Seven O'Clock on a Sunday, With Two Hours of Street
The lowboy is across two lanes with a 2,400 kg packaged unit chained to the bed, the all-terrain is set up on mats at the corner, and the closure order runs from seven until nine. Under the picking radius is a three-storey building with a two-year-old TPO roof on a no-dollar-limit warranty and a dental practice on the top floor that opens again on Tuesday. The eight minutes the unit spends in the air will be the easiest part of the day.
The two things that go wrong on this job go wrong months later and arrive in someone else's language. One is a phone call about a hum in the ceiling that nobody can hear standing on the roof. The other is a stain in a ceiling tile, blamed on the roof, and traced eventually to a boot print, a dropped self-tapper, or a length of gas pipe resting on a timber offcut. Neither is a rigging failure. Neither is visible on the day the crane leaves.
So the work worth doing sits either side of the pick: proving the curb the unit is going to meet, choosing springs against the fan's frequency rather than against its weight, and drawing the line the service crew will walk for the next twenty years so that the membrane survives being walked on. What follows takes the day in the order the unit meets the building.
What the Cabinet Actually Lands On
A curb-mounted unit never touches the roof. It bolts to a curb, the curb bears on the structural deck, and the membrane climbs the outside of the curb and terminates under a counterflashing. Every kilogram and every cycle per second the unit produces travels down through the curb into the deck. The roofing carries none of it, and any detail that asks the sheet to take load is a failure with a date already on it.
Four suppliers meet at that stack and none of them are on site at the same hour. The unit comes from the equipment manufacturer; the curb from the curb fabricator, usually matched to the model number and occasionally not; the flashing and the pads from the roofer; and the isolation from a fourth party whose lead time is the one nobody tracks. Every joint in the drawing below is a joint between two purchase orders, which is why the dimension that fails on the morning is almost never a dimension anybody calculated wrong.
What a curb-mounted rooftop unit stands on
- Unit base rail and cabinet — the footprint the curb has to match, plus the service clearance the manufacturer requires on every side that opens HVAC Equipment Pad/Curb Footprint Calculator
- Spring isolation rail — springs selected corner by corner against the load that corner actually carries, never against a quarter of the unit weight HVAC Vibration Isolation Spring Static Deflection Calculator
- Insulated roof curb — ordered against the unit model and set months earlier; its level and its opening positions are frozen by the time the unit arrives
- Curb base flashing — membrane taken up the curb sides and terminated high, quantified off the perimeter of every unit standing on the roof Rooftop Unit Curb Flashing Calculator
- Cover board and insulation — the build-up between the finished surface and the deck, and the layer that takes the punishment when somebody walks off the pads Roof Insulation Cover Board Calculator
- Steel roof deck — where the load and the vibration both end up; its span and stiffness decide how much isolator deflection the unit really needs Steel Deck Support Beam Spacing Calculator
The Weight on the Docket and the Weight on the Hook
The figure on the submittal is usually a base unit. Then the schedule picked up an economiser, hail guards, a powered exhaust section, a gas heat exchanger and a fourteen-inch curb, and every one of those has its own line in the weight table at the back of the product data. Rigging weight is the unit as it stands on the trailer plus the spreader, the slings, the shackles and anything already strapped to the top rail. Where the shipping paperwork carries a weighed figure, use it — a weight carried across from the last job of the same model is the classic late discovery, and it gets discovered with the load in the air.
Manufacturers put lifting holes in the base rail and then tell you not to use them without a spreader. The reason is geometry. Slings running straight from four base lugs to a single hook squeeze the cabinet as they close, crease the panel edges, and load lugs designed for vertical pull with a large horizontal component. The installation instructions state either a minimum spreader dimension or a maximum sling angle, and that sentence is a requirement rather than a suggestion.
Leg tension is the vertical share divided by the sine of the angle to horizontal, so a leg hanging at thirty degrees carries twice the load it appears to support. Four legs on a rigid frame do not divide by four either: with fixed-length legs the arrangement is statically indeterminate, and the practice behind ASME B30.9 credits two unless the assembly demonstrably equalises. A rooftop unit then adds an off-centre centre of gravity, because the compressors and the heat section sit at one end of the cabinet, so the pair of lugs at that end takes the larger share and sets the whole sling selection.
