I've watched a lot of estimators bid a rooftop swap like it's a like-for-like part replacement. Pull the old 10-ton, set the new 10-ton, hook up the gas and power, done. On a good day, sure. But the unit price of the equipment is maybe half the real cost of the job, and the other half is hiding in five places. Miss them at bid time and you're either eating the overage or having an ugly change-order conversation with a customer who already thinks you're expensive.
Here's where the money actually goes.
1. The curb doesn't match
This is the number one budget killer, and it's almost always avoidable. The new unit's footprint rarely lines up perfectly with a curb that was set 15-20 years ago for a different make. When it doesn't, you need a curb adapter — a fabricated steel transition that sits between the old curb and the new unit and re-routes the supply and return openings.
An adapter isn't a disaster, but it's real money: a few hundred to well over a thousand dollars fabricated, plus the lead time to get it built to the exact old and new model numbers. The killer is when you didn't price it because you assumed a direct fit. Now you're paying for the steel and a second crane day because the adapter wasn't on the roof when the crew showed up.
Pull both model numbers and compare base dimensions before you quote. Our curb adapter decision tool will tell you in ten seconds whether you're likely in adapter territory. If you are, get the adapter quoted and on the roof before crane day.
2. Electrical that isn't actually there
New units pull differently than what they replace. A higher-efficiency unit might have a larger compressor inrush, a different MCA (minimum circuit ampacity), or a different voltage/phase configuration than the disconnect and whip that are sitting up there. I've seen brand-new units land on a roof next to a disconnect that's too small and a feeder that won't carry the load.
Check the MCA and MOCP on the new unit's nameplate against the existing breaker, disconnect, and conductor size. If the building's panel is already full or the feeder is undersized, that electrical upgrade can add thousands you didn't see coming. Electric-heat units are the worst offenders — a 60 kW electric heat section is a serious electrical load that a gas unit never demanded.
3. Structural — the new unit is heavier
Equipment has not gotten lighter. A new unit with a bigger coil, a powered exhaust, and economizer can outweigh the 1990s box it replaces by a couple hundred pounds. That load goes into the curb and the roof structure. On most buildings it's fine. On a marginal roof, or where the original curb was barely adequate, you now need a structural review and possibly reinforcement.
It's not your call to make by eyeball. If the new unit is meaningfully heavier — our calculators flag a 15%+ jump — get a structural engineer to bless it. Budget for the review even if the answer turns out to be "you're fine."
4. Crane and roof access
The crane is often the single biggest line item after the equipment, and it's the one estimators lowball the most. The cost isn't just the crane — it's the setup. Where does it sit? Do you need to close a lane or a parking area? Is there a permit for the street closure? Can the boom even reach over the building, or do you need a bigger (much more expensive) crane because of the building's depth?
And it's a per-mobilization cost. Every time something forces a second trip — missing adapter, wrong unit shipped, failed inspection — you're paying to bring the crane back. That's why items 1 through 3 matter so much: they're what turns one crane day into two.
Walk the site and look up and around before you bid the crane. The cheapest crane day is the one you only do once.
5. Code triggers you weren't expecting
A like-for-like swap feels like it should be exempt from the big code items. Often it isn't. Replacing the unit can trigger current-code requirements that the old install was grandfathered out of: a economizer where one is now required, updated refrigerant detection, fresh-air ventilation rates under current mechanical code, seismic or wind restraint on the curb, or a full mechanical permit and inspection even on a "simple" replacement.
This varies hard by jurisdiction — which is exactly why I wrote the permits-by-state guide. The point at bid time: don't assume the swap is grandfathered. Call the AHJ (authority having jurisdiction), ask what a replacement triggers, and price the code-driven items in.
The bottom line
None of these five are exotic. They're just invisible from the parking lot. The estimators who make money on commercial rooftop work are the ones who get on the roof, pull the model numbers, check the electrical and the structure, plan the crane once, and call the AHJ — before the number goes to the customer. Do that and a rooftop swap is a clean, profitable job. Skip it and you're financing the customer's building out of your margin.
About the author: Gregory Frazier is a heavy commercial HVAC estimator working Maryland, Pennsylvania, Delaware, Virginia and Florida for 25+ years. Read his story →