Essential structural considerations in roof design

by arslan_ahmed | January 6, 2023 5:22 pm

Photo courtesy KTA Structural Engineers Ltd./Teeple Architects Ltd./Kaisian Architecture

By David Thompson, P.Eng.

Design professionals often work in isolation, with each discipline (architectural, structural, mechanical, and electrical) doing their own thing and only getting together after they have completed most of their individual design contribution to co-ordinate dimensions. This approach frequently limits the amount of detailed information exchanged between the design team until the final stages of the project. Crucial data is unintentionally missed, and this can significantly affect the structural design of roof systems. Anticipating what the structural engineer needs to consider to make design choices and how the other disciplines influence these decisions (many times without realizing it) will be addressed in this article, as well as:


Roofs prevent weather from affecting the lives of people inside a building. Extreme weather events occur either when a large-scale system (cold or warm front), short-lived storms, or snow squalls move through an area. Short-lived storms can develop into thunderstorms, hailstorms, or tornadoes. Thunderstorms can cause extreme wind events, hailstorms, and/or tornadoes (Figure 1).

Environment Canada’s website has several interesting facts on these weather conditions:

Environment Canada has supplied design values in Table C-2 of Volume I, Division B, Appendix C, of the National Building Code of Canada (NBC). Appendix C gives a good explanation of the background for the design values. The appendix notes, for locations not listed in Table C-2, designers should contact Environment Canada. One item that needs to be clarified is the term “1/50 year,” meaning there is a probability of two per cent of the value occurring during the year. It is possible for the value to be exceeded one or more times in a year.

Codes and standards

In Canada, building codes are a municipal responsibility. Most municipalities do not want to have their own code, so they rely on the provincial government product and maintain a provincial building code. Most of the provinces and territories use the NBC as a basis for their building codes. Having different provincial building codes has led to different editions of the NBC being used. Presently, the following building codes are in force:

Fortunately, the structural provisions of the NBC 2020 are very similar to NBC 2010. In the U.S., building codes are also a municipal responsibility. Most municipalities do not want to have their own code, so they rely on the state government to maintain building codes. Most of the states use the International Building Code (IBC) as a basis for their state building codes. The IBC has adopted the American Society of Civil Engineering’s Standard, ASCE-7, Minimum Design Loads for Buildings and Other Structures.

However, each state having its own building code led to situations similar to Canada’s, where different states used different editions of the IBC.

Figure 1 Gust Front. All illustrations of this page courtesy


The use of various editions of IBC presents a challenge. The provisions of the most recent edition of ASCE-7 -22 have been adopted for the next edition of the IBC.

ASCE-7 -22 has made significant changes to how rain and snow loads are handled. ASCE-7 -10 changed the specified wind speed from a 1/50-year wind speed to an ultimate wind speed for each category of building importance.

Figure 1 Tornado.

While these editions are not accepted in all locations, this article will discuss the provisions from the NBC 2020 and ASCE-7, Minimum Design Loads for Buildings and Other Structures.


Several types of winds are behind the wind loads used in Canada. Synoptic winds (winds associated with large-scale events, such as warm and cold fronts) make up everyday weather. In four provinces (Alberta, Saskatchewan, Manitoba, and Southern Ontario), short-term storm events cause the major wind events. Short-term storms start as thunderstorms, but they can cause microbursts which can turn into tornadoes or hailstorms. In Alberta, on the east side of the Rocky Mountain Range, there are areas where downslope winds (known as Chinooks) contribute to extreme wind events. A comparable situation occurs on the maritime coasts. The U.S. (and rarely the Atlantic maritime provinces) have tropical storms, hurricanes, and/or typhoons.

Figure 1 Microbust.

The wind speeds of all these weather events have been recorded by Environment Canada. As mentioned earlier, this department of the government has supplied design values in NBC.

Code provisions

The wind load provisions in Canada and the U.S. are different, but they also have some provisions that are similar. In Canada, NBC covers wind load in Article 4.1.7. The wind load equation is:

P = Iw q Ce Ct Cg Cp

This article will focus on four of the variables:

Figure 1 Hail Storms

Importance factor (Iw): accounts for whether the structure is low occupancy, regular use, or high importance. For example, the structure could be used as a post-disaster shelter or a critical structure such as a hospital.

