Meta description: Learn what an FM 1-90 fastening pattern means, why there is no universal layout, and how to find and install the approved roof assembly for a project.
An FM 1-90 fastening pattern is not one standard spacing diagram. It describes the attachment configuration used in a specific roof assembly that has earned an FM Approval for a 1-90 wind-uplift rating. The correct fastener type, spacing, and placement depend on the roof deck, insulation, cover board, membrane, building zones, and tested assembly. Choosing a pattern by name alone can lead to an unapproved installation or inadequate wind resistance. The reliable approach is to identify the complete approved assembly and follow its current installation details and project requirements.
What FM 1-90 Means
FM 1-90 refers to a roof assembly rating associated with FM Approvals, an independent testing and certification organization affiliated with FM. In general, the number indicates the assembly’s tested wind-uplift resistance rating in pounds per square foot. It is a rating for a complete roof construction, not a guarantee that any particular fastener or roof covering can withstand that pressure on its own.
A tested assembly may include the structural deck, fasteners, insulation, cover board, adhesive or membrane attachment, and other components. Changing one of these elements may change the assembly’s performance. For example, substituting a different fastener, using another deck type, or changing the number of fasteners per board can make the installed roof different from the approved construction.
FM 1-90 should also not be mistaken for a universal building-code wind-speed category. Wind speed, exposure, building height, roof geometry, internal pressure, and local code requirements all affect design. A project’s required assembly rating should be established by the design professional using the applicable code and FM guidance, not inferred from the rating alone.
Why There Is No Universal FM 1-90 Fastening Pattern
Searches for an “FM 1-90 fastening pattern” can suggest that a single diagram applies to every roof. It does not. FM Approval applies to specific, documented combinations of materials and installation methods. Two assemblies with the same 1-90 rating can use different fastening details because they have different decks, insulation layouts, membranes, or attachment methods.
The pattern may refer to fasteners securing insulation or a cover board, membrane plates, a mechanically attached membrane, or the roof deck itself. These are separate attachment functions. A roof system may use one fastening pattern for insulation and another for membrane attachment, and the details cannot safely be treated as interchangeable.
Building geometry also matters. Wind pressure is commonly higher near roof perimeters and corners than in the field of the roof. An engineered design may therefore require different attachment densities in different zones. The exact zone dimensions and fastening requirements depend on the project and the approved assembly; they should not be guessed from a generic chart.
Where to Find the Correct Pattern
The most dependable source is the current listing and installation documentation for the exact FM-approved roof assembly. FM’s FM Approvals website provides information about its certification program, while the RoofNav roof system database is used to search FM-approved roof assemblies. Access and available details may depend on the database and user account.
Before selecting a pattern, gather the project information that determines whether a listing applies:
- Deck type and thickness: Identify whether the substrate is steel, concrete, wood, or another approved construction, and confirm any required deck gauge or thickness.
- Roof components: Record the membrane, insulation, cover board, vapor retarder, adhesives, plates, and fasteners specified for the assembly.
- Attachment method: Determine which layers are mechanically fastened, adhered, or secured by another approved method.
- Wind design requirements: Confirm the required rating and any project-specific perimeter or corner enhancements.
- Manufacturer details: Obtain the current installation instructions and written confirmation that proposed substitutions remain within the approved system.
FM Global’s property loss prevention guidance, including FM Global Data Sheets, can provide additional design and installation context. A data sheet does not replace the assembly listing or project documents. The approved assembly and the construction drawings should be checked together.
How Fastening Patterns Are Described
A fastening pattern is normally specified by the fastener type, plate or washer, number of fasteners, spacing, and position relative to the material being secured. Depending on the assembly, a drawing may show fasteners at board corners, along board edges, in interior rows, or in a prescribed grid. The approved detail may also specify edge distances, fastener penetration, and how adjoining boards are arranged.
For a mechanically attached membrane, the pattern may be expressed as row spacing and fastener spacing within each row. For insulation, it may be expressed as a minimum number of fasteners per board or a layout shown on a diagram. These descriptions are not equivalent: a count per board does not automatically establish the appropriate membrane attachment pattern, and vice versa.
Installers should use the pattern exactly as documented for the selected assembly. A field adjustment—such as moving fasteners to avoid a seam, reducing the number of fasteners, or changing plate dimensions—should be reviewed and approved through the project’s design and manufacturer process rather than improvised.
