Updated October 2026 · Acme Construction, Oklahoma City
Rooftop unit framing is the structural support that holds a packaged HVAC rooftop unit (RTU) above a roof opening: the curb, the framing members under the curb, and any added steel or blocking that carries the unit’s weight down into the roof structure. When a rooftop unit is replaced, that framing has to match the new unit’s footprint, weight and duct openings, or the job stalls on the roof.
Most owners focus on the HVAC equipment and assume the new unit will sit where the old one did. Often it will not, and that is where a general contractor that handles metal framing, roof openings and coordination earns its keep.
This guide explains how rooftop unit framing works, when a curb adapter is enough, when the roof opening needs to be reframed, which code items come up during a rooftop unit replacement, and how to plan the work on an occupied commercial building in the Oklahoma City metro.
What Is Rooftop Unit Framing?
Rooftop unit framing is everything between the roof structure and the bottom of the rooftop unit. It includes the roof curb, the support framing that the curb bears on, the framed opening for supply and return ducts, and any added strength the roof needs to carry the equipment safely.
A typical installation has three layers:
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The roof structure. Steel joists, beams or wood framing that already carry the roof deck.
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The opening and support framing. Angles, channels or headers that frame the hole in the deck and transfer the unit’s load to the joists or beams.
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The curb. A raised metal frame, usually supplied to match a specific unit, that the rooftop unit sits on. The roofing membrane is flashed up the sides of the curb to keep water out.
When all three line up, the unit sits level, the ducts connect cleanly, and the roof stays watertight. When one of them is off, you get leaks, sagging deck, vibration, or a unit that does not fit at all.
Why Rooftop Unit Framing Matters During a Replacement
Replacing a rooftop unit sounds like a one-day crane pick. The framing question decides whether it stays that way. Here is what usually changes between the old unit and the new one:
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Footprint. Manufacturers change cabinet sizes between models and over the years. A new unit from a different brand rarely ships with the same base dimensions as the unit it replaces.
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Duct openings. Supply and return openings may sit in different positions, so the existing opening in the roof deck may not line up.
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Weight. A different unit can weigh more or less than the old one, and the load may sit on different points of the curb.
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Height. Code-required clearances, roof insulation thickness and drainage can all affect how tall the curb needs to be.
Each item touches the rooftop unit framing. If nobody checks them before crane day, the installer improvises on the roof or the pick gets cancelled, and a tenant space may sit without heating or cooling.
Curb Adapters vs. Reframing the Roof Opening
The first decision in most RTU replacements is whether a curb adapter can bridge the gap or whether the roof opening and curb need to be rebuilt.
What Is a Curb Adapter?
A curb adapter is a fabricated sheet metal transition that sits on the existing curb and gives the new unit the footprint and duct connections it needs. It lets the new rooftop unit “match” an old curb without cutting the roof. Curb adapters are custom built for a specific old-curb and new-unit combination, so accurate field measurements and the new unit’s submittal drawings are essential.
Curb adapters are popular because they can:
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Avoid cutting and reflashing the roof membrane
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Keep the existing opening and support framing in place
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Shorten the time the space is without HVAC during rooftop unit upgrades
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Reduce roofing work on roofs that are still under warranty
When a Curb Adapter Is the Right Call
An adapter usually makes sense when:
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The existing curb and the framing under it are sound, level and dry
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The new unit’s weight and load points can be carried by the existing curb and supports
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The adapter’s added height still leaves the required clearances and keeps the roof drainage working
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The roofing around the curb is in good condition
When the Opening Needs to Be Reframed
Reframing the roof opening is the better path when:
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The deck around the old opening is rusted, soft or water damaged
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The new unit’s duct openings fall outside the existing roof opening
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The new unit is heavier, or its load lands on points the old framing was never designed to carry
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The adapter would make the unit so tall that it creates access, clearance or wind problems
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The roof is being replaced anyway, so the curb can be set and flashed correctly from the start
Reframing means cutting the deck, adding angles or headers between joists, setting a new curb and flashing it in. It is more work up front, but on an older Oklahoma City building it often removes a long-running leak point.
Structural Checks Before You Order the Unit
The most important step happens before the purchase order: confirming that the roof can carry the new rooftop unit where you want to put it.
Weight and Load Path
The roof structure carries dead loads (the roof itself and everything attached to it), live loads, and environmental loads such as wind and, occasionally in Oklahoma, ice. Adding or relocating a rooftop unit changes those loads. A structural engineer looks at:
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The operating weight of the new unit from the manufacturer’s data
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Where the curb bears on the framing and how that load reaches the joists, beams and columns
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Whether existing joists near the opening need reinforcing or new support members
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Whether the original building drawings are available, or if field verification is needed
A like-for-like swap with a lighter unit may need only a short review. A heavier, larger or relocated unit usually needs an engineer’s letter or drawings for the permit set, and someone has to coordinate the engineer, mechanical contractor and roofer so the design matches the roof.
Wind Loads on Rooftop Unit Equipment
Oklahoma City sees strong straight-line winds and severe storms, so rooftop equipment has to be tied down properly. The structural standard most building codes reference, ASCE 7, has a specific procedure for wind loads on rooftop structures and equipment (Section 29.4.1 in recent editions). It calculates both a lateral force and an uplift force on the unit based on its size and the building’s height, and engineering references note that HVAC units are the most common equipment designed this way (Meca Enterprises summary of ASCE 7 Section 29.4.1).
In plain terms, the curb, the attachment of the unit to the curb, and the curb’s attachment to the roof framing all need to resist those forces. Hurricane clips or engineered attachment details are not just for the coast. Before the unit ships, confirm who is designing the attachment and that the framing details meet the engineer’s wind requirements.