Above the hook, the crane work sits under OSHA 29 CFR 1926 Subpart CC, and any spreader or lifting beam is a below-the-hook device under ASME B30.20 with its own rated capacity and its own inspection record. The mechanical contractor is normally the crane's customer rather than the lift director, and being explicit about who holds that role — and who signs the plan when the picking radius crosses a public street — is a conversation for the pre-lift meeting and not for the pavement.
- Verify the unit weight as configured, then add the rigging: spreader, slings, shackles, and anything already strapped to the top rail.
- Get the curb gasket, the closure and any adaptor onto the curb before the unit leaves the bed — none of them can be fitted afterwards.
- Set tag lines early; a cabinet that broad turns in the smallest breeze and a turning load lands on a curb corner first.
- Bring the unit over the curb high, then descend slowly with a hand at each end aligning base rail to curb rail rather than cabinet to parapet.
- Seat it fully and check the rails are down all round before slacking the slings, and leave the crane hooked until the hold-down fixings are in.
The angle the spreader gives you decides what the worst leg carries, and the compressor end of the cabinet decides which leg that is — worth settling before the sling sizes are ordered rather than at the kerb on the morning.
The weight hanging below the hook, as weighed or as certified.
How many legs of the sling assembly are attached to the load.
The angle between a sling leg and the horizontal plane of the load.
How far the centre of gravity sits from the midpoint, as a share of the span between attachment points.
Tension in the most heavily loaded leg
1,270 lbf
With more than two legs the equal-share figure is the optimistic case. Compare it against the two-leg line in the breakdown, and use the two-leg figure unless the arrangement genuinely equalises.
- Load angle factor at this sling angle
- 1.15 multiplier
- Vertical share carried by the most heavily loaded leg
- 1,102.5 lbf
- Tension if only two legs are credited
- 2,546.11 lbf
- Minimum rated capacity needed for each leg
- 2,546.11 lb
- Sideways pull into the load at each attachment
- 636.53 lbf
They open the calculator with your figures already in it
Sling Leg Tension and Angle Factor Calculator: 1,273 lbf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1,270 lbf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Sizes the sling legs only. Shackles, the master link, the lifting points on the load and the structure behind them each have their own rating, and the assembly is governed by the smallest of them.
- Applies no design factor. A sling's working load limit already includes one, so the tension here is compared against the rated capacity at the angle of use, never against a breaking strength.
- Static only. Snatching a load, freeing one that is stuck, hoisting fast or swinging hard all put forces through the rigging that this figure does not contemplate.
- Says nothing about the crane. Rigging weight is a deduction from the chart capacity, and this page does not compute what remains for the load.
A Curb Somebody Else Set Last Autumn
The curb is usually the only part of this assembly the mechanical contractor inherits rather than buys. It went in with the deck or with the roof, it was flashed months ago, and by the time the unit arrives it is dimensionally frozen. An out-of-level curb cannot be corrected with a unit standing on it, and a curb whose openings miss the duct drops is a day of sheet metal on a roof with no power and no shelter.
Levelness is the check that pays for itself twice. The installation instructions state a tolerance, generally both across the full curb and over a shorter length, and a curb outside it does two things at once: it opens the cabinet gasket at the high corner, letting weather into the unit and conditioned air out of it, and it moves the low point of the condensate pan away from the drain connection, leaving standing water in a pan designed to empty itself.
Height is the other frozen dimension. The NRCA Roofing Manual recommends base flashing not less than eight inches above the roof surface, and every later decision eats into that: tapered insulation built up around the unit, an added cover board, a recover membrane. Measure from the finished surface as it will be, not as it is today. Then check the rails — curb rail width against the unit's base rail, opening positions against the supply and return drops, and whether the curb's internal framing crosses an opening it was never meant to cross.
| What to prove | How it gets proved | What it costs if it waits |
|---|---|---|
| Curb against the unit as configured | Submittal and curb fabricator's drawing side by side, not the model number on the label | A pick that goes back on the trailer, and a second road closure |
| Level across the curb and along it | Rotating laser or a levelled straightedge on the rails, before anything is buried | An open gasket at one corner and a condensate pan whose low point has moved |
| Flashing height above the finished surface | Tape from the roof surface as it will be, allowing for any overlay still to come | Base flashing short of the NRCA recommendation on a roof already under warranty |
| Opening positions and internal framing | Curb drawing against the duct drop layout, plus a look down inside the curb | Transitions improvised on the roof after the unit is landed |
| Gasket, closure and adaptor physically present | A count on the deck at first light, not a line on a delivery note | A unit sitting on a curb it cannot be sealed to until it is lifted again |
Springs Get Chosen Against a Frequency, Not a Weight
The complaint that follows a rooftop unit into an occupied building is almost never noise on the roof. It is a hum somewhere in the ceiling of the room below, worst at night when everything else has gone quiet, and it is structure-borne. The fan and the compressors put a periodic force into the base rail, the curb hands it to the deck, and the deck radiates it into the space below as sound from a surface far larger than the unit that caused it.
