HVAC roof curb repair fixes the raised frame a rooftop unit sits on and the flashing that seals it to the roof. When water appears below a unit and the HVAC technician finds nothing wrong inside, the cause is usually on the roofing side: split flashing corners, a curb too short for the roof, rust, rot or a poorly flashed adapter.
Water dripping under a rooftop unit right now? Call 1-888-346-7663; our 1-hour arrival window guarantee covers emergency calls. For a drip that keeps coming back, book a roof diagnosis online.
Where the roofer's job ends and the HVAC contractor's begins
Curb leaks drag on when each trade assumes the other is responsible. Up close, the dividing line is usually clear:
- Roofing side: the curb walls and top, the base flashing that runs from the roof membrane up the curb, the metal counterflashing or cap, the corners and the cricket on the high side.
- HVAC side: the unit cabinet, the gasket between the unit and the curb, the drain pan and condensate line, and the ductwork inside the curb.
A cracked drain pan inside the unit and a split flashing corner can drip onto the same ceiling tile, so tracing the water path always comes first.
Factory curbs, field-built curbs and equipment rails
Many newer package units sit on prefabricated metal curbs sized to the unit. Older buildings often have curbs framed from lumber on site, wrapped with roofing and capped with sheet metal. Split-system condensers and some fans sit on equipment rails or sleepers instead, and each support needs its own flashing.
HVAC roof curb repair: matching the fix to the failure
Split corners and open laps
Corners are where flashing is folded, cut or patched, and they open first as the roof heats and cools.
When a corner patch is enough
If the flashing is still bonded along the curb walls and only a corner has opened, a properly primed corner patch or pre-formed corner, compatible with the existing membrane, can restore the seal. Smearing mastic over the gap is not a repair, because it dries and cracks long before the roof wears out.
Flashing that has pulled away
Base flashing that has sagged, shrunk or lost its fasteners at the top leaves an opening behind the counterflashing. The fix is to strip the old flashing and install new base flashing up the curb, fastened at the top and covered by counterflashing so water cannot get behind it.
Rusted metal and rotted wood
Salt air in coastal cities such as Huntington Beach, Newport Beach, San Clemente and San Pedro corrodes galvanized curbs and caps, often behind the flashing where moisture sits. On field-built curbs, water that gets behind the flashing rots the lumber and the wood nailers at the base. Either condition means the curb itself has to be repaired or rebuilt, which usually requires lifting the unit.
Curbs that are too short or catch water
If the top of the flashing sits too close to the roof surface, ponding water and wind-driven rain can top it. A wide curb with no cricket on its high side dams water against the curb wall during every storm. Both are design problems that patching won't solve.
Curb height when you reroof
Reroofing changes the height of the roof surface. New insulation, tapered insulation to improve drainage or a cover board under a recover can all raise the finished surface, and every inch it rises is an inch of flashing height lost on each curb. Many membrane manufacturers call for base flashing to extend at least 8 inches above the finished roof, so curbs that were adequate before can come up short afterward.
The fix is to raise the curb, either with a curb extension or by rebuilding it, while the roof is open. Because the unit has to be lifted to do that, ask how old each unit is: if one is near the end of its service life, replacing it during the reroof means one crane day instead of two. Our article on reviewing roof drainage before equipment changes covers related planning.
Adapter curbs when a rooftop unit is swapped
A replacement unit rarely matches the footprint and duct openings of the old one. Rather than cutting a new opening in the roof, HVAC contractors often set an adapter curb on top of the existing curb to transition to the new unit.
What the roofer needs to see
- The joint between the old curb and the adapter is flashed and counterflashed, not just caulked.
- The existing curb is sound enough to carry the adapter, with no rust-through or rot.
- The combined height still leaves proper flashing above the roof surface.
- The membrane around the curb is repaired after rigging and foot traffic.
When removing the old curb is better
If the old curb is deteriorated or the new unit goes in a different spot, the old opening is framed in and roofed over, and a new curb is installed and flashed into the membrane. That is more roofing work up front, but it avoids stacking new equipment on a failing base. Questions about added weight belong to the HVAC contractor or a structural engineer.
Coordinating with your HVAC contractor on crane day
- Before: have the roof inspected around every curb being touched, with photos, so existing conditions are on record.
- During: the HVAC crew disconnects and lifts the unit, the roofer repairs, raises or reflashes the curb while it is exposed, and the unit is set back on a new gasket.
- Protection: walkways and rigging paths are protected so dropped parts and dragged equipment don't puncture the membrane.
- After: have the flashing checked once the unit is set, and again after the first good rain.
When we are brought in before the crane arrives, our 4-Phase Roof Inspection report gives both trades the same photos to work from. Rescue Roofer (St. Lic #1137524) has worked on Orange County roofs since 1993. Our commercial roofing service sends T.E.A.M. Certified technicians, trained in-house, and commercial maintenance contracts can put every curb on a regular checklist. For leaks from condensate lines, ducts and line sets rather than the curb, see our guide to rooftop HVAC units and roof leaks.
Frequently asked questions
Should I call the HVAC company or a roofer first?
If water appears only while the unit runs on a dry day, start with HVAC, since condensate is the likely source. If it leaks during rain, call a roofer. When in doubt, ask both to look the same week.
Can a curb be repaired without removing the unit?
Flashing, corner and counterflashing repairs usually can. Rebuilding a rotted or rusted curb, or raising its height, requires lifting the unit.
Do homes have roof curbs too?
Yes. Some mid-century homes, condos and townhomes in Orange County have package units or old evaporative coolers on flat roofs, and the same curb and flashing details apply.
Is sealing the curb with caulk a fix?
Only as a stopgap in an emergency. Caulk cannot make up for short, split or detached flashing, and it can make the permanent repair harder.
Planning a unit swap or a reroof, or chasing a leak under rooftop equipment? Book your roof diagnosis online or call 1-888-346-7663. We will inspect each curb, document it in a photo report you can pass to your HVAC contractor and give you a free estimate for the HVAC roof curb repair.