Design pressure (q): this is calculated from a design wind. The design wind in Canada is the average wind pressure taken over 60 minutes with a probability of exceedance of two per cent (return period of 50 years) at a height of 10 m (33 ft) above ground in open terrain.

Gust factor (Cg): design pressure is a function of the sample period and the size of the area considered. The pressure increases with a shorter sampling period and the smaller the sampling area. However, the gust factor converts the design wind pressure to a pressure taken over a sample for a time of three seconds. The simplified gust factor for a whole building is 2.0 and it is increased to 2.5 for smaller areas (cladding).

Exposure factor (Ce): the exposure factor accounts for the effect of the surrounding terrain. Rough and open terrain are the only two exposure categories considered.

Over the years, the wind provisions have evolved with these significant changes:

In the U.S., ASCE-7 has several chapters dealing with wind provisions. Chapter 26 has the general calculation for wind loads.

The wind load calculated using the equation is:

Over the years, the criteria of the basic velocity have changed. Table 1 shows the ASCE-7 edition with the basic wind speed and a conversion equation between ASCE-7 to NBC.

In the U.S., it is accepted (by the construction and design communities) that areas where the basic wind speed is greater than 201 km/hr (125 mph) 1/700-year wind, ASCE-7-22, (160 km/hr [100 mph] ASCE-7-05), the area was considered a high-wind zone requiring special detailing. Converting design wind speed for use in Canada, the same criteria is used for buildings in areas where the 1/50-year pressure is greater than 0.55 kPa (11.5 psf). Special framing needs to be considered.

Figure 2 shows a roof section of a project in southern Alberta. The detail shows attention being paid to building science consideration. The project was constructed in an area where the design wind pressure is 0.9 kPa (18.8 psf). This pressure is the equivalent to winds from a category 3 hurricane or an EF-2 tornado.


Tornadoes are rotating columns of air from the ground to the base of a thunderstorm. They cut a narrow path of destruction about 100 m (328 ft) wide, extending out as wide as 1500 m (1640 ft). The greatest wind intensity is in the central path. Wind speeds decrease rapidly away from the vortex of the tornado, but they can still be damaging.

With large tornadoes, more damage can occur in the periphery than from the central path. Field surveys performed after tornadoes have discovered common structural failures. Some of these include:

Tornadoes are classified based on the level of damage they cause. In the 1970s, the Fujita scale was developed and enhanced in the 2000s (Enhanced Fujita scale). Figure 4 provides a description of each scale, estimated wind speeds, and equivalent gust pressures.

Table 2  indicates tornado categories, wind speeds, and wind pressure.

Design for tornadoes is an evolving situation with extensive ongoing research. Tornadoes are different than other wind events as their highest wind speeds are near the ground, are very localized, and cause up to three times larger uplift forces compared to other types of wind events. With the high winds at the ground, windborne debris becomes a major concern.

In Canada, tornadoes are addressed in the “Structural Commentaries of NBC 2015.” The following is the key guidance for the commentary:

  1. Tornadoes account for the greatest incidence of death and serious injury of building occupants due to structural failure and cause considerable economic loss. However, while the probability of tornado occurrence per km2 can well exceed 1 x 10-5 per year, the probability of any one particular building being hit by a tornado is very small (less than 10-5 per year). With some exceptions, such as nuclear power plants, it is generally not economical to design buildings for tornadoes beyond what is currently required by NBC Subsection 4.1.7.
  2. Key details such as those indicated above should be designed on the basis of a factored uplift wind suction of 2 kPa (41.8 psf) on the roof, a factored lateral wind pressure of 1 kPa (20.9 psf) on the windward wall, and suction 2 kPa on the leeward wall.

Structural commentaries of the NBC 2015 list three levels of risk for tornado-prone regions in Canada as follows:

(1) “regions prone to significant tornadoes” are defined as regions where the estimated probability of occurrence of a significant tornado (F2–F5 with three-second wind gust speeds in excess of 180 km/h [111 mph]) per km2 per year exceeds 10-5.