An isolator works by putting something soft in that path, so the frequency the machine forces at sits well above the natural frequency of the machine on its mounts. That natural frequency follows the static deflection: roughly 3.13 divided by the square root of the deflection in inches, in hertz. A fan turning at 900 rpm forces at 15 Hz; an inch of deflection puts the mounted natural frequency near 3.1 Hz, and a ratio around five is where isolation is doing real work. Below a ratio of about 1.4 the mount amplifies instead of isolating, which is why a pad that is too stiff is worse than no pad at all.
Deflection itself is Hooke's law and nothing more — supported weight divided by spring rate. The trap is in the words supported weight. It is not the unit weight divided by four: the cabinet's centre of gravity sits toward the compressor end, and manufacturers publish a corner-by-corner weight distribution for exactly this reason. Select each isolator against the corner it carries, and expect the springs at one end of the unit to be a different part number from the springs at the other.
Which configuration you are buying matters more than the number. Some packaged units are internally isolated, with the fan section on its own mounts inside a cabinet that is hard to the curb — that treats the fan and does nothing about compressor-borne force. An isolation curb is a two-part curb with springs built into its upper rail and a flexible seal that lets the top move while keeping weather out, and on a ducted, curb-mounted unit it is the arrangement that works, because it solves the duct connection at the same time as the mount. Spring rails set between a conventional curb and the unit raise the cabinet and leave the duct connection and the weather seal to be solved separately — fine when it is designed that way, and a leak when it is improvised on the roof.
Springs free to move vertically still have to be held down. The wind provisions for rooftop equipment and the seismic requirements for nonstructural components both land on this connection, and the answer is a restrained or snubbed isolator that permits the vertical travel the isolation depends on while carrying uplift and sliding into the curb. ASCE/SEI 7 governs the loads; the SMACNA Seismic Restraint Manual is where the attachment gets detailed. Neither of them accepts a spring mount held in place by nothing but the weight sitting on it.
Treat the ranges below the way the standards treat them — a starting point, not a selection. The isolation efficiency calculation compares the disturbing frequency at the equipment's real operating speed against the isolator's natural frequency, and the isolator manufacturer's tables are what turn that answer into a part number. A deflection figure on its own tells you what a spring does under load; it does not tell you whether that spring solves the tenant's problem.
| Where the equipment sits | Deflection usually targeted | Why it moves |
|---|---|---|
| Slab on grade | about 0.25–0.35 in | The support barely deflects, so a comparatively stiff mount still gives a usable frequency ratio |
| Upper floor of a framed structure | about 0.75–1 in | The floor deflects under the load, and the isolator has to stay soft relative to it |
| Fans generally | up to about 2 in | Low-speed fans force at frequencies close enough to the mount to need the extra travel |
| Rooftop units on a framed roof | about 1–4 in | A long-span roof deck is usually the most flexible support anywhere in the building |
Put the corner load and the candidate spring rate in and the deflection falls straight out, which is the quickest way to see whether the spring a supplier has quoted lands anywhere near the range this mounting position needs.
The share of equipment weight carried by this single spring isolator.
The isolator spring's stiffness — the load required to compress it by one inch.
Static deflection under load
1.67 in
Static deflection alone doesn't confirm adequate vibration isolation — actual isolator selection should follow the full isolation-efficiency calculation comparing the equipment's disturbing frequency (from its operating speed) against the isolator's natural frequency, per ASHRAE Applications Handbook and the SMACNA Seismic Restraint Manual. Typical target deflection ranges (0.25-4 in / 6-100 mm, depending on mounting location and equipment type) are a starting point for isolator selection, not a substitute for that calculation.
They open the calculator with your figures already in it
HVAC Vibration Isolation Spring Static Deflection Calculator: 1.67 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1.67 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- It assumes the weight is shared equally, and it almost never is. Equipment centers of gravity sit off to one side — the motor end of a fan, the compressor end of a chiller — so the loaded mounts take far more than total weight divided by mount count. Select every mount from that average and the heavy corners compress toward coil bind while the light ones barely move, the unit sits out of level, and the isolation is worst precisely where the load is greatest.
