Rooftop Equipment Supports and RTU Structural Analysis
Roof framing checked, supports designed and rooftop units anchored for wind and seismic loads, stamped for your mechanical or building permit.
- Stamped by a PE licensed in your state
- Built to IBC, IRC and local amendments
- Plan-check comments answered
RTU structural analysis for replacements and new equipment
Swapping an old rooftop unit for a new one sounds like a mechanical job, but the roof has to carry it. New units are often heavier, larger or a different shape than the ones they replace, and the curb may land in a different place. Adding a unit where none existed puts a concentrated load on joists that were designed for snow and roofing alone. That is why many building departments ask for an RTU structural analysis before they will issue the mechanical permit.
PE Engineer checks roof framing for new and replacement rooftop units, condensers, exhaust fans, makeup air units, generators, cooling towers and solar or telecom equipment. Where the existing roof cannot carry the load, we design reinforcement, steel dunnage frames or curbs that distribute the load to stronger members. We also design the anchorage that holds the equipment down against wind and earthquake forces. Every set is sealed by a Professional Engineer licensed in the state where the building is located.
Mechanical contractors, building owners, property managers and design firms use this service for single unit swaps and whole-building equipment upgrades. Send the equipment schedule, roof plan and photos, and we return a fixed-fee proposal, usually the same day, followed by a stamped letter or drawings ready for permit.
What you receive
- Roof framing analysis for equipment loads
- Structural letter or reinforcement drawings
- Curb or steel dunnage frame design
- Equipment anchorage details
- Wind and seismic calculations
- PE stamped letter or drawing set
Rooftop equipment support services
We cover the structural side of putting equipment on a roof, from a quick replacement letter to a full dunnage design.
Replacement RTU letters
Comparison of the new unit with the existing one, with a framing check where weight or location changes, delivered as a stamped letter the permit office accepts.
New equipment roof checks
Analysis of joists, girders, beams and decks for new units, including snow drift against the equipment and the concentrated loads at curb or support points.
Roof framing reinforcement
Added joists, sistering, steel angles, bridging or new beams where existing framing is overstressed, detailed so the work can be done from below the roof.
Steel dunnage and platforms
Steel frames that span between primary members and raise equipment above the roof, with posts, flashing coordination and a design for equipment and service loads.
Curbs and adapter curbs
Review of manufacturer curbs and adapter curbs, with attachment of the curb to the framing and of the unit to the curb for uplift and lateral load.
Wind and seismic anchorage
Anchorage for rooftop equipment designed to ASCE 7 wind and seismic provisions, including attachment hardware, welds and fastener patterns sized for the forces at your site and building height.
Your Stamped, Permit-Ready Plans Start With One Upload
Send your drawings or describe the project. A licensed engineer reviews it and sends a clear, fixed-fee proposal, often within hours.
- Free and no obligation
- Reviewed by a licensed PE
- Secure file upload
When an RTU structural analysis is required
Building departments generally want structural review whenever rooftop equipment adds load, changes location or relies on new attachments. Practices vary, but these situations usually trigger a request for engineering:
- A replacement unit is heavier than the existing unit, or sits on a new curb in a different location.
- New equipment is added to a roof that did not carry it before.
- The roof is framed with open-web steel joists, wood trusses or light wood framing that has little reserve capacity.
- The jurisdiction is in a high wind or seismic region and requires engineered anchorage.
- Multiple units are being added as part of a tenant improvement or building conversion.
For a like-for-like replacement with a lighter unit on the same curb, a short engineering letter may be enough. When the weight goes up, the unit moves or the framing is questionable, a calculation package and reinforcement drawings are usually required. We tell you which applies after reviewing the equipment data and roof framing, so you do not pay for more than the permit needs.
An RTU structural analysis is also good practice even where it is not required. Overloaded roofs tend to show it through ponding, sagging and leaks long before anything more serious happens. Owners who check the roof before a unit is craned up avoid the cost of removing it later.
How we check roof framing and design supports
We start with the equipment: operating weight, footprint, curb dimensions and the location on the roof. The existing framing is identified from original drawings when they exist, or from photos and field measurements of joist sizes, spacing, spans and joist tags. Open-web steel joists are evaluated using Steel Joist Institute load tables and design methods for concentrated loads, wood members to the NDS and steel beams to AISC 360.
Loads include the equipment weight, roof dead load, roof live load or snow, and snow drift that builds up against tall equipment in cold climates. Drift can add significant load next to a unit, and it is one of the most common items reviewers check in snow country.
If the framing works, we document it. If it does not, we choose the simplest fix:
- Add a joist or beam beside the overstressed member.
- Reinforce joist chords and webs with welded angles, or add bridging.
- Use a steel dunnage frame that carries the unit to girders or columns instead of individual joists.
- Relocate the curb to a stronger part of the roof when the layout allows.
Reinforcement is detailed so it can usually be installed from below while the building stays in use.
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Wind and seismic anchorage for rooftop equipment
Equipment on a roof sits where wind speeds are highest and where seismic accelerations are amplified compared with the ground. ASCE 7 includes specific wind provisions for rooftop structures and equipment, along with seismic design requirements for nonstructural components such as mechanical units. Those forces must travel from the equipment through its attachment, through the curb or dunnage and into the roof structure.
Our anchorage designs cover:
- Wind uplift and lateral forces on the unit based on its size, height above the roof and building height.
- Seismic forces based on the site’s seismic design category, the component importance factor and the unit’s location on the building.
