Guides

Does a Rooftop HVAC Replacement Need Structural Calculations?

Often, yes. When a replacement unit is heavier or larger than the old one, needs a new curb, or draws a plan-check comment, a California building department will want stamped calculations showing how the unit is anchored for earthquakes and wind and whether the roof can carry it. Here is what that involves.

By Anthony Sutter, P.E. · Licensed Professional Civil Engineer ·

When a replacement needs engineering

Swapping a rooftop unit for one of the same size and weight on the same curb is often handled with a mechanical permit alone. Structural calculations usually come into it when:

  • The new unit is heavier than the one it replaces
  • The footprint changes and the unit needs a new curb or an adapter curb
  • The unit moves to a new spot on the roof, or goes on a roof that never carried one
  • The plan reviewer asks for seismic and wind anchorage, a common comment on mechanical permits in California

Requirements vary by building department. If you have a plan-check comment, it is the best guide to what the reviewer needs.

Seismic anchorage

Chapter 13 of ASCE 7-22, which the 2025 California Building Code adopts, sets the earthquake force on mechanical equipment. The force depends on the site’s seismic hazard, the unit’s operating weight, how high in the building it sits, and factors for the type of equipment and its importance. Equipment on a roof shakes more than the same unit at grade, so rooftop units carry some of the highest forces in the chapter.

ASCE 7 exempts some equipment, depending on the building’s Seismic Design Category, the equipment’s importance factor, and its weight, but a packaged rooftop unit at a typical Sacramento-area site rarely qualifies. Buildings permitted under the 2019 or 2022 CBC were designed to ASCE 7-16, which uses a different equation, so I confirm with the plan reviewer which edition applies.

Wind on rooftop equipment

Chapter 29 of ASCE 7-22 gives a horizontal force and an uplift force on rooftop equipment, based on the unit’s size and the site’s wind speed. On lighter units the wind case can govern, especially the uplift on the hold-downs.

Following the load path

Anchorage is only as strong as its weakest link, so the calculations follow the force from the unit down into the building:

  • Unit to curb. Hold-down clips or brackets and their screws, checked for shear and tension
  • The curb. Manufactured curbs may come with their own rating; adapter curbs raise the unit, which increases the overturning force
  • Curb to roof. Lag screws or bolts into wood framing, or screws and welds to steel deck and joists, checked for the uplift and shear at each corner
  • Roof framing. The joists, beams, or trusses under the unit, checked for its weight and for the anchorage forces

On a recent replacement in Sacramento, the package answered a city plan-check comment and detailed anchorage for both a steel curb and a wood curb, so the contractor could use either.

The existing roof framing

Most replacement units go on buildings designed decades ago, and the California Existing Building Code governs the existing structure. When an alteration adds more than 5 percent to the gravity load on an existing member, that member must be shown to carry the new load or be strengthened. Where the original structural drawings aren’t available, I work from field measurements and photos of the framing below the unit.

What plan reviewers look for

  • The unit’s make, model, and operating weight from the manufacturer’s data
  • The design criteria: code edition, seismic values, and wind speed
  • The force on each connection and its capacity, with the evaluation report cited for any manufactured anchor
  • Details showing every connection, from unit to curb and curb to framing
  • A check of the roof framing under the new unit

What to send for a quote

  • Make and model of the existing and new units, with their weights
  • The curb: existing, new, or adapter
  • Photos of the roof framing below the unit, and the original structural drawings if you have them
  • The project address and the reviewing city or county
  • The plan-check comment, if you have one

This guide is general information, not engineering advice for a specific site. Requirements vary by city, county, and fire district, so confirm them with the reviewing agency for your project.