Rooftop Support FAQs

An engineered and code compliant rooftop support is part of the structural load path. It is not just a stand holding equipment on the roof. Depending on the project, the support may need to resist:

• Dead loads
• Wind uplift and lateral loads
• Seismic loads
• Snow and ice
• Maintenance walking loads
• Tornado loads
• Thermal expansion/contraction

The IBC (International Building Code) requires structural systems to be based on rational analysis and to provide a complete load path from the point where a force originates to the elements resisting that force. IBC 1604.4 also specifically addresses overturning, uplift, and sliding. When it comes to code compliance, the question isn't simply, "Will the rooftop support hold the equipment?" The question is: "Can the complete rooftop support system safely transfer all applicable forces to the building structure?"

There isn't one single "rooftop support code." Depending on the project, the design may involve some or more of the following codes and standards:

• IBC for structural design and loads
• ASCE 7 for wind, seismic, snow, and other design loads
• IMC for mechanical equipment, piping, supports, wind, and seismic requirements
• IPC for applicable plumbing systems
• IFGC for applicable fuel/gas systems
• NFPA 70 (NEC) for applicable electrical requirements
• SMACNA standards for applicable duct construction and support
• ANSI MSS standards for applicable piping supports
• OSHA regulation requirements for walking/working systems

The exact requirements depend on the codes adopted by the project jurisdiction where the project is located.

The IBC is a model code constructed by the International Code Council that is adopted throughout the United States, usually with state or even local municipal and county amendments. Each project may use a specific edition of the IBC, amendments to that edition, or additional local requirements. The important question is: "Which codes has the local jurisdiction adopted for this specific project?" This is what controls the specific code-compliant design.

It is a coordinated mechanical (or plumbing, electrical, etc.), MIRO support design, and structural coordination process. The mechanical, plumbing, or electrical engineer or architect typically establishes:

• What is being supported
• Where the equipment is on the roof
• Equipment and component weights
• Equipment dimensions
• Piping and duct materials
• Clearances
• Access requirements
• Thermal movement requirements

After the equipment types and locations are established, MIRO specialty engineer designs and evaluates the support assembly, connections, and applicable loads. The structural engineer then evaluates the building structure by receiving the support reactions from MIRO Industries. The responsibilities all connect together. A typical load path: supported component, through the support and connections, into the roof framing or structure. The International Mechanical Code, as an example, specifically requires exterior mechanical equipment, appliances, and supports exposed to wind to resist the applicable wind pressures. The mechanical engineer would pass the duct, mechanical units, etc. to MIRO to evaluate wind and dead load reactions. Then MIRO would pass the reactions from those loads to the EOR to ensure the provided loads work with the building structure.

Past or common practice does not demonstrate that the same solution works on the next project. Many engineers, architects, and contractors are unaware that supports must meet specific building codes, so all unaware parties continue to use non-engineered supports. Wind speed, building height, exposure, equipment size, roof construction, support geometry, connection locations, and seismic requirements can all change from project to project. These are some of the examples of why wood and non-engineered supports do not work; supports must consider the above mentioned variables and still meet project specific design (wind, snow, seismic, etc.) loads. There is a difference between: "We've never had a problem." and "We've demonstrated that this system works for this project." Engineering provides proof that the systems work. The new question should be, "Do the supports need to meet code?" If they do need to meet code, which all projects in North America should, wood block and non-engineered supports fall short of these code requirements.

Direct attachment provides a clear and identifiable load path. Chapter 16 of the IBC requires this load path to be established. A typical attached support can be evaluated through this load path:

• Rooftop MEP equipment
• MIRO support
• Base plate
• Fasteners or anchors
• Building structure

The MIRO engineer can evaluate the support, base connection, anchorage, and resulting building reactions as part of one coordinated system. This is especially important for wind. ASCE 7 Section 29.4.1 addresses the lateral and vertical wind forces on rooftop structures and equipment.