(2) “regions prone to tornadoes” are defined as regions where the estimated probability of occurrence of a tornado (F0–F2 with three-second wind gust speeds in excess of 60 km/h [37 mph]) per km2 per year exceeds 10-5.

(3) “regions where tornadoes are possible” are defined as regions where tornadoes have been observed, but where the estimated probability of tornado occurrence per km2 per year is not more than 10-5.

United States

In the U.S., ASCE-7 covers the design for tornadoes in Chapter 32. The first part of the chapter describes the types of buildings which do not need to be specially designed for tornado winds:

If the building is required to be specially designed against tornadoes, Chapter 32 provides the basic provisions to calculate the design wind speeds. The other wind parameters in Chapters 27, 29, and 30 can be used in the same manner as with other wind loads to calculate wind loads on the structure.


Hail is not considered a structural load. However, the occurrence of hailstorms raises one of the key specifiers’ criteria: Is the product durable?

In Canada, hail occurs in the same areas as thunderstorms. Figure 7 shows the location of hail activity, with Alberta being the most active area.

In the U.S., Factory Mutual has identified the high-risk areas in Property Loss Prevention Data Sheets 1-34 (Figure 8).


Designing for rain is vastly different compared to other roof loads, and for several reasons. The structural engineer is dependent on decisions made by the mechanical engineer and architect on water drainage from the roof. Water moves and follows the geometry of the roof structure. The code provisions give the designer a volume of water, for example, a one-day rain of 100 mm (4 in.) over a roof of a 1000 m2 (10,764 sf) gives 100 m3 (26,417 gal) of water. The designer is then responsible to work out how the water would sit on the roof accounting for roof slopes and how much the roof structure deflects. The rain loading is dependent on the stiffness of the roof system and its components, if the decking and supporting members are not stiff enough, the water will flow to that area and overload the structure. This is called a ponding failure.

Ponding calculations are quite complex and iterative. In the past, guidance was provided on which minimum stiffness to use and what geometry should be avoided. With the additional computing capabilities now available, it is feasible to calculate ponding loads.

Figure 9 Areas where rain loads can exceed snow loads. Illustration courtesy Dr. F.M. Bartlett

In his presentation of the new NBC: “Companion Loads, Wind/Snow Loading,” Dr. F.M. Bartlett discussed areas in Canada where the rain load can exceed roof snow loads. These areas are susceptible to “ponding” (Figure 9).

The NBC does not have many provisions for rain loading; they are extremely broad and general and require a lot of work to satisfy them. Section 4.1.6 states:

“The Rain load, S, due to the accumulation of rainwater on a surface whose position, shape, and deflection under load make such an accumulation possible, is that resulting from the one-day rainfall whether the surface is provided with a means of drainage. Where scuppers are provided as secondary drainage systems and where the position, shape, and deflection of the loaded surface make an accumulation of rainwater possible, the loads due to rain shall be the lesser of either the one-day rainfall or a depth of rainwater equal to 30 mm [1.18 in.] above the bottom of the scuppers.”

The NBC structural commentaries go on to advise designers:

“It is considered good practice when locating roof drains to take into account not only the roof slope but also deflection of the roof due snow and rain. Drains should be provided with suitable devices to prevent clogging by leaves or, where appropriate, suitable overflows should be provided through parapet walls. There is potential for the primary drainage system for a roof to become blocked due to freeze-thaw conditions. Roofs should be designed accordingly.”

In the U.S., the rain loads are covered in ASCE-7-22, Chapter 8.

Rain loads are calculated based on obtaining an equilibrium of the rainstorm rainfall and how fast the water can drain off the roof. There is a requirement for the roofs to have two drainage systems so if the primary drains are blocked, the secondary system will drain the water. The equation for rain loads used is:

R = 5.2 (ds + dh + dp)

The 2022 edition of the ASCE-7 differs from previous editions by requiring that deflections due to ponding be calculated. In past editions, the standard rules were provided of what minimum stiffness and geometry should be avoided (similar to what is in the 2015 edition of the NBC).