Code Items That Come Up on Rooftop Unit Jobs
A rooftop unit replacement is mechanical work, but several code items land on the framing and roof side of the job. Oklahoma City’s Development Services reviews permits for this type of work, so build these items into the plan early. Our permit checklist for commercial construction in OKC covers the broader permit process.
Guards Near Roof Edges
The International Mechanical Code requires guards when equipment that needs service is located within 10 feet of a roof edge and that edge is more than 30 inches above the floor, roof or grade below. The guard must be at least 42 inches high, extend at least 30 inches beyond each end of the equipment, and stop a 21-inch sphere from passing through (ICC CodeNotes on worker safety on roofs). The code allows certain fall arrest anchors as an alternative.
If the new unit sits closer to the edge, or the old one never had a guard, expect this to come up.
Permanent Roof Access
The same code requires a permanent means of access when service personnel would have to climb more than 16 feet to reach rooftop equipment, measured to the top of any parapet. That access cannot require climbing over obstructions taller than 30 inches or walking on roofs steeper than a 33 percent slope (ICC CodeNotes). On many two-story commercial buildings, that means a fixed ladder or roof hatch, sometimes with its own guard.
Fall Protection During Construction
During the work itself, OSHA’s construction rules require fall protection for workers on walking and working surfaces with an unprotected side or edge 6 feet or more above a lower level (OSHA 29 CFR 1926.501). Open roof penetrations during reframing also need covers or guarding. Your contractor’s site safety plan should address both.
Rooftop Unit Framing Step by Step
Every project is a little different, but most rooftop unit framing jobs follow the same sequence.
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Survey the roof. Measure the curb, photograph the roofing and deck, and note ducts, gas, power and drainage.
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Get the new unit’s submittals. Footprint, curb dimensions, operating weight and duct locations for the exact model.
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Decide: adapter or reframe. Compare old curb to new unit and check deck condition.
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Structural review. Engineer confirms load path and wind attachment when needed.
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Permit and schedule. Submit the mechanical permit; book crane, roofer and mechanical crew for one window.
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Pre-fabricate. Build the curb adapter or support framing ahead of time.
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Remove, frame and set. Lift the old unit off, install new framing, set the curb level and attach it per the detail.
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Flash and seal. The roofer flashes the curb into the membrane, the step that decides whether the roof leaks.
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Set, connect and close out. Crane the unit on, connect ducts, gas, power and controls, then finish inspections, punch list and roof warranty paperwork.
Planning an RTU Swap on an Occupied Building
Most rooftop unit replacements happen on buildings that stay open: offices, retail, medical suites, warehouses with office space. That changes the plan.
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Pick the season. Mild spring and fall weeks are the easiest time to have a unit offline in Oklahoma City.
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Plan the crane day. Street or parking lot closures, early-morning picks and coordination with property management all need lead time.
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Protect the interior. Cutting a roof opening means dust and possible water entry below, so plan ceiling protection or after-hours work.
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Expect hidden conditions. Once the old unit comes off, the deck under it is visible for the first time in years. Build a small contingency into the schedule for repairs.
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Communicate. Tell tenants when the unit will be off and who to call.
If your building has several aging units, consider phasing them. Replace the worst units first, and use what you learn about the roof framing on the first unit to plan the rest. For bigger programs, the delivery method matters too; see our comparison of construction manager at risk vs. design-build.
Common Rooftop Unit Framing Problems We See
On commercial roofs around Oklahoma City, Edmond, Norman, Moore and Midwest City, the same problems show up again and again:
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Stacked adapters. A curb adapter set on top of an older adapter, making the unit tall, wobbly and hard to flash.
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Rusted deck at the opening. Years of condensation or small leaks around the curb corrode the deck and the angles under it.
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Shimmed units. Units leveled with loose shims instead of a properly framed curb, which shift and vibrate over time.
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Ducts forced into place. Openings that do not line up, so the duct connection is crushed or poorly sealed, wasting conditioned air into the plenum.
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No guard or access. Units moved near the roof edge without the guard or ladder the code calls for.
Each is easier to fix during a planned replacement than after a leak or failed inspection.
About Acme Construction
Acme Construction is a commercial general contractor based at 219 SE 29th, Oklahoma City, OK 73129. We handle facility remodeling, metal framing, masonry, concrete and custom metal fabrication for owners and property managers across the Oklahoma City metro. On rooftop unit projects, we coordinate the structural engineer, mechanical contractor and roofer so the framing, curb and flashing are planned together instead of improvised on crane day.
Rooftop Unit Framing FAQs
What is a curb adapter?
A curb adapter is a custom sheet metal transition that sits on an existing roof curb and gives a new rooftop unit the footprint and duct connections it needs. It lets you replace the unit without cutting a new roof opening, as long as the existing curb and framing are sound.
What is the difference between a curb and a curb adapter?
The curb is the raised frame that is flashed into the roof and supports the rooftop unit. A curb adapter sits on top of an existing curb to match a different unit. The curb is part of the roof; the adapter is a bridge between an old curb and a new unit.
Do I need a structural engineer for a rooftop unit replacement?
Not always for a like-for-like swap. You should involve one when the new unit is heavier, larger, moving to a new location, or going on an older roof with unknown framing. An engineer confirms the load path and the wind attachment detail.
Plan Your Rooftop Unit Framing With Acme Construction
If you are pricing a rooftop unit replacement or adding new rooftop equipment, call Acme Construction at 405-451-4555 before you order the unit. We will look at the roof, the curb and the framing, coordinate with your mechanical contractor, and give you a clear plan for the curb adapter or reframing work.