Exposure Categories and Surface Roughness

Tue, Dec 05, 2023 at 12:40PM

Riku Ylipelkonen, Owner, Standard Building Advisors and FRSA Technical Advisor

An important consideration for wind uplift calculations is is taking into account where the wind is traveling through to get to the project location. As you can imagine, if wind has a straight path and a slick surface to travel over, the wind will have much more effect than if it must travel over varying surfaces with other buildings, trees and infrastructure to block its path. The way ASCE 7 wind uplift calculations account for this change is with exposure categories. If you were building a new building in a downtown location with taller buildings surrounding this building in all directions, you would expect those taller buildings to protect your new building from wind uplift to a certain degree. In contrast, if you wanted to build a three-story custom home surrounded by one story single-family homes, it would stick out like a sore thumb and be subjected to increased wind uplift exposure. These factors are are taken into account with exposure categories. The three exposure categories are B, C and D. The descriptions will be a little out of order but the explanations will shed light on why that makes sense.

Exposure Category B

Exposure Category BExposure B is tied into the definition of Surface Roughness B. Surface Roughness B is defined in ASCE 7 as urban and suburban areas, wooded areas or other terrain with numerous, closely spaced obstructions the size of a single-family dwelling or larger. Exposure B goes on to declare that this roughness needs to be at least 1,500 ft. in the upwind direction of buildings up to 30-ft. mean roof height and 2,600 ft. for buildings greater than 30-ft. mean roof height. The main features to look for would be bodies of water that are at least 1,500 ft. in dimension, golf courses, large paving or parking stretches and other features that would allow wind to travel long distances before hitting the project location. For most neighborhoods in Florida, this is the exposure category that applies. The calculations adjust for exposure in the Velocity Pressure Coefficient, Kz. For exposure B projects, this can be as much as a 30 percent reduction in wind uplift design pressures. In other words, your close neighbors are helping your roof perform in wind events.

Exposure Category D

Exposure Category DExposure D is tied into the definition of Surface Roughness D. Surface Roughness D is defined in ASCE 7 as flat, unobstructed area and water surfaces. This includes smooth and mud flats, salt flats and unbroken ice. Exposure D goes on to declare that this surface roughness condition must extend at least 5,000 ft. in the upwind direction. This would also apply if the project were surrounded by surface roughness B or C but is within 600 ft. of a surface roughness D condition in the upwind direction. In Florida, this is prevalent at waterfront locations and even more for islands and seashore developments. For considerations of exposure D, the Kz can be as much as a 31 percent increase in wind uplift design pressures at 60-ft. mean roof height. Even more for higher roofs.

Exposure Category C

Exposure Category CExposure C is somewhat tied to Surface Roughness C. Surface Roughness C is defined in ASCE 7 as open terrain with scattered obstructions that have heights generally less than 30 ft.. This includes flat, open country and grasslands. Exposure C is then defined as all the cases where Exposure B or Exposure D do not apply. The rural, inland parts of Florida have a lot of exposure C. This also shows up in transition areas from the seashore that don’t have enough roughness to be considered B. For considerations of Exposure C, the Kz can be a 13 percent increase in wind uplift design pressures at 60-ft. mean roof height.

While these three categories are well defined in ASCE 7, it is up to the design professional to declare and substantiate the designation of the project exposure category. Some municipalities use a default exposure category definition to boost the robustness of the buildings to perform during a wind event. Another major consideration is when the definitions include “upwind” direction: it is best to apply the term “any” in Florida. While the direction of wind can be predicted in some locations, like downtown urban locations or mountain range valleys, the ability to predict which direction a wind event approaches a specific location within Florida is not rationally definable.

To sum all this up in everyday terms, obstacles work to reduce the overall wind effect on buildings. In Florida, you can make your home cheaper to build if you surround yourself with neighbors or big trees.


Photo Credit: Engineering Express

Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years working for Polyfoam Products. When Polyfoam Products was acquired by 3M, the name was changed to ICP Building Solutions Group. Riku worked at ICP as Technical Services Manager until March of 2023, when he left to begin his own company. Riku is an engineer and is working as a consultant with FRSA. He is a member on FRSA’s Codes Committee, Codes Subcommittee, Tile Committee and on the FRSA-TRI Manual Rewrite Committee. Riku is also a member of the American Society of Civil Engineers (ASCE).

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