Snow Loads and Roof-Mounted Equipment

Why this matters

In snow country, the equipment a shop installs either works with the snow or gets buried, drifted over, iced shut or crushed by it. A sidewall vent set a foot above grade disappears under the first big storm and the furnace locks out at 2 a.m. A heat pump placed under the eave of a metal roof takes a slab of sliding snow on its fan guard. A new rooftop unit becomes the obstruction that a drift piles against, adding load the roof was not designed to carry at that spot. None of these are exotic failures; they are placement choices made by someone thinking about summer.

Snow load in one page

A ground snow load, written pg, is the weight of snow on the ground that the code uses as its design event for a site, read from the adopted code's map or a value the jurisdiction publishes. Mountain and lake-effect areas often have site-specific values and some jurisdictions require a case study instead of the map. Your building department knows the number.

Roof snow loads derive from it. Under ASCE 7, Chapter 7 (the 2016 edition is the one most jurisdictions adopted through the IBC and IRC; the 2022 edition changed the ground snow maps to a risk-targeted basis), a roof's flat snow load is usually less than the ground load because wind and heat take some snow off, then adjusted for slope, exposure and how warm the roof is. Two parts of that chapter matter most to equipment.

Drifts. Wind moves snow across a roof and drops it where the wind slows: against a parapet, at a step down to a lower roof, and around anything sticking up from the roof. ASCE 7 Section 7.8 (2016) requires drift loads at roof projections such as rooftop units, with an exemption for projections shorter than a length the standard sets. A drift is deeper and denser than the snow around it, so the load next to a unit can be several times the load in the middle of the roof.

Density. Snow is not one weight. ASCE 7-16 gives a design density of 0.13 pg + 14 pounds per cubic foot, capped at 30 pounds per cubic foot. Fresh powder is far lighter than that; wet, settled snow and ice are heavier. Density is what lets you turn a load into a depth, which the worked example uses.

What this means for a shop. Adding equipment to a roof in snow country is a structural question as well as a mechanical one: the unit's weight, and the drift it creates, land on framing sized before the unit was there. A replacement of similar size and weight on the same curb is usually not a new question. A new unit, a heavier one, or a unit moved to a different spot is, and the answer comes from an engineer or the building department, not from the installer's judgment.

Terminals and intakes: the snow line is the grade

Direct-vent and power-vent appliances take combustion air and discharge flue gas through the sidewall or the roof. Snow covering the terminal blocks either one. A blocked exhaust trips the pressure switch and the appliance locks out; on some designs a partly blocked pair can pull exhaust back into the intake, which degrades combustion and raises CO at the appliance.

Manufacturers specify a minimum terminal height above grade and above the expected snow level, commonly written as a clearance above grade or the expected snow level, whichever is higher. The model fuel gas codes, as adopted, set their own minimum clearances above grade for mechanical draft terminals. In snow country the manufacturer's snow-level clause is usually the one that governs, because the expected snow level is higher than grade by definition.

Two installation details follow. Raise the terminal with the manufacturer's vertical extension or snorkel kit where the wall leaves no other option, rather than with field-built pipe. And keep the terminal out of the path of roof shedding and away from where a plow or a shovel throws snow, which can bury a terminal that was set well above the natural snow line.

Plumbing vents have their own snow rules. The IPC leaves the roof extension height for the jurisdiction to fill in (IPC 903.1, 2021 edition), and snow-country jurisdictions often set it high enough to stay above the snowpack. Where the winter design temperature is very low, the codes' frost closure provision (IPC 903.2 and IRC P3103.2, as adopted) requires a larger vent through the roof, at least 3 in diameter, with the increase starting inside the heated space, so frost building up inside the opening does not close it.

Ice that falls

A roof sheds. Metal roofs shed suddenly, in slabs; shingled roofs drop icicles and the ice dam that broke loose. Whatever is directly under the eave on that side of the house takes it.

Condensers and heat pumps under an eave get fan guards crushed, coils pierced and fan blades broken. The fix is placement: on a gable end, or far enough out from the eave that the shed lands short of the unit.

Gas meters and regulators under an eave can have a riser bent or a fitting cracked by falling ice, and a regulator vent that ices over can stop venting, which defeats the regulator's protection. Gas utilities ask customers to keep snow and ice off meters and regulator vents and to clear them gently by hand, never by chipping. If a meter or riser has been struck and anyone smells gas, everyone leaves the house, no switches touched, no lights, no phone used inside, and the utility is called from outside.