- A spring only isolates if what it stands on is far stiffer than the spring. On a long-span floor the structure itself deflects under the same equipment load, and once that structural deflection approaches the spring's the two behave as one soft system and the isolation largely disappears. That is why upper-floor equipment gets the deeper mounts, and why the floor's deflection under the unit has to be known before the mount is picked.
- Whatever is bolted to the equipment can undo the entire mount set. A rigidly connected pipe, conduit, drain or duct carries vibration straight past the springs into the structure, and one hard-piped line will short-circuit a correctly selected isolator. Flexible connectors, and resilient hangers on the first several pipe supports away from the machine, are part of the isolation rather than accessories to it.
Half of Every Isolator You Buy Is the Deck Underneath It
Isolation is relative. A spring giving an inch of deflection, sitting on a bay that itself deflects appreciably under the unit, is not an inch of isolation — the structure has joined the spring, and the mounted frequency climbs back toward the frequency you were trying to get away from. ASHRAE's guidance on this is straightforward and routinely ignored: choose an isolator deflection that is large compared with the deflection of the structure at the point of support. It is the whole reason rooftop targets run to several inches while the same machine on grade needs a fraction of one.
Position follows from the same fact. A curb landing over a joist or a beam line is carried by something far stiffer than a curb landing mid-bay, and the difference is real enough that shifting a unit half a metre to clear a drain is a structural decision as much as a drainage one. That call belongs to the engineer and on the drawing — but the crew needs to know it is a call, because on the morning of a pick, "we moved it to miss the sump" sounds like housekeeping.
None of this touches airborne noise. Fan noise travelling down the supply duct into the ceiling plenum arrives by a completely separate route and is cured by lined plenums, silencers and duct geometry; no spring attenuates it. When a complaint survives a correctly isolated unit, the next place to look is the duct, not the mount.
The Boots Take the Same Line Every Time
Nobody wanders on a roof. From the hatch or the ladder there is one sensible route to the unit, and every filter change, every belt, every service call and every controls contractor takes it. The wear that results is a strip rather than a field: scuffing and granule loss along the line, grit ground into the sheet, and the occasional dropped self-tapper standing on its point until the next person puts a boot on it.
Protection starts during the installation, not at handover. The rigging crew, the sheet metal crew and the pipefitters will all stand somewhere, material will be laid down somewhere, and none of it belongs on a bare membrane. Temporary protection boards over a slip sheet along the working route and under any staging area are the cheapest insurance on the job. Compatibility matters as much as coverage — single-ply sheets have documented incompatibilities, PVC with asphalt-based products and EPDM with petroleum oils among them — and the membrane manufacturer's own literature, not general practice, is the list that decides what may sit on the roof.
The permanent pads come out of the roofing system and go down to the membrane manufacturer's detail, welded or adhered by the roofer. Lay them with gaps between panels rather than as a continuous ribbon, so water crossing the route drains between them instead of standing behind a dam made of walkway. Run the path from the access point to the service side of the unit, and widen it where the technician actually stands: at the control panel, at the filter access, and wherever a removed panel gets put down.
Quantity comes off the route as walked rather than as drawn. Measure the real line, including the dog-leg round the drain and the detour round the units already up there, and take the width from what the path has to carry — one person, a panel laid flat, a trolley of filters.
Route length by working width gives the area to protect, and this turns it into whole panels at the size your membrane supplier actually sells — order against that number, then add for the cuts at the turns and the separate pad at the hatch.
The total area of the protective walk path to be covered.
The surface area covered by one walk pad panel.
Walk pad panels needed
25 panels
They open the calculator with your figures already in it
Roof Walk Pad Protection Panel Calculator: 25 panels — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Treats the path as solid coverage, and walk pad is not laid solid. Pads are set with gaps between them so water crossing the roof passes through the run instead of damming behind it — a continuous strip laid across the drainage path ponds water on its uphill side, which is the exact condition the pads were there to keep off the membrane. Those gaps also mean a given route needs fewer panels than a straight area division suggests.
- Says nothing about whether the pad will bond to your roof. Walk pad is chemistry-matched to the membrane: a PVC pad will not heat-weld to TPO, and an adhered pad on a mechanically attached or ballasted roof needs the membrane manufacturer's own attachment detail. This is not merely a loose pad — putting a foreign product onto a warranted membrane is one of the routine ways a roof warranty is voided.