Snow is similar to water in that it moves and follows the geometry of the roof structure. High parapets, roof valleys, snow guards, changes in roof elevations, privacy screens, mechanical units, and ducting can all cause snow accumulations and increase loading. In northern Canada (north of the tree line in exposed areas especially with constant winds), designers have experienced snow drifts up to 9 m (29 ft) in height. Care must be paid to the shape of a building to reduce snow accumulations. Problems occur when snow accumulates in an area not anticipated by the structural engineer.

Calculation of the weight of snow is also a challenge. Snow properties change over time, including the snow’s density. When snow first falls, its density is exceptionally light 1 kN/m3, within 24 hours the density doubles and over time it can increase to 4 kN/m.3 If only the depth of the snow has been measured, the change in density with time makes estimating the snow load difficult.

Calculation of snow loads on roofs is relatively new. The 1953 edition of the NBC, first dealt with design snow loads. The roof loads were equal to the ground snow load, with reductions allowed for sloped roofs only. The load values were approximate and resulted in over-design for some roofs and under-design for others, particularly in areas subject to high-drift loads.

Between 1957 and 1968, The National Research Council of Canada (NRC) undertook a countrywide survey of snow loads on roofs. This survey provided evidence on the relationship between ground and roof loads and enabled the committees responsible for the 1960 edition of the NBC to adjust the code requirements.

In 1960, roof loads were set at 80 per cent of the ground load, and they were adjusted to allow for the increase in the load caused by rainwater absorbed by the snow. At the time, NRC researchers recognized, if a roof was cold and in a sheltered area, the snow load remained at 100 per cent of the ground snow.

However, this possible variation (now Cb) was not documented in the structural commentaries or the NBC. By 1965, all roof loads were directly related to the snow load on the ground. The basic design load remained at 80 per cent of the ground load, but a snow load of 60 per cent of the ground load (Cw) was allowed for roofs exposed to the wind.

Snow accumulations were accounted by means of snow load coefficients or accumulation factors, and these were shown in the form of simple formulas and diagrams, similar to those still used in 2015.

A new slope reduction formula was given for unobstructed slippery-sloped roofs in 1990. The minimum Cw was reduced to 0.5—rather than 0.75—for exposed roofs north of the tree line. Design roof snow loads were separated into snow Ss and rain Sr components. In 2005, the return period for snow loads was increased from 30 years to 50 years. This change harmonized the Canadian and American standard. The increase in loading on large-area roofs (which was previously captured in the accumulation factor Ca) was taken out of Ca and incorporated into the basic snow load factor, Cb.

Presently, in NBC 2022, snow load provisions are found in Article 4.1.7. The equation for snow load is:

S = Sr + Cw Cb Ca Cs Ss

Two of the variables are affected by architected details.

Many engineers believe the value of Cb remains constant at 0.8, no matter what roofing system or amount of roof insulation is used. When this value was first considered, the researchers involved debated whether Cb should be one or less. As Dr. D.A. Taylor reported in his paper, “Roof Loads in Canada,”

“The design load coefficient for a uniformly on a well-insulated or unheated roof in a perfectly sheltered location Cb = 1.0.”

An R30 insulated roof is well-insulated and a ventilated roof is unheated. This is demonstrated in the ponding case study presented in this article.

For the sliding factor Cs to be considered, the snow needs a path to slide off. If snow is prevented from sliding off due to snow guards, parapets, or another structure, the roof must take the full snow load.

The snow load provisions in ASCE-7 are similar to Canada’s provisions, but with some significant differences. Snow loads are dealt with in Chapter 7 and the snow loads calculated from the following equation:

pf = Cb Ce Ct pg

ps = Cs pf

= Cs Cb Ce Ct Is pg


Pg = Ground snow load based on a reliability analysis

Ce = Exposure factor

Cb = Ground to roof factor set at 0.70

Ct = Thermal factor

Cs = Sliding factor 1.0 ≥ Cs ≥ 0.0

The differences are the ground snow load pg and the basic roof factor Cb. In Canada, it is a combination of the basic roof factor and the thermal factor. In the 2022 edition of ASCE-7, the thermal factor was increased by 20 per cent in value to reflect the effects of increased roof insulation or vented roof and increased 30 per cent where a complete building was used as a refrigerator.

