# Why a rooftop HVAC unit should never just sit on its curb

By MarketScale Newsroom · Published 2026-10-29 · Building Management on MarketScale
Canonical: https://www.marketscale.com/industries/building-management/why-a-rooftop-hvac-unit-should-never-just-sit-on-its-curb-1286327

> A blind inspection of an adhered EPDM roof found HVAC units rocking on their curbs and caulk as the only seal over conduit fasteners.

## Key points

- Codes no longer treat unit weight as restraint; many jurisdictions require an engineer to certify curb and restraint compliance.
- A hold-down bracket on a standard curb satisfies no wind code by itself; the rated system runs from unit to curb to structure under a professional engineer's stamp.
- FM Global's uplift ratings carry a safety factor of two, so an FM 1-60 assembly is meant for a 30 psf design load, and the rating number is not a wind speed.

Brandon Reynolds put one hand on a rooftop HVAC unit and pushed. The unit rocked under one hand. Nothing held it to the curb beneath it, and behind him, past the gutter edge, was a narrow sidewalk and the lot where the building's employees park.

Reynolds, OEM business manager at Anchor Products, was recording a rooftop equipment blind inspection for a MarketScale Studio segment. From the ground he had seen parapets and some equipment and did not know what membrane was up there. On the roof he found adhered EPDM, a rubber sheet glued down to the insulation rather than screwed through it, and a building owner who told him the roof was about ten years old.

The timing comes down to a bead of caulk. Reynolds pointed at the cracks in it and expected them to widen as winter set in.

The question he was asking is also live in the trade. In January 2026 curb maker MicroMetl wrote on its own blog that its sales desk keeps fielding requests for hold-down brackets from buyers who believe the brackets alone satisfy wind code, and in May 2026 rooftop support maker MIRO Industries published its own guide to meeting ASCE 7-22 wind loads without cutting a membrane. Both are manufacturer material, and both exist because facility managers keep asking what Reynolds asked on that roof.

## Codes stopped counting unit weight as restraint

Many U.S. jurisdictions require a licensed engineer to certify that curbs and restraints meet the building code, Professional Roofing reported in July 2007. The magazine's explainer by Paul Selman traced how that came about. Before comprehensive codes, crews set rooftop equipment without attaching it to the structure on the theory that a unit's weight would keep it put, and research into natural disasters showed that resisting wind and seismic loads means restraining the unit from leaving its curb and tying the curb to the building.

A curb is the raised frame, flashed into the roof, that a rooftop unit sits on. When wind hits the unit, the connection between unit and curb has to resist tension, shear, moment and uplift, and the curb has to carry those forces into the deck and framing, according to Professional Roofing. Selman described the design as three steps: identify the governing code for the site, calculate the design force from wind or earthquake (both, where it is not obvious which is larger), then specify how the unit attaches to the curb and the curb to the roof.

> These HVAC units are not actually attached to the curb. They simply are sitting on the curb. — Brandon Reynolds, OEM Business Manager, Anchor Products

Anchor's own site says most rooftop units in service were set before anyone asked how they would hold in high wind, and that the question usually arrives with a code review or an insurance inspection. For the facility manager, years of trouble-free operation answer nothing. The thing to find out is whether anyone ever ran the calculation.

## A hold-down bracket by itself is not a wind rating

MicroMetl wrote in January 2026 that a hold-down bracket added to a standard curb fulfils no wind code requirement on its own. A hold-down bracket screws into the base rails of the unit at one end and into the curb at the other, and a series of them keeps the unit from lifting off the curb, the company explained in its own material. What code looks for is the whole path: brackets sized for the unit, a curb sized for the brackets, and an anchorage from curb to structure, with screw types, quantities and metal gauge specified, all reviewed and stamped by a professional engineer registered in the project's state.

Here the vendor complicates Reynolds's thesis as much as it supports it. In many parts of the country, MicroMetl wrote, wind pressures are low enough that a unit's weight keeps it in place for its service life, and it is hurricane regions that need more than weight. That qualification leaves a facility manager to ask where this roof's design pressure falls, and Reynolds did not say where the building stands.