Everything That Stays on the Roof After the Crane Leaves
Condensate is the item that quietly damages a roof it was never meant to touch. A draw-through unit needs a trap deep enough to overcome the fan's negative static, or the pan holds water whenever the fan runs and the drain does nothing; that depth comes from the unit's static pressure, not from a standard fitting off the van. Where the water goes afterwards is a code question — the International Mechanical Code's general regulations cover condensate disposal in whichever edition your jurisdiction has adopted — and discharging it straight onto the membrane stains the sheet, feeds growth along the run, and in a freezing climate builds ice exactly where the roof needs to drain.
Gas, power and controls all arrive after the unit is set, and all of them want to rest on something. Gas piping falls under NFPA 54 / ANSI Z223.1 with its shutoff and sediment trap at the appliance; the disconnect within sight of the equipment comes from NFPA 70 Article 440, along with the service receptacle the code requires for rooftop equipment. All of it is carried on supports the membrane manufacturer accepts, spaced so that nothing sags onto the sheet between them — never on timber offcuts, never on a paver laid straight onto the membrane, and never hung off the curb flashing.
Placement is decided before all of this and regretted afterwards. The outdoor air intake wants clear distance from plumbing vents, from kitchen and toilet exhausts, and from the discharge of anything else already on the roof, at the separations ANSI/ASHRAE Standard 62.1 sets out. A unit sited to suit the crane and the duct drop, then found to be drawing its fresh air off a vent stack, is a problem no isolator and no flashing detail can answer.
A Spring Doing Nothing Looks Exactly Like a Spring
Spring isolators ship blocked or bolted solid so they are not working in transit, and blocking that never gets released turns the assembly into an expensive hard mount that looks entirely normal from three metres away. The tell is the gap. A released, loaded mount has clearance between the equipment rail and the housing, with the load on the spring rather than on steel touching steel. Check every mount on every corner after the unit is landed, and again once it is charged, running and at its real operating weight.
The same visit levels the cabinet on its mounts, brings the springs to a consistent free height, and proves that condensate actually leaves the pan with the fan running rather than only when it is switched off. Photograph the isolators with the gap visible and the shipping blocks in a pile beside them — it is the only record that survives a change of service contractor.
The roofing handover is a separate document and it matters as much. The roofer closes, inspects and photographs every penetration, the warranty inspection gets booked, and those photographs go into the O&M file with the pad layout. What you leave behind is what the next crew works from: the ladder or stair the mechanical code requires for equipment at that height, the guarding where the unit sits close to the roof edge, and the anchor points. Once the building is occupied, the technicians using them are working under OSHA 29 CFR 1910 Subpart D rather than the construction rules the installation ran under, and the roof either supports that or quietly does not.
- Release and remove every shipping block, then confirm a visible gap at each mount with the unit running.
- Level the cabinet on its isolators and bring the springs to a consistent free height.
- Prove the condensate trap with the fan running, not with the unit switched off.
- Have the roofer close, inspect and photograph every penetration and the finished pad run.
- Walk the completed route from the access point carrying a full bag, and move any pad that is in the wrong place while the roofer is still on the roof.
- File the corner loads, the isolator part numbers and the curb drawing with the O&M — they are what the replacement unit gets selected against in fifteen years.
Settled before the road closure is booked
The quantities on this job are small and the cost of missing one is not, because almost everything here has to be on the roof before the unit is — or ordered after the crane has already gone home.
- Corner loads from the unit's own weight distribution — Four different numbers, taken from the manufacturer's distribution table for the unit as configured, never the shipping weight divided by four.
- Isolators, by corner and by restraint type — Deflection chosen against the fan and compressor frequencies and the flexibility of the deck; restrained mounts wherever wind or seismic requirements reach this connection.
- Curb gasket, closure and any adaptor — A one-shot item — none of it can be fitted once the cabinet is seated, and none of it is worth a second pick.
- Temporary protection over the working route — Boards and a slip sheet wherever crews will stand or stage material, in a material the membrane manufacturer accepts against that sheet.
- Permanent walk pads, access point to service side — Measured on the walked line rather than the drawn one, and laid with gaps so the route drains instead of damming.
- Rigging: spreader, slings and shackles — Sized on the tension in the worst leg at the angle the spreader gives, with the compressor end of the cabinet setting the selection.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.