Rooftop HVAC Unit on Wood-Frame Building
A two-story wood-frame office building (Type V-B) is proposing to install a new packaged rooftop HVAC unit. The unit weighs 8,500 pounds and will be placed on the roof, which is framed with wood trusses at 24 inches on center. The existing roof was designed for a snow load of 30 psf and a dead load of 15 psf. The owner's contractor claims that no permit is needed because it's 'just equipment.' The building official disagrees and requires a permit. The contractor submits a one-page drawing showing the unit location and a curb, but no structural calculations. The building official must determine the scope of review and what structural provisions apply. The building is in seismic design category B with a wind speed of 110 mph. The HVAC unit will be anchored to the roof structure. The building official must decide whether a registered design professional is required to seal the structural plans and what inspections are necessary.
1. Is a building permit required for the installation of the rooftop HVAC unit? Explain the code basis.
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Yes, a building permit is required. IBC Section 105.1 requires a permit for any construction or alteration that affects structural members, egress, or fire-resistance. Installing an 8,500-pound HVAC unit adds significant dead load to the roof and requires anchorage for wind and seismic forces. This is not minor work exempt under IBC Section 105.2. The contractor’s claim that it is 'just equipment' is incorrect because the unit alters the structural loading and anchorage. The building official should also require a separate mechanical permit, but the structural aspects trigger the building permit. The official should not approve the installation without structural review.
2. What structural evaluation should the building official require before approving the permit?
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The building official should require structural calculations sealed by a registered design professional. The calculations must demonstrate that the existing wood trusses can support the added dead load of the 8,500-pound unit plus the curb, considering the existing 15 psf dead load and 30 psf snow load. The evaluation must also address wind uplift (110 mph) and seismic anchorage for Seismic Design Category B. Because the trusses are at 24 inches on center, the unit may need additional framing or reinforcement to distribute the load. The official should require details of the curb, flashing, and anchorage to the roof structure. If the trusses cannot handle the load, the design professional must propose reinforcement. The official may also require a structural observation during installation.
3. What inspections should be performed during and after installation?
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The building official should require several inspections. Before the unit is set, an inspection of the roof framing and any reinforcement should be performed to verify compliance with the approved plans. After the unit is placed, an inspection of the anchorage and curb connection should be made to confirm that the unit is secured for wind and seismic forces. A final inspection should verify that the unit is installed per the approved plans, that flashing is complete, and that the roof membrane is sealed. If special inspections are required by the design professional, the official should verify that they are performed. The official should not issue a certificate of occupancy or final approval until all inspections are passed and any required structural observation reports are submitted.