MicroMetl traced the modern curb rules to Hurricane Andrew in August 1992, which it said brought 165 mph winds and $25 billion to $26.5 billion in damage in Florida alone. Since then, the company wrote, the Florida Building Code's wind provisions have become the template across the United States.

Reynolds's answer is the position of this particular unit. It sits at the back gutter edge with a sidewalk and parked cars below.

The standard behind every one of those calculations is ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. MIRO's May 2026 guide lays out which parts apply: Chapter 26 sets the design wind speed and exposure category for a site, Chapter 29 covers wind loads on rooftop structures and equipment, and the standard is adopted by reference in the 2021 International Building Code. ASCE's own contents for 7-22 list Chapter 30, Components and Cladding, alongside those, with errata to the standard effective February 2026.

So there is no single wind speed at which anchoring becomes mandatory. The threshold is a design pressure computed for each site and each piece of equipment, which is why the engineer's stamp matters more than the hardware.

## Caulk is the only seal over the conduit fasteners

Along the back edge Reynolds found a run of small electrical conduit held by brackets screwed straight through the membrane, each screw head buried under caulk. A shutoff box nearby was fastened through the parapet membrane with no caulk at all. Water is getting into that wall whether the owner knows it or not, Reynolds said.

PHP Systems/Design, a rooftop support maker, wrote on its own site in April 2016 that every membrane penetration adds a weak link to the roof. Contractors seal each one, the company wrote, but most sealants dry out and crack over time and give moisture a path into the building, with mold and leaks the result. MIRO's May 2026 guide makes the parallel point about coverage: a penetration may affect the membrane warranty unless it is installed to the membrane manufacturer's approved details, meaning the maker's own drawings for how that penetration is flashed.

Carlisle SynTec now warrants its 90-mil EPDM membrane for 40 years, it announced in October 2024. The company said in a press release that the non-prorated material warranty on its Sure-Seal and Sure-White 90-mil sheets rests on accelerated weathering tests and on roofs past 40 years still in service, and that ballasted 60-mil systems built to 30-year detailing now qualify for 30-year warranties.

Reynolds did not name the membrane maker on this roof, and the owner's hope for two to three decades, as Reynolds relays it, sits well inside what the market now sells. The caulk over each screw head, by Reynolds's count, needs replacing roughly every year, or every other year at best. That gap is the story of this roof.

## What an engineered attachment does that a caulked screw cannot

An engineered attachment moves the load off the sealant and onto a part designed to carry it. MIRO describes two non-penetrating mechanisms on its site. Ballasted bases counteract uplift with weight on a wide footprint, with the amount of weight calculated from the site's ASCE 7-22 design wind speed and the base geometry, while cable bracing and stanchions tether equipment laterally to the building perimeter or to structural members instead of fastening through the deck.

The company adds two cautions. A base that concentrates too much weight on too small a footprint depresses the insulation and creates a bowl where water pools, so the pounds per square inch on the membrane have to be checked against the roof's limit. And a support that shifts in wind will abrade the membrane under it.

Anchor's approach, described on its own site, flashes an anchor into the roof instead of resting on it. The U-Anchor base is a galvanized steel plate with rolled edges that sits under a cover membrane matched to the roofing brand, type and color, leaving a stainless stud at the surface for hardware to bolt to. For a unit already in place, the company's Existing Mechanical Tie Down runs cable from the unit to bases set around it and tensions the cable against uplift, so the unit and its curb stay where they are.

Anchor says each base is considered a component of the roof and is included in the roof warranty, and that the base model is chosen by the membrane on the building rather than by the equipment. That is the company's claim about its own product. Whether a given membrane maker accepts a given base is a question for that maker's detail drawings.

Set that next to what Reynolds found. A field-fabricated pocket is a small open-topped box built on the roof around a pipe or conduit and filled with sealant, and uncured EPDM flashing is rubber that has not been vulcanized, so it stretches around odd shapes but lacks the strength of the cured sheet. A bracket through the membrane with caulk on top has no plate spreading the load and no cover membrane over the hole.

