provide field review, the field reviewer takes on the responsibility to confirm that the construction substantially complies with the plans and supporting documents that were submitted for the building permit. The responsibility for code compliance of the design remains with the original registered professional who undertook the design. In this event, the Schedule C-B must be modified by the field reviewer by crossing out and initialing Clause (b)(i) and providing the effective transition date.

Then, Note A-2.2.7.2. (1)(a) Coordinating Registered Professional states the following (again, red text is by the writer for emphasis):

The coordinating registered professional is responsible to ascertain that all Code related aspects which are relevant to the project are clearly identified by each of the registered professionals in the collection of Schedules B. If a registered professional of record has crossed out any item on their Schedule B, the coordinating registered professional must confirm this item is not applicable to the project or resolve the issue with the registered professional of record.

Back to Section 2.2. of Division C, we read the following in Article 2.2.2.1. General Information Required (blue text is in keeping with the original wording in the Code):

1) Sufficient information shall be provided to show that the proposed work will conform to this Code and whether or not it may affect adjacent property.

2) Plans shall be drawn to scale and shall indicate

a) the nature and extent of the work or proposed occupancy in sufficient detail to establish that, when completed, the work and the proposed occupancy will conform to this Code,
b) the applicable edition of the Code,
c) whether the building is designed under Part 3 or Part 9,
d) the major occupancy classifications of the building,
e) the building area and building height,
f) the number of streets the building faces,
g) the accessible entrances, work areas and washrooms,
h) the accessible facilities particular to the occupancies, and
i) the energy compliance path to which the building conforms, and, where a building conforms to Subsection 9.36.6. or 10.2.3. of Division B, the Step to which it conforms.

3) When proposed work is changed during construction, information on the changes shall comply with the requirements of this Section for proposed work.

This is followed later by Subsection 2.2.4. Structural and Foundation Drawings and Calculations (which, according to Article 2.2.4.1., applies “only to buildings covered in Part 4 of Division B”). Article 2.2.4.3. Information Required on Structural Drawings, concerning structural drawings and specifications, states (red text is for emphasis):

1) Structural drawings and related documents submitted with the application to build shall indicate, in addition to those items specified in Article 2.2.4.6. and in Part 4 of Division B applicable to the specific material,

a) the name and address of the person responsible for the structural design,
b) the date of issue of the Code and standards to which the design conforms,
c) the dimensions, location and size of all structural members in sufficient detail to enable the design to be checked,
d) sufficient detail to enable the dead loads to be determined, and
e) all effects and loads, other than dead loads, used for the design of the structural members and exterior cladding.

Building on Article 2.2.4.3., note that in Article 2.2.5.1. Application the Code states, “This Subsection applies to building materials, components and assemblies to which Part 5 of Division B applies. (See Article 1.3.3.2. of Division A).” This is extremely relevant to the current conversation about design responsibility, since Division B, Part 5 answers the question of “how” that Part 4 raises in specifying what loads the roof will be subjected to. Also, notice that the structural design for “exterior cladding” must be submitted at the time an application to build is made to the Authority Having Jurisdiction (AHJ). “Submitted” presumes that the loads have been calculated and that the design is complete.

The term “exterior cladding” used in Sentence (1) is not defined in the Code but the NRC, who oversees the development of the NBC (the model Code), and who undertook the research and development of what is now the CSA-A123.21 test method for membrane roofs (ref. Division B, Part 5: Article 5.2.2.2.), refers to the membranes as “roof cladding” (Annex H, H.1). By inference, then, Sentence (1) means that the loads to which a membrane roof will be subject must be calculated before the design even goes to tender.

Lastly, notice in Subsection 2.2.5. (Drawings and Specifications for Environmental Separators and Other Assemblies Exposed to the Exterior) that Article 2.2.5.2. (Information Required on Drawings) indicates what kind of information a design shall provide to the constructors (red text is again used for emphasis):

1) Information shown on drawings and in specifications shall be clear and legible, and shall contain sufficient details to demonstrate conformance with this Code. (See Note A-2.2.6.2.(1).).

What shall we make of all this?