In 2020, a major project was launched to generate ground snow loads across the country to reflect a more consistent level of safety matching what is required by ASCE-7. Achieving a more consistent level of safety was done following a “reliability approach.” This method could only be done at this time because of the amount of computing capacity required to do this. The work was done in the following manner:

The snow load was compared to the resistance of a steel roof.

○ The ground snow load probability distribution based on annual maximum snow load records across the country.

○ Snow depth to load conversion equations for different parts of the country at locations where only snow depth was measured.

Using all these distributions, Monte Carlo simulations were done. Snow loads that would cause failures were found at each site for low-occupancy buildings, normal occupancy buildings, and high importance only for an acceptable number of times.

The new snow maps have improved the prediction of ground snow loads across the U.S. The project results improved the prediction of snow loads across the country. The new ground snow load maps have less areas requiring special studies to establish ground snow loads and the new maps have dealt with areas where local codes used different loads than in ASCE-7.

It must be noted, for the ground-to-roof conversion, the data from the snow study performed by the NRCC between 1957 and 1968 was used.

Occupancy loads

Roofs not only have to resist environmental loads but also loads from human use of the roof area. Most of the occupancies are ones which architects are used to dealing with. The one loading that is different, is an allowance for doing maintenance work on the roof. Both Canadian and U.S. codes have requirements for a minimum roof live load for maintenance work on the roofing system to prevent collapse.

The minimum live load requirements for both countries are similar. Prior to 1985, there was confusion about the purpose of the minimum roof live load. The load allowance was considered by many as a minimum snow load. Since 1985, the NBC  clarified the minimum roof live requirements in Article 4.1.5, which directs the designer to Table for an area load of 1.0 kPa (20.9 psf) and Table for a point 1.3 kN (290 lb).

The structural commentaries, since 1985, explain these load allowances are for use and occupancy loads, not a minimum snow load intended to provide for maintenance loadings, workers, and so forth and the load allowances are not reduced as a function of area or roof slope.

The U.S. has had minimum load provisions since 1943. The provisions, in their present form, first appeared in the Uniform Building Code (UBC) in 1949. They allow the minimum load to be reduced based on the tributary area the roof member is supporting and on the roof’s slope. The requirements are:

In 1988, a lower value for the minimum roof load was allowed if repair work is done from the ground and approved by the local authority.

Dead loads

With roofs, the contractor needs to know what weights are always on the roof structure and what allowances are provided for items that may or may not be there. There are many terms for describing these dead loads that might be there. This author prefers the term “collateral dead load.”

The load allowances for mechanical ductwork and architectural bulkheads are examples of collateral dead loads. The reason for identifying these is when:


The locations of mechanical units, ductwork, and architectural bulkheads should be reviewed with the structural engineer. Often, these items are handled by delegated design and the structural engineer is never made aware of them. This is especially true for architectural bulkheads and ceilings.

Connection details for hanging loads have been identified as an area structural engineers should be aware of and not left to a sub-trade to do. As a minimum, the engineer for the sub-trade should know the type of detail that is acceptable to the structural engineer of record.

Project manual

Design of the roof and its systems and components involve all the design disciplines, for example, roofing Division 7, structure Divisions 3, 5, or 6, reflected ceiling plans and bulkheads gypsum board assemblies Division 9, lighting Division 26, and HVAC ductwork Division 23.

Roof design can be handled in many ways: by the owner’s design team, delegated designers, or a combination of both. Previous articles published in Construction Canada have recommended processes for delegated design. The same considerations also apply if the owner’s design team (professionals of record) do all or part of the design work. The transfer of design information is the same for all these approaches.

The author’s preferred approach of interacting and sharing design information is to supply it through drawings and specifications. In the author’s case, they need to supply the loads the roof components must resist and how they should attach to the roof structure. The load information that needs to be conveyed is shown below:

   ○ Constant

        ■ Self-weight, mechanical units

○ Collateral

■ Mechanical ductwork

■ Architectural finishes (ceilings, bulkheads)

○ Minimum live load

○ Load on roof membrane

○ Projections off the roof (mechanical units and screens)

○ Basic snow load

○ Snow buildup due to

■ items on the roof (mechanical units and ductwork), screens

■ roof geometry

○ Distribution due to roof geometry and deflection of members

Article 2.2.4 Division C Volume 1 of the NBC requires the following information to be shown on structural drawings:

  1. a) sufficient detail to enable the dead loads to be determined
  2. b) all effects and loads, other than dead loads, used for the design of the structural members and exterior cladding
  3. c) the dimensions, location and size of all structural members in sufficient detail to enable the design to be checked
Figure 10

The author puts the main portion of the design loads on the roof structural drawings and adds the required information in the general notes (Figure 10).

