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Why Wind Design Is Important for Your Commercial Building

Posted by Art Valentz on December 13, 2018

How Important is Wind Design for Your Commercial Building

If you happen to own a building with a roof, the Bernoulli effect is out to get you. Daniel Bernoulli, the 18th-Century Swiss mathematician, recognized the following principle: The faster a fluid is moving, the lower its pressure. You may recognize this principle as the effect that creates lift under an aircraft’s wing. This principle is also in effect, however, when a very strong wind tries to tear the roof off your walls. The air in a strong wind is moving very fast—therefore, its pressure is very low. The air inside your building is moving very slowly relative to the wind, so its pressure is relatively high. The area of high-pressure air inside your building wants to rise into the low-pressure air outside it—and it doesn’t care that your roof is in the way.

The specific force we’re referring to here is known as uplift. The forces acting to prevent uplift are known as uplift resistance. Every roof is built with a certain amount of uplift resistance in mind, but this amount tends to vary from region to region based on how much wind the area receives. In general, the amount of uplift your building must resist is governed by an international building code known as ASCE 7 (this standard also covers loads related to ice, earthquakes, and even tsunamis). Knowing about ASCE 7 and meeting its requirements are far from the same thing, however. Here are a few of the best ways to meet ASCE 7 requirements and create a wind-resistant roof.

Step One: Know Your Wind Map

ASCE 7 divides the country based on a wind map which demarcates the average maximum wind speed that each region is ever likely to receive. These maps change on a rotating basis—each new revision revises regional wind speeds based on newer and more accurate data. The latest revision, ASCE 7-16, was published in 2017 and is expected to take hold nationwide in 2019 once it is adopted on a state-by-state basis. While this new version of ASCE 7 does revise many calculations for commercial buildings—creating larger perimeter zones for low-slope roofs, for instance—the overall news for roofing contractors is good. When located outside of hurricane-prone areas, the average wind speed for much of the country has been decreased.

Step Two: Design for Compliance

Depending on your location, you will need your building’s roof to resist a certain amount of uplift pressure, expressed in pounds per square foot. In addition, the calculations for uplift pressure will consider how high your building is, how many openings there are, and whether there are surrounding obstacles such as hills. Higher buildings with more windows must resist additional uplift pressure. Buildings surrounded by hills must resist less.

Based on the amount of uplift you must resist, you must design a roof in a certain way, using certain materials, techniques, and reinforcements. In high-wind areas, this might mean:

  • Switching to a metal roof with additional clips and fasteners. According to FEMA, these roofs can withstand winds of up to 170 mph.
  • Using more and better-quality fasteners alongside a single-ply roof.
  • Consider spray polyurethane foam (SPF). When SPF is adhered directly to a concrete roof deck, its uplift resistance has been tested at or around 900 pounds per square foot.

Once design is completed, you will need to test your design. A number of organizations—the largest of them being FM Global—will test roofing systems for wind uplift resistance. To pass the approvals process, your roof deck must generally consist of either 22-guage steel, structural concrete, 3/4” fire-retardant plywood, or fiberglass-reinforced plastic. Building owners often specify FM Global-approved roofing systems and their designs, and since FM Global bases their approvals process on ASCE 7, it is important to understand both organizations and their standards while building or designing a wind-resistant roof.

Step 3: Go Above and Beyond by Building a Resilient Roof

Here’s the thing—ASCE 7 represents a minimum boundary for compliance. It is responsible, perhaps even vital, to design your roof so that it can resist a wind speed higher than code may indicate. This is what is meant by a resilient roof. Resilient roofs are designed with the future in mind—a future where weather gets worse, and major wind events become more frequent.

In terms of construction, resiliency means reinforcing the wind zones that experience the strongest uplift force. According to ASCE 7, the corners of your building’s roof—zone 3—receive the strongest loads during high winds. Zone 2 is the roof perimeter and receives the second-strongest load. These zones should be reinforced with durable materials, including sacrificial layers of material designed to blow off first while preserving the main structure of the roof. Redundant flashings can help hold material down if the roof edge gives way. Non-penetrating fasteners will prevent wind or wind-driven rain from finding a toehold.

Lastly, resiliency means attaching other structures to your roof that will increase survivability after the storm has passed. This may mean installing solar panels to keep the power running in spite of a widespread outage. It could also mean installing skylights so that building tenants can continue to see what they’re doing without power. It could even mean installing rainwater catchment systems to keep the plumbing working and tenants hydrated during a boil-water order.

ASCE 7 is More than Just a Checkbox

Compliance with ASCE 7 is not optional. Every time this standard is updated, it is incorporated into state and city building codes and becomes law, with penalties for non-compliance. Once you understand where your building is in relation to the updated ASCE 7-16 wind map, you should proceed by building to the necessary standard or beyond.

At PHP Systems Design, we strongly recommend going beyond ASCE 7 standards. Building a resilient roof doesn’t just mean you’ll be better able to ride out storms—rather, it means you’ll be able to start working again as soon as they stop.

Topics: Roof Architecture

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