A curb is cheap material and an expensive leak if it is set wrong. That ratio is why we price every unit on the roof as its own line. In Coeur d'Alene, ID the snow that piles against a rooftop unit does not melt evenly. It sits on the upwind side, packs, thaws at noon and refreezes at the curb flashing by dark.
If you are the general contractor carrying a mechanical swap, the roof is where the schedule gets tight. The unit arrives on a crane day, the curb has to be ready the same morning, and the membrane has to be dry behind it.
What We Set: Curbs, Rails and Supports
Four things sit on a commercial roof, and each one gets a different detail. A prefabricated curb carries a packaged unit and its duct opening. An equipment rail or dunnage carries a unit that does not need a roof opening, such as a condenser. A pipe or conduit support carries lines across the membrane without crushing it. A sleeper carries light loads on a protected pad.
Over a membrane such as TPO or EPDM, we replace curbs that have rotted, rusted or been set at the wrong height. We add new ones when a tenant brings in a unit. We re-seat supports that have settled into the insulation. Each is priced with its flashing, its counterflashing and its sealant, so the number you see is the number that closes the penetration.
The equipment mix varies by building. A hospital such as Kootenai Health, with its 329 beds, would have air handling that cannot go down for a roof job, so curb swaps there would be staged unit by unit. A flex building in Post Falls may have one packaged unit per tenant bay, each with its own curb and its own lease.
What the City Sheet Says: Snow, Wind and Seismic
The City of Coeur d'Alene Building Services sheet is the document your roof submittal is checked against. Its numbers are 60 psf of ground snow, 115 mph of design wind, Seismic Design Category C and a 24 inch frost line. Each number touches equipment. Snow drifts against the unit and adds load to the deck around the curb. Wind tries to lift a unit and slide it, so the anchorage must hold. Seismic Design Category C is the figure the mechanical engineer uses to restrain the equipment itself.
We are the roofer, not the mechanical engineer, so the unit's anchorage belongs to the mechanical trade. What belongs to us is the curb, the flashing, the curb height above the snow line and the drift pocket on the windward face. Where that interface is unclear, we call a hold point so it gets settled before the crane is booked.
What It Costs: The Lines That Move the Bid
Five lines move the number. Curb count. Curb height, since a taller curb costs more and stands clear of deeper snow. Deck condition under the old unit, which we only learn after the unit is lifted. Crane time, which is a fixed cost the mechanical trade usually carries, though sometimes the roofer does. And access, because a unit set over an occupied tenant space needs protection below.
We break these out as base scope and bid alternates, with the insulation question handled under insulation and cover board. Replacing a curb that is sound but ugly is an alternate. Replacing one with a rotted nailer is base scope. If we open the deck and find saturated insulation, that is a change-order trigger with a unit price already in the bid, so nobody argues about it on the roof.
Where It Fails First: Corners, Counterflashing and Condensate
The corners go first, a pattern that also shows up in skylight and penetration flashing. A curb flashing is a box with four inside corners, and each corner is a fold in a membrane that moves with the temperature. In a place with a record low of minus 30 F and a July mean near 70 F, the fold works every day. Sealant that was acceptable in summer cracks in January.
Second is the counterflashing, the metal that covers the top of the flashing. It loosens when the unit vibrates and when snow slides off the cabinet. Third is condensate. A drain line that discharges on the membrane puts a permanent wet spot in the same place, and the membrane there ages years ahead of the field. We reroute the discharge, add a splash pad and note it in the record.
Drift Pockets: Why the Windward Side Matters
A rooftop unit works like a small parapet. Wind carries snow across the roof, hits the cabinet and drops its load on the leeward side, sometimes a foot or more deep against the curb. Saturated snow weighs roughly 20 pounds per cubic foot, per the City's roof snow guidance, so a drift pocket is a real load on the deck and a soaking against the flashing.
We look at where the units sit relative to the prevailing wind and the nearest parapet. On a tilt-up of the kind found at Riverbend Commerce Park, a row of units set close together can make one long drift. We recommend spacing, taller curbs or a snow diverter where the layout allows, and we price those as alternates so the owner picks.
Schedule: Crane Day and the Dry-In
Curb work has a hard sequence. Disconnect and lift the old unit. Cut out the old curb. Inspect and repair the deck. Set the new curb. Flash it. Set the new unit. Close the roof opening. Each step needs dry weather, and the roof must be sealed by dark. We stage the curb, the flashing membrane and the sealant on the roof before the crane arrives, and we hold the crew until the lift is complete so the dry-in happens the same day.
Winter picks up the risk. A mechanical swap in January can leave an open duct on a roof with snow forecast. We prefer to schedule swaps before the first heavy storm or after the spring thaw, and when a failed unit forces a winter swap, we plan the dry-in around the forecast and carry a temporary cover. For the larger picture of a roof where several units are being replaced, our roof replacement scope folds the curbs into the new membrane, and a recover raises them to suit the added layer.
Commercial Roofing Questions
Who pays for the curb when a tenant adds a unit?
That depends on the lease and on who ordered the unit. We price the curb and flashing as a separate line so the owner and the tenant can split it as their lease says. The number is the same either way.
Can we reuse an existing curb for a new unit?
Only if the new unit has the same footprint and weight and the curb is sound. We check the nailer, the corners and the height. A curb that has rotted or sits too low gets replaced, and that goes in as base scope.
How high should a curb stand above the roof?
Higher than the usual minimum where snow drifts. A taller curb keeps meltwater from running over the flashing top. We set the height from the roof's slope, the drift risk and the manufacturer's flashing requirements.
What happens to the warranty when a unit is swapped?
The swap can void it if the penetration is detailed outside the manufacturer's rules. We follow the membrane maker's published curb details and record the work with photographs. If the roof is still under warranty, send us the document before the swap is scheduled.
Do pipe supports need to be replaced during a reroof?
Usually yes. Old supports sit on worn pads or directly on the membrane. We replace them with supports that spread the load and protect the sheet, and we add them to the bid as a line item with a count.