Mechanical ducting or gypsum assemblies are often supported off the roof structure. This can be done in two ways: typical details on the structural drawings with the specifications cross referencing the details or sketches included in the specification section of the sub trade. Often, the latter are missed and catch the sub trades by surprise. Commonly, architectural sub trades do not look at, or are not given, the structural documents by the general contractor.

One of the greatest challenges is co-ordination of the roof design and getting mechanical, electrical, and architectural information supplied to the structural designer. Often, co-ordination is seen as how all the different trades geometrically fit together. Co-ordination is not thought of as the cost implications of what is being designed by the different trades.

In the case of roof design, the impact of hanging loads have a significant impact on the structure if the clear spans of the roof structural members exceed 9 m (29.5 ft). The author recommends the project’s co-ordinating professional receive line drawings showing weights for mechanical ductwork, fire sprinkler lines, lighting, and architectural finishes. Transferring this information is best done during design development but can also be part of the submittal process during construction. The coordinating professional can identify the impact of these weights on the structure and optimize the layout of the suspended loads. In the author’s experience, there is significant structural savings if the loads are evened out across the roof structure.


Design of roofs require co-ordination and cooperation of all the design disciplines to ensure no design details are overlooked. Each separate discipline impacts the other often more than designers recognize; therefore, communication is critical.

Considerations, all of which must be recorded in the project manual, include extreme weather systems such as wind gusts, tornadoes, hail, rain, snow, and both occupancy loads and dead loads on roofs.







Future Weather

Author’s note: I would like to thank the following people who reviewed this article, provided advice and emendations on the details, Jan Dale P.Eng., technical director, principal with RWDI; Keith Robinson FCSC, RSW, LEED AP, associate, specifier with DIALOG; and Dr. Brennan Bean, associate professor, Utah State University.

Discussion on structural considerations in roof design in the U.S. has also been included as products produced in the U.S. are often proposed for use in Canadian projects. Ensuring that technical information is interpreted correctly can prevent costly mistakes.


[10]David Thompson is a principal at KTA Structural Engineers Ltd. of Calgary, Alberta. He has been a professional engineer for more than 35 years. He has been involved in the design of hospitals, student residences, office buildings, film studios, and sports facilities. Thompson specializes in the design of tension membrane structures and has dealt with projects in 55 countries. He was a member of the Canadian Standards Association (CSA) committee for CAN/CSA S157, Strength Design in Aluminum/Commentary on CSA S157-05, Strength Design in Aluminum Design of Aluminum Structures, and he serves as a member of ASCE Standard Committee for ACSE-55 Tension Membrane Structures. Thompson has been a member of CSC since 1990 during which he served on the Canadian Construction Documents Committee (CCDC) for 10 years representing the Association of Consulting Engineering Companies. He can be reached at

How loads are specified
Nothing in safety, resistance, or loads for structural design is cut and dry. Most of the variables shown in this article can vary, and each variable has a probability distribution. For example, roof snow loads S = Sr + Cw Cb Ca Cs Ss Cw Ca Cs are educated approximations while for Sr, Cb, Ss can be modelled using probability distributions.

Considering the period building codes have been used, snow loads have been specified in the National Building Code of Canada (NBCC) since 1953. Technology has advanced exponentially while more environmental data has been collected. The improvement in computing capability has allowed loads and safety to be addressed in a more sophisticated manner than in earlier years.

The following is a brief history on the development of loads, but first, here are some terms that will be used that need explanation:

  • Specified load—load set so the probability of exceeding the specified load is only between one and 10 per cent. The load is usually set with a two per cent chance of exceedance.
  • Characteristic or nominal resistance—resistance set so the probability of the nominal resistance is only between one and 10 per cent. The resistance is usually set with a five per cent chance of being less.
  • Accidental (rare) load—load the structure must support during the life of the structure.
  • Ultimate (factored) load level—a combination of loads the structure must support during the life of the structure.

