Building a Curb Detail That Stays Closed
A repaired curb starts with the structure under it. We confirm the curb is plumb, fastened to the deck and framed to carry the unit, and we replace rotted wood or bent steel before any membrane goes on. The curb gets insulation matched to the roof, a cant at its base and a flashing sheet cut as one wrapped piece, with the corners hand-formed instead of stacked from scraps. We keep every lap away from the corner itself, since a lap at a corner is a leak waiting for the next heat wave.
The top of the flashing gets a termination bar and a sealant joint tooled flat, and a sheet metal curb cap or counterflashing shields the whole edge from sun and runoff. Cap metal is formed to fit the particular curb, with laps soldered where the metal allows and sealed where it does not. A flashing that fits holds up better than one that was stretched to fit.
Pipe Supports, Pitch Pockets and Boots
Pipes, conduit and gas lines cross the roof on supports, and each support is a spot where foot traffic and thermal movement keep working on the membrane. A support set directly on the sheet without a protection pad wears a hole in it. A rubber boot hardens in the sun and cracks around the pipe, and a pitch pocket packed with sealant pulls away from its box as the filler dries. We replace those with supports that lift the line clear of the surface on proper bases, pads beneath every foot, and welded or formed metal boots that suit the membrane, whether it is TPO or another weldable sheet.
Long runs are the hardest to flash. A gas line or refrigerant bundle that crosses a dozen feet of roof is better carried on a series of low supports than flashed through one heavy collar, so one failure never opens a long path for water. Our penetration flashing work follows the same rule, one clean detail per opening.
Heat Cycles and Foothill Wind Around Equipment
Pasadena roofs heat hard. The city has recorded 113 F in June, and the two-day heat advisory in October 2023 showed that late-season spells reach the roof as well. Curbs bake in that sun, and the flashing on them expands and contracts more than the field of the roof does, which is why curb flashings open before field seams. We choose flashing materials with UV exposure in mind and keep sealant joints small and tooled, the same concern behind our heat and UV damage repairs.
Wind is the second force. Santa Ana gusts forecast at 60 to 80 mph ahead of the Eaton Fire were the load that roof edges, flashings and rooftop units in the foothills had to ride out. A unit not strapped to its curb can shift enough to tear its own flashing. In the mapped zones along the Arroyo Seco and in upper Hastings Ranch, the covering around a curb must belong to a Class A assembly when more than half the roof is replaced, and ember-resistant vents are part of the same rules, so we flag every vent that must change with the covering.
Equipment Swaps and Service Traffic
Roof equipment is replaced far more often than roofs are. A new unit set on an old curb of a different size often gets an improvised adapter, and the improvisation is where leaks begin. Older offices re-tenanted for life science users, like the 48,882 SF that Arrowhead Pharmaceuticals leased in downtown Pasadena in late 2025, can mean extra exhaust fans and cooling equipment, and with them new curbs, new supports and new openings in the roof above. Buildings added across the years, as at Huntington Hospital in the HeArt district (an east tower in 1998, a west tower in 2008 and an enlarged emergency department in 2014), tend to end up with curbs and supports of different ages, and each age asks for its own flashing check.
Service traffic is the other wear. Technicians walk the same paths to the same units, and crushed membrane shows up beside every access panel. We lay walkway pads along those routes and photograph each curb and support before and after the work, so the owner has a record for the next unit change. Curbs and supports belong in a regular maintenance visit.
Commercial Roofing Questions
Can a rooftop unit be replaced without opening up the curb flashing?
Sometimes, when the new unit has the same footprint and rests on the existing curb. Most replacements are not that tidy, because a different footprint or a different height forces an adapter or a rebuilt curb. We check the new unit's dimensions before the crane day and say which it will be.
Why does the same curb keep leaking after every repair?
Usually because each repair treated the stain and not the corner. A bead of sealant over a fish mouth buys one season, then the curb moves and the joint opens again. A lasting fix rebuilds the cant, the flashing sheet and the counterflashing as a set, and checks that the curb itself is sound and fastened.
Do curbs have to be rebuilt when the roof is replaced or recovered?
A replacement is the natural moment to rebuild them, since the old flashing comes off anyway and the curb can be checked and raised. On a recover, the new roof surface sits higher, so curbs often need extensions to keep the flashing at a proper height. We measure each one before the scope is written.
Who is responsible for the roof when an HVAC contractor cuts a new penetration?
Whoever flashes the opening owns the water-tightness of it, which is why we prefer to flash every new cut ourselves. Check the roof manufacturer's terms before any opening is made, and give us notice so the detail is built properly.
How are curbs handled on a roof in a Fire Hazard Severity Zone?
The curb flashing has to sit inside an assembly that holds its Class A rating, so the covering, underlayment and deck are judged together. We confirm with the city which work triggers the rule and plan vents to match before materials are ordered.