Reasoned Conclusions

Based on the administrative requirements in Division C of the Building Code, we can take away the following conclusions:

  1. The person designing the roof assembly (the designer, or the Design Authority), who could be the person responsible for coordinating the design of the project (the coordinating registered professional), must ensure that the assembly design (Part 5) capably resists the loads calculated in Part 4. This is true regardless of the Design Authority’s profession.
  2. Calculating the Specified Wind Loads (a Part 4 requirement in Division B) must be undertaken or at least reviewed by a registered professional skilled in the work concerned. This person does not have to be the “coordinating registered professional” (read Design Authority), but that person is ultimately responsible to ensure the design complies with the Code.
  3. Because of the inter-relatedness of Part 4 and Part 5 in Division B, with respect to the design of the roof, compliance with Part 5 logically falls to the “coordinating registered professional”. This is because the application of the structural loads in the design of a load-resistant roof falls under Part 5 of Division B.
  4. Drawings and specifications for roofing must include sufficient details to build the roof system/assembly to resist the Specified Wind Loads5. Ideally, the architectural drawings will also provide load information and indicate the size and location of roof zones; at the very least, architectural specifications and drawings will reference structural drawings that show load values and zones6.
  5. The design of the membrane roof assembly should identify the Specified Wind Loads, and ought to provide clear instructions for how the roof must be constructed to resist those loads, since the roof forms part of the building enclosure that separates different environments7.
  6. Drawings and specifications for the “proposed work” must be complete by the time the building design is “submitted with the application to build” (that is, before anyone bids on a scope of work).

None of the above precludes the Design Authority from assigning to the constructor the task of selecting a suitable roofing system using the options provided in Article 5.2.2.2. (Division B, Part 5, Section 5.2). However, assigning this aspect of the design is not a panacea for each project. As we will see in forthcoming articles, what the Design Authority assigns to the roofing contractor proportionately impacts the look and performance of the end-product.

Big Rocks

Let us stop for a moment to digest a few things learned from this and the previous articles. First, it helps to remember that the 2018 Code shifted away from a prescribed model to performance-based requirements. Within that framework we can identify two ‘big rocks’ of roof design:

  1. All roofs (regardless of how they are secured) must be designed to resist Specified Wind Loads, which are structural in nature.
  2. Roofs that are not ballasted must be designed to resist Specified Wind Loads using one of the following three options (ref. Part 5 of Division B: Article 5.2.2.2.):
    a) a Tested Assembly.
    b) an assembly with proven past performance.
    c) custom-engineered securement (perhaps by using the ANSI-SPRI WD-1 wind design standard).

Labouring Together

Keeping the “Big Rocks” in mind, how does one specify such a roof with good thermal performance and comply with the Code requirements for wind? Speaking of rocks, how should ballasted roofs be secured? What do we do about membranes that are liquid-applied? And how can a design account for the effect of wind on “green roofs”? All good, important, and necessary questions. The next article, Part 4: Flexibility in roof design, will answer some of those questions, explain how a design specification can leverage “Tested Assemblies”, and discuss options and trade-offs when assigning final design work to the roofing contractor.

Building good roofs requires the vision of architects, the direction of structural engineers, the expert technical advice by material manufacturers and knowledgeable consultants, and the competent experience of a professional roofing contractor who can turn transform it all into something real, beautiful and lasting. What binds them together? Collaboration (literally, working together). Alone, no one succeeds. Together, they make great things happen.

Footnotes

1 Assignment of design to constructors: A discussion and direction. Robinson, Keith et al. The Construction Specifier: December 3, 2018 (https://www.constructionspecifier.com/assignment-of-design-to-constructors-a-discussion-and-direction/)
2 See the previous article in this series, The Building Code and wind.
3 British Columbia Building Code, Division C, Part 2: 2.2.1.2.(1) Structural Design.
4 British Columbia Building Code, Division C, Notes to Part 2: A-2.2.1.2.(1) Structural Design.
5 While the calculation of Specified Wind Loads is a structural matter, their application is a ‘building enclosure’ concern, which is why they should form part of Division 07, Thermal and Moisture Protection, of the National Master Specification.
6 Roof zones are delineated regions of the roof subject to different Specified Wind Loads. Watch for more on this topic in forthcoming articles as we explore the means and methods of wind-resistant roof design.
7 Ibid.

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