Step 3: Limit States design approach allowed loads to be combined in a manner that provided a more consistent margin of safety than allowable strength design.1

Step 4: With the additional environmental data, the accidental (rare) load could be better defined. Use of the accidental load captures unique situations that were not caught by using a specified load with a load factor. For example, on the coastal areas of the U.S., there are significant spring snow falls. The effect of these storms was not accounted for previously but are now. The 2020 snow study increased the design snow loads two to three times in some coastal areas. This change reflected local knowledge of snow loads for those municipalities.

Step 5: Is an exciting development, it shows multiple probability distributions can be used to establish design loads, where in the past, only one probability distribution was used for each load type.

  1. Loads were first estimated and if they were used in combination with material design standards of the time, they produced safe, robust, and durable structures, for example, U.S. minimum roof loads.
  2. In the 1960s, statistics were used to calculate specified loads. Usually, these were set so the probability of the loads would only exceed two per cent of the time over a year. With Allowable Stress Design, a factor of safety would be used to arrive at a member size. For example, Canadian snow loads.
  3. In the 1980s, Canada used Limit States design. So, instead of using a Factor of Safety, the loads were increased by a load factor and the resistance was reduced by a Resistance Factor. The term Factor of Safety was replaced with Margin of Safety.
  4. Over the past 10 years in the U.S., instead of using the specified load set at two per cent, it was set as an accidental load. For example, U.S. wind loads.
  5. In 2022, the U.S. design snow load was set based on the consistent probability of failure (where the load value exceeds the resistance capacity). It considers both Cb and Ss probability distributions.


1 For more on Limit States Design in Canada, read the paper “Safety and Limit States Design for Reinforced Concrete” by J.G. MacGregor published in the Canadian Journal of Civil Engineering, V. 3, No. 4, Dec. 1976, pp. 484-513.

Ponding case study
This case history is of a building in Alberta shows ponding failures are not isolated to steel structures and not solely caused by rain. It also demonstrates the possible consequence of upgrading the roof insulation and the need to check the impact of the colder roof.

This building was originally constructed 1968 and has had three additions. The roof structure was constructed level using sloped insulation to provide drainage. The structure system was 38 mm (1.4 in.) wood decking, supported by 89 x 280 purlins, spaced at 2400 mm (94 in.) on centre, connected to 178 x 743 glue-laminated timber (glulam) beams spaced at 4800 mm (188 in.) onto 225 x 826 glulam beams.

The analysis of the as built condition of the roof found the structure met the requirements of the timber design standard CSA O86- 1970 and the National Building Code of Canada (NBCC) 1970. In 2018, using ponding theoretical calculations, the author found the roof was susceptible to ponding failure.

The cause of the roof members’ failure was due to excessive snow and water on the roof. Environmental records indicate the maximum-recorded ground snow depth in the area, in 2018, was between mid-February to the beginning of March. However, historical weather records for the area showed higher ground snow depths since 1968 occurred in 1978 not 2018. The only change identified was the roof was upgraded using R30 insulation in the early 2000s.

The amount of snow depth on the roof was higher than the recorded ground snow depth. The roof system was overloaded when the maximum ground snow loads were mid-February causing some permanent deflections and cracking in the wood purlins. When the snow on the roof melted, water ponded in depressed locations. The additional load due to the ponded water caused the purlins to have a complete rupture failure.

Luckily, the problem was caught before there was a total collapse and the building is again operational with addition purlins supporting the roof deck.

Snow management case study
In the early 1990s, commercial snow guards were installed on the roofs of Alberta’s Chateau Lake Louise which failed. The firm the author worked for was asked to investigate the failure and, over time, the assignment grew into a snow management plan and replacement of the snow guards. The project provided vast experience in designing snow guards, dealing with microclimates, and managing snow.

Lake Louise is a glacier-fed lake in the Rocky Mountains. The Chateau Lake Louise is located at the east end of the lake and is 165 m (541 ft) higher and 1.8 mi (3 km) west of Lake Louise Village. In the Alberta Building Code 1990 Edition, Lake Louise Village was listed as having a design snow load of Ss = 6.3 kPa (132 psf) and Sr = 0.3 kPa (6.3 psf).