## FM Global measures uplift in pressure, and a pressure is not a wind speed

FM Global's test loads a roof assembly from below in 15 psf steps, each held for one minute. The Canadian Roofing Contractors Association (CRCA) summarized the method in a June 2007 bulletin: pressure rises in those increments until something fails, and the assembly earns a rating from FM 1-60 up to FM 1-999 matching the last pressure it survived. In a fully adhered system, any separation at all, including a field seam letting go, counts as failure.

A psf is a pound of pressure per square foot of roof. Because FM applies a safety factor of two, CRCA noted, an assembly rated 1-60 is recommended for a building whose design wind load is 30 psf, and the rating number has no relation to wind speed.

Those ratings describe the roof cover, insulation and deck, not the unit on the curb. They still frame the risk on an adhered rubber roof, because the same pressure difference that lifts a membrane is what Reynolds was describing when he rocked the unit.

CRCA also set out where FM's authority begins and ends. FM Approvals does its testing and certification in-house, and only an FM engineer can certify that a roof meets FM requirements, which applies when a project specifies them. The guidance sits in Property Loss Prevention Data Sheets 1-28, Design Wind Loads, and 1-29, Roof Deck Securement and Above-Deck Roof Components, which CRCA described as engineering guidelines.

That is the line between require and recommend. A code official enforces the adopted building code, while an insurer's data sheet is a condition of coverage and rating where the policy calls for it. Neither FM's guidance as CRCA summarizes it nor the ASCE standard publishes a count of how often unanchored units actually leave their curbs.

## The conduit that pulls on its own flashing

The last stop on Reynolds's walk was a conduit that nothing held up. Its weight was dragging on the side wall of a field-fabricated pocket wrapped in uncured flashing, and the pocket was visibly leaning. On another unit, conduit rested on an old roof pipe and had bent downward between its first 90-degree turn and the next; when Reynolds lifted it, it floated free, which means its whole load had been sitting on the pipe flashing.

Neither the standards bodies nor the support manufacturers put a public price on lifting a conduit run onto proper supports or tying down a unit, and none of them prices the leak that follows a failed pocket. What a facility manager can compare is maintenance per penetration against the roof's remaining life. Every caulked screw head is a recurring line item, and a support that carries its own load is not.

Reynolds's walk suggests what a facility manager can do before winter without hiring anyone: put a hand on each rooftop unit, look under each conduit bracket for cracked caulk, find any fastener into a parapet with no seal over it, and ask whether a professional engineer ever stamped the attachment from unit to curb to structure. Anchor says the next segment in its blind inspection series will show the crew supporting and securing the equipment on this roof. The test Reynolds points to is the one he performed: a unit that moves under one hand is not attached.

## Sources

- [ASCE/SEI 7-22, Chapter 30, Wind Loads: Components and Cladding](https://amplify.asce.org/content/standard/9780784415788/part/provisions/standard-chapter/s30) (ASCE)
- [Considering curbs, Professional Roofing, July 2007](https://www.professionalroofing.net/Articles/Considering-curbs--07-01-2007/1101) (Professional Roofing)
- [Changes to FM Global Loss Prevention Data Sheets, CRCA, June 2007](https://roofingcanada.com/bulletin/changes-to-fm-global-loss-prevention-data-sheets/) (Canadian Roofing Contractors Association)
- [HVAC Hold Down Brackets and Wind Certification, MicroMetl blog, January 2026](https://blog.micrometl.com/hvac-wind-bracket-hurricane-bracket-wind-certification/) (MicroMetl)
- [SpecTips: New Extended Warranty Options for EPDM, Carlisle SynTec, October 2024](https://www.carlislesyntec.com/en/Resources/Media/Blog-Landing-Page/SpecTopics/2024/10/08/New-Extended-Warranty-Options-for-EPDM) (Carlisle SynTec)

Tags: Anchor Products, rooftop HVAC, EPDM roofing, wind uplift, facilities management, roof penetrations

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