In a spring mount roof curb, the spring mounts sit between the base curb and the upper curb, fastened to the base curb’s interior rail under the upper curb’s load points, so the HVAC unit’s weight reaches the roof only through the springs. On a data center build, that small interface repeats under every rooftop cooling unit, and how it’s bought decides whether those units set on schedule. (New to the product? See what a spring isolation curb is.)
Why does the mount interface matter on a data center roof?
Because one detailing error gets copied under every unit on the roof. Why isolation is non-negotiable here is covered in our mission-critical spec checklist; scale is the newer pressure. Lawrence Berkeley National Laboratory’s 2025 Update (June 2026) projects data centers could reach 11.8% of U.S. electricity use by 2030. A mount that misses its rail on one curb misses it on the whole package.
Where does the spring mount sit in a spring isolation curb?
Each mount is a captive spring: housed, fastened top and bottom, and adjustable for level. ASHRAE’s HVAC Applications Handbook (2023), Chapter 49, classes these curb isolation systems as type D: spring isolation in a watertight assembly.
| Interface | Its job | What to check on the submittal |
|---|---|---|
| Base plate (welded or bolted) | Ties the mount to the interior rail | Full bearing; isolation if the curb is aluminum |
| Mounting stud, leveling adjustment | Carries and levels the upper curb | Stud size, adjustment range |
| Restraint housing, limit stops | Stops spring extension when load comes off | Stops clear in operation |
| Snubbers / restraint clips | Resist wind and seismic load | Restraint in each horizontal direction |
| Weather seal | Keeps the opening watertight | Seal doesn’t bridge the springs |
Mount count follows unit weight and footprint, spaced so each spring carries its share; changing the count changes every mount’s load, so rerun the deflection selection at each mount.
Which spring mounts does a data center roof call for?
Restrained spring mounts, ASHRAE type 4, with seismic restraint at the mount, not the curb. Manufacturer specifications call for housed mounts with limit stops on roof-mounted air handlers, so springs can’t extend when wind lifts the unit or weight comes off for service. ASCE 7-22, Table 13.6-1, requires a snubber or bumper restraint in each horizontal direction on vibration-isolated components, and doubles the design force when the air gap exceeds 0.25 in. Check the unit, too: a unit manufacturer’s curb guide warns against adding curb springs under internally isolated fans without an engineered analysis.
Why do data center builds expose the split-scope problem?
Because the spec can put the curb and its springs in different divisions, and different trades buy them. Many owner master specifications list roof curbs, including spring-isolated ones, under Section 07 72 00, Roof Accessories. Federal guide specifications place spring isolators and curb-mounted isolation bases in the HVAC division (VA Section 23 05 41; UFGS 23 05 48), inside the mechanical contractor’s package.
Data center schedules leave no room for that gap. HVAC manufacturers’ supply chains are already strained, so units often arrive late; a curb package that needs rework at set adds a second delay. The result is two suppliers, two submittals, and one connection nobody owns. Mismatches show up with the crane booked: a mount wider than the rail, a stud pattern that misses the upper curb, a stack height that moves the duct connection.
Should a data center GC buy spring mounts with the curb?
Yes, unless a stamped isolation design already fixes the mount.
| Integrated (curb + mounts) | Separate (mounts via mechanical scope) | |
|---|---|---|
| Interface ownership | One supplier | Split; GC coordinates |
| Submittals | One drawing set | Two, checked against each other |
| Field-fit risk | Resolved in the shop | Found at set |
| Phase-to-phase consistency | Same detail every phase | Depends on two vendors |
One check first: some Division 23 specs require every isolator on the job to come from a single isolation manufacturer. If yours does, get the integrated curb package approved under that section before the PO, not after.
Custom Curb supplies the spring isolation system inside the curb package, and the person who designs your curb is the person who answers your RFI. See our spring isolation curbs for data centers.
How should a data center GC write the mount scope?
Name one supplier as owner of the curb-to-mount interface, in the PO itself. If the PO is silent, the GC coordinates it by default. A scope line along these lines closes the gap:
Furnish spring isolation curbs complete with restrained spring mounts and seismic restraint, designed to the HVAC manufacturer’s curb requirements for each unit. Include mount data, restraint ratings, and the mount-to-rail attachment detail in the curb submittal.
On a phased campus, that ownership pays twice: the first building’s approved curb-and-mount detail becomes the template for every later phase, so the last curb matches the first.
Frequently Asked Questions
Are roof curbs specified in Division 07 or Division 23?
Often both. Roof curbs commonly sit in Section 07 72 00, Roof Accessories, while vibration isolators sit in Division 23 (Section 23 05 48). Check where your spec puts the spring isolation curb before buying.
Can the mechanical contractor supply the spring mounts separately?
Yes. Then require the curb supplier to design the rail to that mount and include its data in the curb submittal.
Are seismic restraints part of the curb or the mount?
The mount. ASCE 7-22 requires horizontal snubbing at vibration-isolated components.
Who is responsible if the spring mounts don’t fit the curb?
Whoever the PO names. With no scope line assigning the interface, the GC carries the coordination.
Do spring mounts need the roof to be level?
Yes. On a sloped roof, the base curb is pitch-corrected so mounts bear level.
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