FM 1-90 Pattern Versus Roof-Zone Design
Wind uplift does not act uniformly across every point of a roof. Wind moving around a building can create localized suction at corners and perimeters, with different pressures in the interior field. Roof-design standards divide the surface into zones so the design can account for these differences. The applicable building code and project engineering determine the required pressures and zone boundaries.
As a result, an FM 1-90 assembly may be suitable for one area or design condition but not automatically adequate for every location on a particular building. A project may call for a higher-rated assembly, additional perimeter attachment, or other approved enhancements. The specific solution must follow engineering calculations and the assembly manufacturer’s instructions.
FM Global publications discuss roof wind design and loss prevention, but a general recommendation should not be used as a substitute for project-specific calculations. The building’s location, height, shape, openings, roof edge conditions, and occupancy-related requirements can influence the design. The authority having jurisdiction and the project’s licensed design professionals may also impose requirements beyond a minimum rating.
Installation Checks for an Approved Assembly
Even a correctly selected assembly can underperform if installed incorrectly. Before work begins, the roofing contractor should verify that materials delivered to the site match the approved specification. Product names, fastener lengths, plate sizes, insulation thicknesses, and deck conditions should be checked against the listing and project submittals.
During installation, the crew should confirm that fasteners are compatible with the deck and achieve the specified engagement. Fasteners that spin, fail to seat, or do not secure the component properly need attention under the manufacturer’s repair instructions. Overdriving a fastener can damage a board or plate; underdriving can leave the assembly loose. The required condition is the one shown in the applicable installation detail.
Other practical checks include keeping fastener rows straight where the detail requires it, maintaining specified spacing, installing plates in the correct orientation, and documenting concealed work before it is covered. Weather, wet substrates, damaged materials, or deck irregularities may require a pause and review. Product-specific instructions take precedence over assumptions based on another roofing system.
Common Mistakes to Avoid
- Using a generic spacing chart: A chart for another product or deck may not be part of the selected FM-approved assembly.
- Treating the rating as a fastener specification: FM 1-90 identifies assembly performance; it does not, by itself, state a universal fastener count or spacing.
- Mixing components: Substituting insulation, plates, adhesives, or fasteners can take the installation outside the documented assembly.
- Ignoring roof zones: Applying one attachment density across the entire roof may conflict with the engineered design.
- Confusing uplift pressure with wind speed: A pressure rating is not a direct conversion to a single wind-speed value for every building.
- Relying on outdated paperwork: Listings and manufacturer instructions can change. Confirm that project documents reflect current products and details.
FM 1-90 Pattern Selection Checklist
| Check | What to confirm |
|---|---|
| Assembly listing | The exact roof system is listed for the required FM rating and applicable deck. |
| Components | Every specified layer and attachment component matches the listing. |
| Fastening detail | Fastener type, plate, spacing, layout, and penetration match the current instructions. |
| Roof zones | Field, perimeter, and corner requirements follow the engineered project documents. |
| Substitutions | Any proposed change has written manufacturer and design-professional approval where required. |
| Quality records | Substrate checks, installation inspections, and concealed-work photographs are retained. |
Frequently Asked Questions
Does FM 1-90 mean 90 fasteners per roof section?
No. The “90” is part of the assembly’s wind-uplift rating, not a fastener count. Fastener quantity and spacing come from the details for the exact approved assembly and the project design.
Can a contractor use a pattern from another FM 1-90 roof?
Not without confirming that the other pattern applies to the same approved assembly and conditions. Identical ratings do not establish identical components or installation requirements.
Is an FM 1-90 rating always enough for a building?
No. Adequacy depends on the building’s design pressures, applicable code, roof zones, deck, and other project conditions. The required rating and assembly should be determined during design and verified with the appropriate professionals.
Who should approve a change to the fastening pattern?
The change should be reviewed through the project’s design professional and the roofing-system manufacturer, with the authority having jurisdiction involved when required. Written documentation helps establish that the revised detail is acceptable.
Bottom line: An FM 1-90 fastening pattern is specific to a tested roof assembly, not a universal spacing rule. The correct layout comes from the current FM-approved assembly documentation, manufacturer instructions, and project-specific wind design. Matching the full assembly and documenting installation are essential to achieving the intended performance.