- Fasteners, clips, straps or welds connecting the unit to its curb or frame.
- Connections of the curb or dunnage to joists, beams and decks.
Many manufacturers offer factory hurricane or seismic curbs. We can review their data, confirm it meets your site’s forces and design the attachment to the roof that the manufacturer leaves to others. For equipment at grade or inside the building, our equipment anchorage design service covers the same calculations for pads, slabs and floors.
Anchorage also has to suit the roof assembly. We coordinate attachment points with roofing and flashing so the anchorage does not create leaks, and we specify hardware that can be installed with the unit set in place by the crane crew.
Coordinating with the mechanical permit
Rooftop equipment projects usually involve both a mechanical permit and a structural review. The mechanical side covers the unit’s capacity, ductwork, ventilation, refrigerant and energy code items, while the structural side covers the roof, supports and anchorage. Reviewers also look for practical items such as service access, clearance around the unit and guards when equipment sits close to a roof edge, which the mechanical code addresses.
Our mechanical and HVAC design team can prepare the mechanical drawings, and the structural work in this service fits into the same package, so both reviews see consistent equipment weights, locations and curb sizes.
To prepare a fixed-fee proposal, send us:
- The building address and the type of roof framing, if known.
- Cut sheets or submittals for the new equipment, including operating weight and curb data.
- Data on the existing unit if this is a replacement.
- A roof plan or aerial image marking the unit locations.
- Photos of the roof framing from below, including joist tags when visible.
- Any existing structural drawings of the building.
Plan-check comments are answered and the drawings revised within the original scope at no extra charge.
You likely need this if
- The new RTU is heavier than the old one
- Equipment is going on a roof for the first time
- The city asked for a structural letter
- The roof uses open-web steel joists or trusses
- You need wind or seismic anchorage details
- A curb or dunnage frame must be designed
How it works, start to finish
From your first message to stamped drawings, you work directly with the engineering team.
- 1Share your projectSend the address, scope and any drawings, photos or plan-check comments. Two minutes is enough.
- 2Receive a fixed-fee proposalAn engineer reviews the scope and replies with a firm price and delivery date for your rooftop equipment supports drawings.
- 3Engineering and draftingWe run the calculations and prepare drawings to the codes your jurisdiction enforces, checked before sealing.
- 4Stamped set deliveredYou receive a sealed PDF set ready to submit, and we answer any plan-check comments within scope.
Codes and standards we design to
Rooftop equipment supports are designed to the editions your jurisdiction has adopted, including local amendments:
- International Building Code (IBC) and International Existing Building Code (IEBC)
- ASCE 7 for roof loads, snow drift, rooftop equipment wind loads and nonstructural seismic design
- Steel Joist Institute (SJI) specifications for open-web steel joists
- AISC 360 for steel dunnage and reinforcement
- NDS for wood framing
- International Mechanical Code (IMC) for equipment access and clearances
How licensing works. Engineering drawings must be sealed by a Professional Engineer licensed in the state where your project is located. PE Engineer works with a network of state-licensed PEs, so every set is reviewed and stamped by an engineer with an active license in the state where your project is located. Their name and license number appear on the seal, exactly as your building department requires.
Related services
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Mechanical & HVAC DesignHeating, cooling and ventilation design with Manual J load calculations, equipment schedules and duct layouts, sealed by a licensed PE for permit.Explore More
MEP PE StampMechanical, electrical, plumbing, fire protection and energy drawings checked against code and sealed by engineers licensed in your state for each discipline.Explore MoreFrequently asked questions
Do I need a structural engineer to replace a rooftop unit?
Often, yes. Many jurisdictions require a structural letter or calculations whenever a replacement unit is heavier, larger or placed on a new curb. If the new unit is lighter and uses the same curb, some departments accept a short letter confirming that. We tell you what your project needs after reviewing it.
What information do you need for an RTU structural analysis?
The new equipment cut sheet with operating weight and curb size, data on the existing unit if it is a replacement, a roof plan with locations, and photos of the framing below. Existing structural drawings help a great deal but are not always required.
What is steel dunnage?
Dunnage is a steel frame that supports rooftop equipment and spans between strong points in the roof structure, such as girders or columns, instead of loading individual joists. It is a common solution when the roof framing cannot carry the unit directly. Dunnage also raises equipment so the roofing below can be maintained and replaced.
Can you reinforce open-web steel joists?
Yes. Typical reinforcement includes welded angles on chords or webs, added web members at concentrated load points, added bridging or a new joist alongside. Details are designed so the work can usually be done from below the roof. Where field welding near occupied space is a concern, we can use bolted options instead.
Do rooftop units need seismic anchorage?
In many regions, yes. ASCE 7 sets seismic requirements for nonstructural components, including mechanical equipment, based on the site’s seismic design category and the component’s characteristics. We calculate the forces and design the attachments when your jurisdiction requires them. Many low seismic regions require little more than positive attachment.
Can you stamp the manufacturer's hurricane curb?
We can review the manufacturer’s curb data against your site’s wind loads and design the attachment of the curb to the roof framing. The curb itself is a manufactured product, so we rely on its published capacities and design everything the manufacturer leaves to the engineer of record.
Get your free rooftop equipment supports proposal
Tell us where the project is and what you need. Attach any drawings, photos or plan-check comments. A licensed engineer reviews every request.
- Upload your filesPlans, sketches, photos or comments
- Get a fixed-fee proposalUsually the same day
- Receive stamped drawingsSealed by a PE licensed in your state