HVAC Curb Replacement on Commercial Roofs: When and Why

Why HVAC Curbs Fail on DFW Commercial Roofs

A rooftop HVAC curb is a raised metal frame that lifts the HVAC unit above the roof surface and provides a waterproof transition between the unit and the membrane. Here's the problem: curbs are typically fabricated from 14-gauge galvanized steel, and in DFW's climate — extreme heat cycles, UV exposure, and those acidic condensate drains from the HVAC units themselves — galvanized steel curbs have a real-world lifespan of 12-20 years. Meanwhile, a good TPO membrane lasts 20-30 years. So you end up with a roof where the membrane is fine but every curb is rusting through. The flashing at the curb-to-membrane transition starts lifting, water gets under it, and you have a leak that looks like it's coming from the middle of the roof but traces back to a $1,200 curb problem.

Signs a Curb Needs Replacement vs. Repair

Minor flashing separation where the membrane meets the curb base can often be repaired with new flashing and sealant — that's a $200-$500 repair. Replace the curb when you see: visible rust-through or perforation in the steel frame, flashing that's been re-caulked multiple times (the underlying problem won't go away with more caulk), the curb is visibly pulling away from the roof deck, or the unit itself has been replaced and the existing curb doesn't properly match the new unit footprint. During a roof replacement, all curbs should be evaluated — replacing them during a reroof costs about 40% less than returning to do it separately.

The HVAC Contractor Problem

We see this constantly in DFW commercial work — an HVAC contractor swaps out a unit, notices the curb is corroded, slaps some caulk on it, and calls it done. Or they modify the flashing without coordinating with the roofing contractor, creating a configuration that voids the membrane warranty. HVAC contractors are experts at HVAC. They are not membrane roofers. Any time a rooftop HVAC unit is serviced, replaced, or relocated, a roofing contractor should inspect and sign off on the penetration seals and flashing. JRH works with several commercial HVAC companies in DFW on a coordinated basis for exactly this reason.

Curb Adapter Systems: The Efficient Solution

When full curb removal isn't required, a curb adapter kit (sometimes called a curb cap or curb extension) slides over the existing deteriorated curb and creates a new waterproof base — without lifting the unit off the roof. For most situations, this cuts curb replacement cost by 30-50% and eliminates the need for an HVAC crane. We stock standard curb adapter kits in DFW and can typically complete a curb adapter installation in 2-3 hours per unit. Call JRH at (469) 888-6903 for a commercial roof penetration assessment.