What are wind pressure zones 1′, 1, 2, and 3 on a low-slope roof?

Direct Answer

Wind pressure zones 1′, 1, 2, and 3 divide a low-slope roof into areas that experience different levels of wind-induced pressure and uplift. This four-zone labeling is commonly associated with FM Global roof wind-design guidance. In general, Zone 1′ is the interior roof field, Zone 1 is the outer field area, Zone 2 covers the perimeter, and Zone 3 covers the corners, where wind effects are typically most severe.

These zones help engineers and roofing professionals determine the wind-uplift resistance needed for the roof deck, insulation, membrane, fasteners, adhesives, and other components of the complete roofing assembly. Perimeter and corner areas generally require greater uplift resistance than the interior because wind can create stronger localized suction around roof edges and corners.

For Florida commercial buildings, the applicable design must follow the governing Florida Building Code and referenced wind-design standard, or the specified FM Global criteria where applicable. The four-zone FM Global scheme should not be confused with the roof-zone diagrams used in every edition or application of ASCE 7. The correct zone layout and design pressures depend on the applicable standard, building geometry, roof height, wind exposure, and other project-specific factors.

References: FM Global Data Sheet 1-28: Wind Design and the Florida Building Code, Chapter 16: Structural Design.

What Does Each Low-Slope Roof Wind Zone Mean?

Wind zone	Typical location	Wind-design significance
Zone 1′	Interior roof field on sufficiently wide roofs under the applicable FM Global layout.	Typically experiences lower localized uplift demand than the outer roof areas.
Zone 1	Main field area closer to the roof perimeter, outside the designated perimeter and corner zones.	Requires a roof assembly rated for the calculated field-area wind pressure.
Zone 2	Roof perimeter strips along the edges, excluding the separately designated corner areas.	Usually requires greater uplift resistance because wind flow separates and creates increased suction near roof edges.
Zone 3	Roof corners, where two perimeter edges meet.	Often has the highest localized uplift demand and may require the strongest assembly rating and enhanced attachment.

The exact dimensions of these zones are determined by the applicable design method. They should be taken from the project’s wind-design calculations or the relevant standard rather than estimated from a generic diagram.

Why Are Wind Pressures Different Across a Roof?

Wind does not act uniformly on a flat or low-slope roof. As wind moves over a building, it accelerates around edges, separates from roof surfaces, and produces localized pressure fluctuations. These effects can create upward forces that try to pull the roofing membrane, insulation, and other roof components away from the structure.

Corner regions are especially important because wind approaches and flows around two intersecting building edges. Perimeter areas also experience significant suction. The central roof field is generally less exposed to these localized effects, although it still must resist its calculated design pressure.

In Florida, these considerations are especially important for buildings exposed to hurricane-force winds. Roof-system design must account for the applicable wind criteria and the capacity of the entire assembly, not merely the membrane’s strength.

How Do Wind Zones Affect Roofing System Selection?

1. Roof Membrane Attachment

Mechanically attached membranes rely on fasteners and plates to transfer wind-uplift forces through the roof assembly to the supporting deck. Fastener type, spacing, deck capacity, and the attachment pattern must suit the calculated design pressures in each roof zone.

Adhered membranes rely on an approved adhesive system and the strength of the supporting layers. Adhesion alone is not proof of adequate uplift resistance; the complete assembly must have the required tested or approved performance.

2. Insulation and Cover Board Attachment

Insulation boards and cover boards can be lifted or separated when the attachment system cannot resist the applicable pressure. The number and placement of fasteners or the specified adhesive application may differ between the field, perimeter, and corner zones.

Roofing contractors should follow the approved assembly’s installation instructions and the project’s attachment schedule. They should not assume that a pattern suitable for Zone 1 is adequate for Zones 2 and 3.

3. Roof Deck and Structural Connections

Wind uplift loads are transferred through the membrane, insulation, attachment system, and roof deck into the building’s structural framing. A strong membrane cannot compensate for a weak deck connection or inadequate supporting structure.

Where calculations identify high uplift pressures, the structural engineer may need to evaluate deck attachment, supporting members, and connections as part of the complete load path.

4. Perimeter and Corner Detailing

Edges and corners require careful attention to membrane terminations, flashing, parapets, edge metal, and other details. A roof assembly can perform poorly if the field attachment is adequate but its perimeter terminations or edge components are not designed for the applicable wind loads.

For Florida projects, these details should be coordinated with the required product approvals, tested assembly, installation instructions, and applicable building-code provisions.

