Quick answer
Many building departments require a structural letter when a rooftop HVAC unit is replaced with a heavier unit, moved to a new location, set on a curb adapter, or added where none existed. The engineer confirms the roof framing can carry the unit’s operating weight with snow, wind and seismic forces, and specifies reinforcement or anchorage where needed.
Rooftop unit (RTU) replacements look routine, but newer, higher-efficiency units are often heavier and have different footprints. The roof joists under them were sized for the original unit decades ago.
When a structural letter is usually required
- The new unit is heavier than the existing one
- The unit moves to a new location, or a new unit is added
- A curb adapter raises the unit and increases wind forces
- New roof openings are cut for ducts
- Visible roof sag, ponding or damaged joists near the unit
What the engineer checks
- Roof joist or truss capacity for the unit’s operating weight plus roof loads, including drifting snow against the unit in snowy areas
- How the load is distributed through the curb to the framing
- Wind and seismic anchorage of the unit to the curb and the curb to the roof
- Framing around new roof openings
What you get
A sealed letter confirming the roof is adequate for the new unit, or a reinforcement detail such as added framing under the curb, plus the anchorage requirements for the mechanical contractor.
Cost and turnaround
Typical RTU structural letters fall in our review and stamp tier, from $249 as a fixed fee, and most are returned in 48 to 72 hours. Mechanical contractors with regular replacements can ask about ongoing support. See engineering for contractors.
What to send us
- Old and new unit model numbers and weights
- Roof plan or photos showing the unit location
- Photos of the roof framing below the unit, if accessible
- Building address
Common questions
Does a like-for-like RTU replacement need a structural letter?
Often not, if the new unit weighs the same or less and sits in the same place. Many departments still ask for confirmation, so check with yours.
Who usually orders the structural letter?
The mechanical contractor or the building owner, as part of the mechanical permit.
Can steel bar joists carry a heavier unit?
Sometimes. The engineer checks the joists for the concentrated load and adds reinforcement or supports where needed.
Do curb adapters change the structural check?
Yes. Raising the unit increases wind forces on it and on its anchorage.