Systems can have some supports that land on the roof membrane in some locations and then have attached supports in other locations. The key is that the system has been analyzed to meet the code requirements and the different support types are compliant in the MIRO calculations for the specific project. It is important that structural system be properly analyzed and provide a complete load path capable of transferring forces from their point of connection to the elements they support. Looking at the code, IBC 1604.4 does not simply say that everything must be bolted. It requires an engineered structural analysis with a complete load path. The MIRO calculations go through this load path and demonstrate which locations are required to be attached. Direct attachment is the simplest and most cost-effective way to demonstrate this load path as required by code.

Typically, non-attached supports on rooftops do not satisfy all code requirements. The load path required in Chapter 16 of the IBC and loading in the ASCE 7 requires complete analysis of transferred loads. A ballasted system unfortunately relies on:

• Self-weight
• Added ballast
• Friction
• The roofing assembly
• Other resistance mechanisms

If these items must resist the design forces, they become part of the load path. Along with load path issues, industry lacks published data specific to rooftops for coefficients of friction from rooftop supports and membranes that account for dust, temperature variance, water, snow, and ice, etc. The engineer performing the rooftop support analysis would need to demonstrate adequate resistance to:

• Sliding of bases
• Overturning of frames
• Uplift of system
• Load transfer through the roof system (all components meeting uplift and lateral loading)
  • Membrane to coverboard
  • Coverboard to insulation
  • Insulation to insulation layering
  • Insulation fasteners to decking, etc.

"It's heavy enough" is not an engineering calculation that meets code requirements.

A directly attached system can provide a direct load path through the support and its connection to the structure. A ballasted system may depend on several additional interfaces, including:

• Support to membrane
• Membrane to adhesive
• Adhesive to insulation
• Insulation to fasteners
• Fasteners to deck
• Deck to structural framing

If the roof assembly is part of the resistance mechanism, those components need to be considered in their entirety and must be capable of resisting combined loading. MIRO recommends direct structural attachment for engineered rooftop support applications as these methods are straightforward and always meet code.

Adding weight can increase resistance to sliding and overturning, but it also increases the load placed on the roof. Additional ballast may require evaluation of:

• Roof deck capacity
• Structural framing
• Localized roof loading
• Roof system capacity
• Existing structural conditions
• Maximum compressibility on insulation (long-term)
• Membrane puncture resistance

Ballasted supports are a potential method for floating systems, but it is not automatically a substitute for structural attachment or meeting code.

Friction depends on the actual materials and conditions at the roof interface. It can be affected by:

• Roofing membrane
• Base material
• Surface texture
• Age and weathering
• Temperature
• Moisture
• Dust and dirt
• Oils or chemicals
• Contact pressure
• Contact area
• Direction of movement
• Roof slope
• Static versus kinetic conditions

A generic coefficient of friction should not automatically be assumed to represent every rooftop system; these values are not available for the different states that are found on rooftops. If friction is critical to the design, the EOR needs a technically defensible basis for the value being used. Depending on the application, that may require testing or other recognized technical evidence that can change drastically depending on project location, time of year, and even the time of day when interactions occur. The roof is being used as part of the resistance mechanism, if needing to meet code.

The horizontal wind force on a free-floating support may have to be resisted through:

• Support base
• Friction at base
• Roofing membrane
• Adhesive
• Insulation
• Insulation fasteners
• Roof deck
• Deck fasteners
• Structural framing

If any part of that system cannot transfer the required force, the assumed load path is interrupted. The roof cannot simply be assumed to have structural capacity because it supports the weight of the equipment. Supporting the weight of the equipment (dead load) and meeting code lateral and uplift loads are not the same.

Yes. A system can be designed using a combination of:

• Positively attached supports
• Free-floating supports
• Fixed points
• Guides, if needed
• Sliding supports
• Thermal expansion/contraction

This can be useful when the system needs both structural restraint and thermal movement. The important point is that the entire configuration needs to be engineered as a system.

