A rooftop unit sits on a curb, and the curb is where the roof around it usually gives up first. On a Bronx retail strip or a mid-rise office block, the stain on the ceiling tile directly under an air handler is rarely a mystery. The flashing at the foot of the curb has split, the sealant has pulled back, or the unit was swapped years ago and the new footprint never got proper flashing. A property manager should look at the base of every unit before anyone talks about a new roof. One failed curb can pass for a failed roof, and the repair costs a fraction of the difference.
We rebuild and replace curbs, equipment rails and pipe supports on low-slope roofs in the Bronx, NY, and flash each one into the membrane.
What a Curb Is and What It Carries
A curb is a raised frame, wood or metal, fastened to the deck and built up above the roof surface so a unit sits clear of standing water. The membrane runs up the outside as base flashing, and a metal counterflashing covers the top edge. Skip a layer and the curb becomes a funnel. Rails, sleepers and pipe stands are different: they hold the load off the membrane so a condenser line or gas pipe does not crush the insulation underneath.
On a five or six story apartment building in Belmont or Bedford Park, the penetrations are usually an exhaust fan, a boiler flue and a handful of vent stacks, each with its own flashing. The same rules apply to apartment roofs and to commercial roofs with a heavy air handler.
Where Curbs Fail First
Four spots repeat. The corner, where flashing is folded and the seam opens under movement. The top edge, where the counterflashing loosens and water slips behind it. The unit itself, where a missing access panel gasket lets rain run down the curb wall. And the low side, where water gathers and sits.
Weather presses on all four. Sealant on a curb rides a swing from 95.7 F July highs at the Central Park station to a record -15 F, and that fatigue shows first at the corners. The low side also collects drifted snow that melts, refreezes and works at the flashing from underneath. A curb that stood in water after Hurricane Ida in 2021, which dropped 3.15 inches in a single hour, needs its flashing failures repaired before the next storm.
What We Check on a Flat-Roof Walk
We put hands and a probe on each curb, photograph it and tag it on a roof plan, so the work order lists every curb by number.
- Flashing height. Water ponding at the base needs enough height to stay below the top edge, and old curbs are often cut too low.
- Slope around the curb. A saddle on the uphill side keeps water from stopping against the wall.
- Pitch pockets and pipe boots. These older fixes for round penetrations dry out and open.
- Hold-downs. Loose fasteners let a unit vibrate, and vibration cracks flashing.
Gas, conduit and refrigerant lines need a pad under each leg, or the pipe settles onto the membrane and wears through it.
We also look at what is stored near the units. Stacked filters, scrap duct and dead conduit lying on the membrane trap water and cover the seams we have to read.
Repair, Rebuild or Replace
A sound curb with failed flashing gets the old base flashing cut away, new membrane set and a metal cap on top. That is a commercial roof repair done in a single visit. If the wall is rotted or short, we rebuild it. When a unit is going out anyway, swap the curb under the same work order, since a fresh unit on an old curb picks up every leak the old one had.
Near the end of a roof's life the answer shifts. Six rebuilt curbs on a membrane due for replacement in two years is money spent twice, so we fold the curb work into the replacement scope. On a roof with good years left, curbs are the right place to spend, and an infrared moisture survey shows how far water traveled from the penetration. Where the existing system allows it, we flash with the same membrane, such as TPO, so the new flashing bonds cleanly.
Access and Neighboring Trades
Curb work puts us next to the mechanical contractor. The mechanical trade must disconnect and lift the unit aside before the curb can be rebuilt, so we book one lift between us and keep the downtime short. On Fordham Road, a day without cooling is a lost sales day, and we schedule around it. The crew reaches the roof by stair hatch, bulkhead door or ladder, so we confirm the haul route with the superintendent and lay walk pads along it. Drains near the curbs get cleaned in the same visit, and roof drains and scuppers that clog undo the curb repair.
Hoisting over a parapet means protecting the coping stone and the roof edge, so we pad the contact points and keep rigging clear of tenant windows. In cold weather we stage the curb rebuild so the opening is covered by the end of each shift, because a dry deck costs less than a wet one.
Permits and Records
The Department of Buildings publishes the 2022 NYC Construction Codes. Filings go there. The city publishes the 2022 NYC Construction Codes, including Chapter 12 for existing buildings. We supply curb dimensions and flashing heights for the filing and leave the filing itself to the owner's expediter.
After the work, we send photographs of each curb with materials and dates, which is useful when a tenant claims a leak later. A repeat check under a preventive maintenance visit catches the next seam to open. Send the roof age, the unit list and the last tenant complaint, and we will walk it. Owners in Fordham can send the same three items.
Commercial Roofing Questions
Can a leaking curb be fixed without replacing the whole roof?
Usually yes, when the membrane around it is still sound. We cut out the failed flashing, rebuild the corners and water-test the area before we leave. The exception is wet insulation spread across the deck, where the curb is only the first symptom.
Who flashes a new rooftop unit, the mechanical contractor or the roofer?
Flashing belongs to the roofer, and the contract should name that before the unit is delivered. Mechanical crews set the unit. Flashing the curb into the membrane is roof work, so we coordinate the lift date and flash the opening the same day.
How often should curbs be checked on a Bronx building?
A flat-roof walk twice a year, in spring and again in fall, plus one after any hard storm. Winter movement opens seams and summer heat softens sealant, so each walk catches what the season did.
Does a heavy unit need extra support under the roof?
Often it does. A large air handler can push hard on insulation never meant for a point load, and the membrane splits at the curb edge. Rails spread the weight onto the structure. The building's engineer gives us the load figure, and the support is built to it.
What should happen with an unused unit nobody services?
Take it off and close the opening. A dead unit still rests on seams that can open, and the duct beneath it leads water straight to the ceiling. We remove the curb, patch the deck and finish with new membrane.
Send the Unit List and the Last Tenant Complaint
Give us the building type, the roof age and which units sit on the roof. We will schedule a flat-roof walk, mark every curb and support, and give you a written list of which to repair and which to leave.
Request a Roof Assessment