Does Zone 1′ Always Appear on a Low-Slope Roof?

No. Zone 1′ is part of particular wind-design systems, including the FM Global scheme, and is used where the applicable building dimensions and zoning criteria call for it. It should not be assumed to exist on every roof or to be interchangeable with similarly named zones in another standard.

In the FM Global approach, the interior Zone 1′ applies to relatively wide buildings under the conditions described in the relevant data sheet. Smaller roofs may not have a separate Zone 1′ area because the perimeter and corner regions occupy more of the roof surface.

By comparison, the 2023 Florida Building Code references ASCE 7-22 for wind-load determination. The zone labels and procedures in that standard must be applied according to the relevant building type, roof geometry, and design method. A drawing using FM Global Zone 1′ should therefore be read in the context of the specified FM criteria rather than automatically interpreted as an ASCE 7 diagram.

How Are Roof Zone Dimensions Determined?

The zone boundaries are not universal fixed-width strips. The applicable standard uses building geometry and other design parameters to establish the dimensions of the field, perimeter, and corner areas. Relevant factors can include:

• Building length, width, and mean roof height.
• Roof slope and geometry.
• Applicable wind-design standard and edition.
• Building enclosure classification and internal pressure.
• Site wind speed and exposure conditions.
• Parapets, roof-height changes, and adjoining structures where the design method accounts for them.

The design professional should identify the governing method and show the applicable zones on the roof plan. The contractor should use those zones when applying the specified attachment patterns and approved roof assembly.

How Do You Determine the Required Wind-Uplift Rating for Each Zone?

Each zone’s required resistance must be based on the project’s calculated design pressure and the applicable assembly-rating method. A typical process is:

1. Establish the governing criteria. Identify the applicable Florida Building Code edition, referenced ASCE 7 provisions, or FM Global requirements specified for the project.
2. Determine site-specific wind parameters. Establish the applicable design wind speed, exposure, risk category, building enclosure, and other required inputs.
3. Calculate zone pressures. Determine the applicable uplift demand for the field, perimeter, and corner areas.
4. Select a suitable roof assembly. Confirm that the complete assembly and its attachment method can meet the required pressures under the applicable rating procedure.
5. Verify the supporting structure. Confirm that the deck and connections can transfer the uplift loads into the building structure.
6. Document installation requirements. Show the zone boundaries, attachment patterns, approved products, and relevant details on the project documents.

For FM-rated assemblies, the FM Global Wind Design Data Sheet 1-28 explains the applicable methodology. For code-regulated Florida projects, consult the applicable code and structural design documents.

Common Mistakes When Interpreting Roof Wind Zones

• Using one attachment pattern everywhere: The field, perimeter, and corners may have different design demands.
• Assuming Zone 3 has the same pressure on every building: Actual pressure depends on project-specific design inputs.
• Confusing FM Global and ASCE 7 zone labels: The labels and boundaries must be interpreted under the specified standard and edition.
• Checking only the membrane: The entire assembly, including insulation, fasteners, deck, and structural connections, must resist the applicable loads.
• Ignoring roof edges and terminations: Perimeter flashings and edge details are critical parts of wind-uplift resistance.
• Using a generic diagram as a construction detail: The project-specific roof plan and approved assembly govern installation.

Bottom Line

Wind pressure Zones 1′, 1, 2, and 3 help distinguish the interior roof field, outer field, perimeter, and corners under the applicable FM Global wind-design approach. Wind uplift demand generally increases toward the perimeter and corners, so these areas often require stronger attachment and higher-rated roof assemblies.

For Florida commercial roofing projects, use the zone layout and pressure calculations required by the governing design standard. Verify that the complete roof assembly, including its supporting deck and connections, meets the calculated wind-uplift requirements before installation.

Related Questions

• How does building height affect roof uplift pressures?
• What’s the difference between static and cyclic wind-uplift testing?
• What is a fastener pull-out test, and how many tests are required?
• What are fastener pull-out testing requirements for existing decks?
• What happens if a commercial roof is installed without proper product approval?

Sources

• FM Global Property Loss Prevention Data Sheet 1-28: Wind Design
• Florida Building Commission: Wind Loads — Impacts from ASCE 7-22
• 2023 Florida Building Code, Building — Chapter 16: Structural Design
• FM Global Property Loss Prevention Data Sheet 1-52: Field Verification of Roof Wind Uplift Resistance

Last reviewed: October 2026

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