Electric meters and service masts take ice too, and a service mast pulled by a falling ice load or by snow sliding down the drop is the utility's emergency, not the shop's repair. Keep clear of a damaged mast and call the utility.

Snow guards on a metal roof hold snow in place so it melts off rather than avalanching. They also change the roof's load, so adding them is a roofing and structural decision, not a fix a mechanical shop makes to protect its unit.

Worked example: placing a heat pump at a snow-country house

A customer is adding a ducted heat pump. The house has a 12/12 metal roof on the long sides and gable ends east and west. The building department gives a ground snow load of 50 psf for the address. The manufacturer's instructions say to raise the outdoor unit above the expected snow level and keep it clear of roof runoff.

Turn the design load into a depth, and read it correctly. ASCE 7-16 density: 0.13 x 50 + 14 = 6.5 + 14 = 20.5 pounds per cubic foot. The depth of snow at that density which weighs 50 psf is 50 / 20.5 = 2.44 ft, about 29 in. That is the depth corresponding to the code's design event, which in the 2016 edition is roughly a 2 percent annual chance value. It is a structural design number, not the snowpack in a normal winter, and it is not what the manufacturer means by "expected snow level". Using it as a stand height would put the unit on a 30 in stand for an event that may not arrive in the unit's life.

Use the local snowpack for the stand. The shop's own service history and the neighbors' units say the deepest settled snow most winters is around 18 in in this town (illustrative of what a shop learns from its own records). The stand is set so the bottom of the coil is 24 in above grade, which leaves 24 minus 18 = 6 in of clearance over a typical winter's peak and puts the coil 5 in below the 29 in design-event depth. The customer is told plainly that in a design-scale storm the unit will be buried at its base and the area around it needs clearing.

Keep it out of the shed. The long sides drop roof slabs. The unit goes on the west gable end, where the roof does not shed.

  long side (sheds)               gable end (no shed)

   \  metal roof                   /\
    \                             /  \
     \______ eave                /    \
     |      :                    |    |
     | wall : shed lands         |    |  [HP] on stand
     |      :  out to here       |    |   coil 24 in up
     |      :....                |    |   ===== grade

Drain the defrost. A heat pump melts frost off its coil several times a day in cold weather. That meltwater has to drain away from the stand, not refreeze into a ramp of ice under the unit that climbs into the coil. The pad gets gravel underneath and a slope away from the house.

What changes the answer. A house with no gable end clear of shedding needs the unit out beyond the eave by more than the shed reaches, or on a wall bracket high enough to sit above the landing zone, with the bracket fastened to framing and rated for the unit plus a snow load on its top. A roof with snow guards sheds less and the long side may be usable. And a shop with no local history should ask the building department or a long-time neighbor what a deep winter looks like rather than guessing an inch figure.

Clearing equipment without becoming the accident

Customers will be told to keep terminals and units clear, and techs will be called to clear them. Clear at ground level only, by hand or with a soft brush or plastic shovel, never with a metal tool against a coil, a vent or a meter. Nobody goes onto a snow-covered roof to clear a rooftop unit or a vent on a service call: the snow hides skylights and edges, adds to a load the roof is already carrying, and turns a pitched roof into a slide. Roof work in snow is a planned job with fall protection under the shop's plan.

How to tell the placement was right

• The ticket names the ground snow load or the jurisdiction's equivalent and the basis for the stand or terminal height chosen.
• Every terminal and intake on the job clears the expected snow level per the manufacturer's clause, stated in inches above grade.
• Nothing you installed sits under a shedding eave, or the ticket says why it had to and what protects it.
• Any added or relocated rooftop equipment went past an engineer or the building department for drift and load.

References

• ASCE 7, Chapter 7, Snow Loads, including the density relation and Section 7.8 roof projection drifts (2016 edition), as adopted through the IBC and IRC
• International Plumbing Code 903.1 and 903.2 and International Residential Code P3103.2 (2021 editions), as adopted
• Appliance and heat pump manufacturers' installation instructions for terminal and outdoor unit clearances
• See related: Design Events: What the Code Assumes Will Happen to the House; Outdoor and Rooftop Equipment in High Wind Areas; Roof Penetrations and the Secondary Water Barrier