When Environment Canada was asked what the snow load for the Chateau was, the firm was told it was the same as the village. Environment Canada’s ground snow models predicted there would be a greater depth of snow, but it would be lighter due to the higher elevation. The model did not consider the effect of moisture from the lake. The firm received advice from local ski guides, later confirmed by site measurements, and used a much higher design ground snow load. In the 2019, Alberta edition of the National Building Code (NBC), the Chateau Lake Louise site design ground snow load was 0.7 kPa (14.6 psf) higher than Lake Louise Village.

The roofs at the chateau and parkade were standing seem metal roofs to encourage the snow to slide off. However, the roofs would have shed snow on pedestrian walkways or cars below; therefore, the snow needed to be held on the roofs for safety.

When replacing the existing snow guards, there were several challenges. The guards needed to be much higher and larger than anticipated. In addition, the roof structures needed additional reinforcement.

Often, people think of snow as monolithic light material, as it is when it first falls; however, snowpacks are not. Snow compacts with time; if held in place for a lengthy period, it can turn into ice. A 1 m (3.3 ft) depth of fresh snow compacts to 500 mm (19 in.) in a day and can reduce to about 250 mm (9 in.). For example, in Lake Louise, the design ground snow load represents up to a depth of 1.6 to 3.2 m (5.2 to 10.4 ft) of snow on the roof. When using guards, all the snow must be held on the roof over the winter season; one cannot rely on some of the snow sliding off. Over the winter, there will be weather conditions that create weak layers in the snowpack on the roof, similar to how snow behaves on a mountain. The guard must prevent snowpacks from sliding on the roof’s surface in multiple areas.

In the areas the author was assigned to design, his firm used 1200 mm (47 in.)-high hollow structural section (HSS) posts anchored to the roof’s perimeter steel structure, with the top of the HSS post tied back to the peak of the roof by 12 mm (0.4 in.) cables. This sounds excessive, but snow guards did not look out of place when buried in 2 m (6.5 ft) of snow with cars parked directly below the roof.

In high snow load areas, such as the Rocky Mountains, the author recommends architects design the snow sheds safely away from any pedestrian traffic below or consider having a near flat roof to carry the full ground snow load.

Climate Change and Effect on Table C-2 of Volume I, Division B, Appendix C, of the National Building Code of Canada (NBCC)
The design value found in Table C-2 has been derived from historical weather data.

What will happen with the effects of global warning? Environment Canada commissioned a study on how global warming may affect the design criteria. Some findings from the study, “Climate-Resilient Buildings and Core Public Infrastructure 2020: An Assessment of the Impact of Climate Change on Climatic Design Data in Canada” are listed below.

It is certain that Canada’s climate will warm. The report projects temperatures will increase at twice the rate of the global mean temperature. The increase in seasonal temperatures will occur across the country, with greater warming happening in the north of Canada during the winter months.

The report indicated very low confidence in the effects increase of temperature on design wind pressures. The figure, “Changes in Hourly Wind Pressure q50 versus change in Temperature” shows a modest increase in wind pressures with temperature increases. However, the shaded portions of the graph show the increase will not be the same in every part of each region, and the design wind pressures may even decrease in some locations.

The authors of the report had medium confidence in their projections for precipitation. They believe annual, and winter precipitation (rain or snow) will increase everywhere in Canada, with Northern Canada having the largest increase. Summer participation is projected to be less in Southern Canada.

24-hour and 15-minute rainfall
There is even less confidence in the projections for changes in the 24-hour and 15-minute rainfalls. The report recommends accounting for a seven per cent increase in these two values for each degree Celsius increase in temperature.

It is highly likely that snow cover duration will decline over Canada with an increase in surface air temperature. Decreases in seasonal snow accumulation are projected over much of southern Canada. Northern regions of Canada are predicted to have a slight increase in snow accumulation with increased temperature. The changes in snow accumulations are shown in the following two figures.

The report “Climate-Resilient Buildings and Core Public Infrastructure 2020” can be found here,

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