Specify the requirement early. The MPE, Structural, and Architectural drawings and specifications should clearly identify:

• Engineered rooftop supports
• Applicable design loads
• Roof attachment requirements
• Engineering documentation (Deferred Submittal)
• Structural reaction calculations
• Submittal requirements

When this is established during design and shown in RFP drawings and specifications, the contractors can price the correct system, and the structural engineer can coordinate the reactions prior to installation.

Typical information includes:

• Project address
• Applicable code edition
• Building height
• Building risk category
• Wind criteria
• Roof height
• Roof slope
• Seismic criteria
• Snow and ice criteria
• Tornado criteria, where applicable
• Equipment weights
• Equipment dimensions
• Roof assembly and thickness
• Structural deck and framing information

Projects that start out with the correct engineering information typically finish smoothly and meet required timelines.

A support unfortunately cannot be properly engineered without understanding what it is expected to support and how it is to connect to the building. If final equipment has not been selected, the design team should establish documented assumptions for the MIRO engineer to work with. Those assumptions should be verified and provided to MIRO when the final equipment is selected. An assumed load is a design assumption, and it is not included in the final support design reactions required per code.

A metal or wood deck transferred loading connection is not necessarily equivalent to the same reaction delivered directly to a joist, beam, slab, or other structural member. Where the support attaches matter in the code transferred loading. The structural engineer needs to know where the support reactions are at the structural elements so he or she can evaluate the supporting structure accordingly. MIRO engineering analyses provide support reactions and attachment forces necessary for that coordination and structural analysis.

No. MIRO's engineering focuses on the rooftop support assembly and its applicable structural requirements. MIRO engineering will provide support reactions and attachment forces for coordination with the building's structural engineer. The structural engineer then evaluates the capacity of the roof deck, joists, beams, slabs, framing, or other building components receiving those reactions. MIRO engineers the supports. The building's structural engineer evaluates the building structure.

Thermal movement is primarily a mechanical system design item, but it directly affects support selection. MIRO can provide support configurations that accommodate specified movement, including:

• Fixed points
• Guides
• Sliding supports
• Expansion/contraction allowances
• Restraint configurations

The mechanical engineer should establish the required thermal movement and restraint strategy. MIRO will provide the supports to accommodate the required linear movement needed on the project.

Vibration isolation does not automatically eliminate the need for code loading. The codes require supplemental restraint where vibration isolation of equipment and appliances is employed. The support design may need to incorporate:

• Spring isolators
• Elastomeric isolators
• Seismic restraints
• Wind restraint
• Equipment anchorage
• Operational movement

The isolation and restraint systems should be supported in the systems together. Provide this information upfront when working with MIRO's engineering department, and the supports will show the supports that meet the isolation along with the supports meeting code restraint loading.

They need to be addressed since the building codes require it and they are rooftop structures that people will use as means for access. They may need to be evaluated to meet:

• Live (walking) loads
• Dead loads
• Guardrail loads
• Wind, seismic, snow, etc.
• Attachment
• Clearances or code maintenance heights

If maintenance personnel need to cross over piping, ductwork, or other obstructions, the design should provide a safe access and code compliant solution instead of leaving the issue to the installer. If people are expected to use it, it should be intentionally designed. Also, don't forget about meeting OSHA requirements for these systems. MIRO will ensure that the walking support systems meet OSHA requirements and then code requirements when called for.

A standard product may be available, but a catalog rating does not automatically mean that the support is adequate for every project when meeting code specifics. Capacity can change based on:

• MEP geometry
• Equipment span
• Equipment weight
• Wind
• Seismic
• Structure types
• Anchor capacity
• Load direction
• Attachment location

Off the shelf product selection and project-specific engineered supports are not the same thing. Make sure that everyone understands there is a difference and make sure MIRO's engineering department is aware of code requirements when they are needed. A product is manufactured for a particular range of rooftop applications. An engineered system evaluates how the engineered products will perform for a specific project.