Installation of a GREE FLEXX R32 ULTRA central heat pump (by Dettson) on a flat roof in Upton, in the Acton RCM

Two hundred and forty-one pounds on a roof: the Upton project, from structural calculations to the first start-up

The owner of a two-story brick building alongside the main road Upton, in the Acton RCM, called us at the beginning of summer. A commercial space on the ground floor, an apartment upstairs, no usable land on either side of the building, and only one available clearance area: the flat roof.

This kind of project cannot be solved by choosing a unit from a catalog. It is solved by answering, in order, three questions: can the structure support the load, can the roof accommodate the equipment without being penetrated, and will maintenance be possible in February? Here is how we answered them before installing a GREE FLEXX R32 ULTRA central heat pump, distributed in Quebec by Dettson.

Why the roof, and what to check before considering it

Placing an outdoor unit on a roof is an engineering decision, not a space decision. Three checks are required, and none is optional.

The structure

The selected outdoor unit weighs 241.4 lb (109.5 kg) when dry. With its support, treated-wood sleepers, and the snow load that will accumulate around it, the actual load transferred to the roof far exceeds the weight of the unit.

A residential or commercial flat roof is designed for a distributed load, not four concentrated point loads. That is why the sleepers visible at the Upton site are used: they distribute the load across several joists rather than concentrating it between two. In a building with an uncertain structure or whose use has changed over the decades, we require validation by a structural professional before proceeding with anything. This is not a formality: a roof that deflects under a point load creates a depression, and a depression on a flat roof retains water.

The membrane

No screws were installed in this roof. That is the rule at AirGreen for every membrane roof, and it has a simple reason: every puncture is a potential failure point, and the roofing warranty generally does not survive third-party fastening.

The assembly therefore rests on a three-level system:

1. Membrane-protection pads, placed directly on the roofing to prevent any wood-to-membrane contact and abrasion.
2. Treated-wood sleepers that distribute the load perpendicular to the joists.
3. An aluminum support that raises the appliance above the roof surface.

Wood-to-membrane contact without a cushion is one of the most frequent causes of roof failure we see during our inspection visits: wood retains moisture, expands and contracts through freeze-thaw cycles, and eventually wears down the surface beneath it.

The wind

A two-story building beside an unobstructed traffic corridor is exposed. An appliance 1,260 mm high and only 340 mm deep presents considerable wind exposure on a narrow base.

The support was therefore secured to the roof sleepers and the equipment as a single integrated assembly, and the orientation was chosen so that the most exposed face would not be the one offering the greatest wind resistance. An appliance that tips over on a roof costs more than just the appliance: it costs a roof, and sometimes more.

The selected equipment: 54,000 BTU/h and 10.5 HSPF2

The choice almost no one explains: 48,000 or 54,000 on the same chassis

Here is a nuance that every property owner comparing quotes should know.

The outdoor chassis GUD60W2/NhE-D(U) accepts two different indoor units. Depending on which one it is paired with, it produces two systems with distinct characteristics:

• With the 48,000 BTU/h indoor unit: 19 TRÉS2 and 10 CPSC2. This is the best cooling efficiency in the range.
• With the 54,000 BTU/h indoor unit: 18.5 TRÉS2 and 10.5 CPSC2. This is the best heating efficiency in the range.

The same outdoor unit, two different AHRI numbers, two different performance profiles.

The choice therefore depends on the building's actual consumption profile. In Upton, the ground-floor commercial space is occupied year-round, and the heating season in Montérégie lasts about seven months, compared with three to four months of active cooling. The 10.5 CPSC2 combination wins by a wide margin over the system's service life, even though it shows half a point less TRÉS2 on the data sheet.

The opposite would be true for a south-facing, fully glazed retail space with a high internal heat load, where the cooling load dominates. This is exactly the kind of trade-off we document in our quotes, backed by figures, rather than systematically proposing “the biggest” option.

The coil's protective grille

The unit's side coil has been fitted with a metal protective grille. On an accessible roof, it addresses three situations at once:

• Hail. An aluminum-finned coil deforms under sizable hailstones, and a crushed coil permanently loses heat-transfer efficiency.
• Debris. Leaves, roofing gravel, pieces of insulation, bags: anything that blows onto a flat roof eventually gets plastered against the coil.
• Accidental contact. A roofer, sign installer, or telecommunications technician working on this roof will not necessarily see where to place their ladder.

A grille does not replace annual coil cleaning—it only makes it less urgent.

The layout of refrigeration piping on a roof

This is the area where we see the most shoddy work during our corrective interventions, and it has the greatest impact on the compressor's service life.

The insulation that does not survive the sun

Standard elastomeric rubber insulation exposed to direct ultraviolet radiation on a roof cracks within two to three seasons. Once cracked, it absorbs water, freezes, comes loose, and the suction line begins to condense directly onto the membrane. The result is a continuous loss of efficiency and permanent runoff onto the roof covering.

On this project, the exposed piping received UV-resistant protection along its entire length above the roof. This expense of a few hundred dollars prevents having to redo the entire route five years later.

Elevation difference and oil return

The FLEXX allows a maximum length of 98.4 ft (30 m) between the units and a maximum elevation difference of 49.2 ft (15 m). In a single-story house, these limits are theoretical. In a two-story building where the indoor unit is in the basement and the outdoor unit is on the roof, the elevation difference becomes an actual design consideration.

This elevation difference is not merely a matter of distance: compressor oil circulates with the refrigerant and must be able to return upward. A poorly designed vertical route traps oil at the low point and gradually starves the compressor. We therefore measure the actual route, the exact elevation difference, and the transition points before ordering the equipment, never afterward.

