Determining the Loads Acting on a Rooftop

Simply put, a roof assembly must be able to resist the design wind loads acting on the rooftop. The loads acting on a roof must be calculated in order to select a roof system that has the necessary capacity (i.e., wind uplift resistance). Therefore, step one is to determine the loads acting on the roof of a specific building.

There are a number of factors that determine the design wind uplift loads for the field, perimeter and corners of a roof. In order to determine the wind loads acting on a roof, the architect/designer needs to know the following about a building—location; building code that is in effect at the building’s location; height, length and width; exposure category; use and occupancy; enclosure classification; topographic effects; and ground elevation.

Location. The location of the building within the United States tells us two things which must be determined in specific order. The location directs us to the specific version of the IBC or the applicable building code that is in effect for the project. For example, if the 2006 or 2009 IBC is in effect, then ASCE 7-05 governs. If the 2012 or 2015 IBC is in effect, then ASCE 7-10 governs. If the 2018 IBC is in effect, then ASCE 7-16 governs.

Height, Length, Width. Determining the height, length and width of a building should be straightforward and a vast majority of buildings are predominately square or rectangular in shape, or in general, have square or rectangular roof areas. Note: there are methods to determine the wind loads acting on a roof for non-rectangular or non-square buildings; however, that is outside the scope of this article.

Exposure Category. Exposure category is based on the roughness of a building’s nearby terrain. A terrain’s surface roughness is determined from natural topography, vegetation and the surrounding construction.

ASCE 7 uses three Surface Roughness Category types—called B, C and D—which in turn, defines three Exposure Category types, also called B, C and D.

Exposure Categories B, C and D are generally defined as follows:

• Exposure B is applicable to buildings with a mean roof height of less than or equal to 30 feet and where surface roughness B prevails in the upwind direction for a distance greater than 1,500 feet tor buildings with a mean roof height greater than 30 feet, Exposure B shall apply where surface roughness B prevails in the upwind direction for a distance greater than 2,600 feet or 20 times the height of the building, whichever is greater.
• Exposure C is applicable for all cases where Exposure B and D do not apply.
• Exposure D is applicable where surface roughness D prevails in the upwind direction for a distance greater than 5,000 feet or 20 times the building height, whichever is greater. Exposure D also applies where the ground surface roughness immediately upwind of the site is B or C, and the site is within a distance of 600 feet or 20 times the building height, whichever is greater, from an Exposure D condition.

Use and Occupancy. The use and occupancy of a building is used to determine the “Occupancy Category” in ASCE 7-05 or “Risk Category” in ASCE 7-10 and ASCE 7-16. They are effectively interchangeable terms, however, they are addressed differently. ASCE 7-05 uses Occupancy Category to determine the value to use for the Importance Factor. In ASCE 7-05, Importance Factor is a stand-alone factor in the velocity pressure calculations, and why there is one map in ASCE 7-05. ASCE 7-10 and 7-16 incorporates Risk Category (i.e., importance factor) into the wind speed maps, and that is why there are 3 maps in ASCE 7-10, and 4 maps in ASCE 7-16. In general, the greater the importance of a building, the higher the Importance Factor or Risk Category which results in higher uplift pressures.

Exposure Classification. This factor essentially relates to the possibility that a building will become internally pressurized during a wind event. For ASCE 7-05 and ASCE 7-10, there are three classification types: Open, Partially Enclosed, and Enclosed. ASCE 7-16 amended these classification types by adding another type called, “Partially Open” and also revised some of the definitions. The ASCE 7-16 classification types are Open buildings, Partially Open, Partially Enclosed and Enclosed buildings.

Using “Partially Enclosed” as the building type results in an increase in the design wind pressures in the field of the roof versus an “Enclosed” or “Partially Open” building—all other factors held equal. This is significant. Selecting an “Enclosed” or “Partially Open” building when it could become a “Partially Enclosed” building if doors and windows are blown out during a high wind event could result in a roof system without the adequate capacity to handle the anticipated higher loads.

Topographic Effects. Research and experience has shown that wind speeds can increase significantly due to topographic effects. The wind speed increase is known as a wind speed-up effect. An abrupt change in the topography, such as escarpments, hills or valleys can significantly affect wind speed. ASCE 7 addresses these speed-up effects by applying a multiplier to account for topography in the velocity pressure calculations.

For more in-depth information about determining wind loads, read this blog.

An architect/designer needs to know a building’s location; the building code that is in effect at the building’s location; its height, length, and width; the exposure category; the use and occupancy category; the enclosure classification; any topographic effects; and ground elevation in order to determine the wind loads acting on a roof. Some of these selections may seem straight forward, but some impart a higher resultant design wind load, especially when compounded by similar risk-averse choices. For more information about resilient roof systems, read this blog.

The process of determining the loads acting on a rooftop results in design wind pressures for each roof zone and, importantly, determines the dimensions and overall size of each of the roof zones. Providing this information on the construction documents ensures the contractor and manufacturer (together or separately) can provide an appropriate roof system with the tested capacity that exceeds the design wind pressures in each of the roof zones.

Revisions to ASCE 7-16

Eventually, we will all use ASCE 7-16 as the basis for determining design wind loads for our roofs. To that end, we will need to understand what has remained the same, what is changed, and what has been added to the latest version of ASCE 7.

Basic differences between versions of ASCE. There are some noteworthy differences between the three ASCE 7 editions and they include: the wind speed maps, roof zones, enclosure classifications and external pressure coefficients.

Wind speed maps. Simply put, for the contiguous U.S., ASCE 7-05 has one wind speed map and it is based on the Allowable Stress Design (ASD) method. ASCE 7-10 has three wind maps, based on Risk Category I, Risk Category II, and Risk Categories III and IV, and they are based on the Strength Design method. ASCE 7-16 has four wind speed maps, one for each Risk Category and they are also based on the Strength Design method.

The two design methods used in ASCE-7 are mentioned intentionally. Components and cladding for buildings—which includes roof systems—are allowed to be designed using the Allowable Stress Design (ASD) method. When ASCE 7 changed from the ASD method and went to the Strength Design (a.k.a., Ultimate) method in the 2010 version, it was still appropriate to design roof systems according to the ASD method, but it wasn’t specifically stated as such. Currently, in the 2016 version, ASCE specifically states that components and cladding (i.e., roofs) can be designed using the ASD method. The 2018 International Building Code also provides similar language allowing the use of the ASD method when determining design wind pressures for roof systems. This is important because using the ASD method reduces design wind pressures by forty percent.

Roof Zones. ASCE 7-05 and ASCE 7-10 have three roof zones: field, perimeter and corner. See Figure 1. The dimensions of the zones are mostly determined by a building’s length and width. ASCE 7-16 added another zone and it presents the potential to have four roof zones: interior, field, perimeter and corner. ASCE 7-16 also revised how the dimensions of the zones are sized; it is now based solely on a building’s height. Depending on the proportions of a building, roof zones 1 Prime and 1 may not exist using ASCE 7-17. This possibility is shown in the case studies later in this article.

Figure 1: Roof zone layout for ASCE 7-05 and ASCE 7-10.

Enclosure Classifications. As covered previously, ASCE 7-05 and ASCE 7-10 have three classification types: Open, Partially Enclosed and Enclosed, while ASCE 7-16 added Partially Open and slightly modified the definitions. These classifications determine the values to use for the Internal Pressure Coefficient, GCpi. These are shown in Figure 2.

Figure 2: Interior Pressure Coefficients, GC_pi, for ASCE 7-16.