The unit is precharged for 24.6 ft (7.5 m) of piping; beyond that, 0.22 oz per foot (20 g/m) must be added for this model, with a 3/8 in. liquid line and a 3/4 in. suction line. The additional charge is weighed on a scale and recorded in the commissioning report.

The supports

The piping should never rest directly on the membrane. It is supported by supports placed on protective pads at regular intervals, with a slight slope and no low points where moisture could stagnate. A line dragging across a hot roof in July means guaranteed abrasion with every expansion.

Electricity: the 347/600 V trap

A critical point on a commercial building, and one that is often discovered too late.

The FLEXX operates on 208–230 V single-phase power. However, a large proportion of commercial buildings in Quebec are supplied with 347/600 V three-phase power. An appliance of this type cannot be connected to such a system: an existing 120/208 V panel with available capacity or a step-down transformer is required, which substantially changes the project cost.

With an MCA of 39.9 A and a maximum protection of 45 A, verifying the available capacity is not something to assume can be done on-site on the day of installation. We systematically perform the electrical load calculation and validate the supply voltage at the quotation stage. A customer who learns at delivery that they need a transformer has every right to be unhappy.

The disconnect switch was installed on the brick wall, within sight of the unit, with a liquid-tight flexible connection. The wall penetration was sealed and oriented downward to prevent capillary infiltration.

Vibrations, noise, and the neighbour downstairs

A rooftop unit is mechanically coupled to the building structure. Without isolation, its vibrations travel down into the joists and turn into a continuous hum in the upstairs living space—a low-frequency sound that is difficult to locate and makes a bedroom uncomfortable.

Three measures were implemented:

• Anti-vibration pads beneath each of the unit’s four support points on its base.
• Decoupling between the support and the sleepers, rather than a continuous rigid assembly.
• Positioning that avoids direct alignment with sleeping areas.

The unit is rated at 63 dB(A) at high speed. In modulation mode, it runs well below that level most of the time. Nevertheless, sound carries on a roof: we always verify the applicable municipal regulations for sound levels and distances from property lines before determining the location. These rules vary from one municipality to another in Montérégie, and relocating an improperly placed unit comes at someone’s expense.

Defrost water on a flat roof in winter

In heating mode, the unit periodically defrosts its outdoor coil and discharges several litres of water per cycle. On the ground, this water seeps into the soil. On a flat roof, it flows toward the drain—or freezes before getting there.

Two rules apply:

• The clearance beneath the unit must be sufficient to ensure that an accumulation of ice does not reach the coil.
• The location must account for the path of water toward the roof drains, without ever placing the unit directly above a drain, whose access must remain clear for roof maintenance.

What building owners need to know before signing

Financial assistance: LogisVert or Solutions efficaces?

It is the question that costs the most when answered incorrectly, and it arises as soon as the building is not an ordinary single-family residence.

• LogisVert is Hydro-Québec's residential program. It applies to houses, condominiums, multiplexes, and buildings with a maximum of 19 dwelling units. The amount is calculated based on the capacity at -8 °C of the specific AHRI combination installed, and ENERGY STAR certification is mandatory for any installation completed since November 26, 2025.
• Solutions efficaces is Hydro-Québec's program for businesses, with one component for small businesses and another for medium-sized and large businesses. The financial assistance framework is different, as is the application process.

A mixed-use building like the one in Upton therefore requires determining, at the quotation stage, which program applies to which portion and under which account. We prepare the file with the owner before ordering the equipment, because a substitute combination chosen at the last minute due to an inventory shortage can affect eligibility.

Maintenance: the question we forget to ask in July

Rooftop equipment must be accessible twelve months a year. Before accepting this type of project, we verify:

• The permanent access method—hatch, fixed ladder, or stairs—and its compliance.
• Service clearance around the equipment, with the panels open and a technician and their tools present.
• Access availability in winter, when the roof is covered in snow.
• Replacing indoor filters, which remain in the basement and fortunately are not affected by the roof.

Equipment that cannot be properly maintained will not reach its expected service life, regardless of its build quality.

Seven mistakes specific to rooftop installations

1. Drill through the membrane to secure the support. Ballasted and load-distribution solutions are available that do not compromise the roofing system.
2. Place wood directly on the membrane, without a protective pad.
3. Use pipe insulation that is not UV-protected on exposed sections.
4. Ignoring the difference in elevation between the indoor and outdoor units when designing the route.
5. Assuming the building has a 208–230 V power supply because previous ones did.
6. Neglecting vibration isolation on an occupied building, especially beneath a dwelling.
7. Placing the unit above a roof drain or in a low spot on the roof.

Where we work

Our teams cover the Acton RCM — Upton, Acton Vale, Roxton Falls, Sainte-Christine, Béthanie — and all of Montérégie, from Saint-Hyacinthe to Granby and Saint-Jean-sur-Richelieu.

We carry out the same rooftop installations in Longueuil, Brossard, and Saint-Hubert, in the mixed-use and commercial buildings of Montreal, in Laval, and on the North Shore, from Terrebonne to Saint-Jérôme. Triplexes and mixed-use buildings on the South Shore and in Montreal account for a significant part of our work, and these are precisely the projects that require the most preparation before installation day.

The owner in Upton wrote to us in July: “The business is air-conditioned without the upstairs tenant hearing a thing. That was my condition.” It was ours too.

Let's move on to your project

If your building has no ground clearance, if you manage a mixed-use building or commercial space, or if you simply want to know whether your roof can support a central heat pump, the answer starts with an on-site assessment, not a phone quote.

At AirGreen, every quote for this type of HVAC installation includes a structural and roofing inspection, a load calculation, verification of the voltage and electrical capacity, and an assessment of eligibility for applicable assistance programs. Contact us for your project in Upton, in the Acton RCM, in Montérégie, on the South Shore, in Montreal, in Laval, in Longueuil, or on the North Shore.