A roof permit in Calgary starts from a ground snow figure of 1.1 kilopascals, not from anything you can measure by stepping outside after a storm. That number gets run through a formula that adjusts for your roof’s slope, exposure and shape before it becomes the load a truss actually has to hold. Start any framing or re-roofing decision by pulling the current design values for your address from your municipal building department, not from a guess based on a bad winter you remember.
What is the ground snow load in Alberta?
At Calgary, the ground snow load used for design is 1.1 kPa (Ss), with an associated rain load of 0.1 kPa (Sr) added on top, both published in Table C-2 of the National Building Code of Canada 2020. That pairing matters: Canadian code assumes rain can fall onto standing snow, so the rain component is carried alongside the snow figure rather than treated separately.
Ground load versus roof load
This is a ground number, not a roof number, and that distinction is where most confusion on this subject starts. The code converts it using the formula S = Is[Ss(Cb·Cw·Cs·Ca) + Sr], where basic, wind, slope and shape factors move the figure up or down depending on the actual roof. A steep, exposed roof in an ordinary residential setting typically ends up carrying less than the ground value. A flat roof, a valley, or a spot downwind of a taller wall can end up carrying more. Nobody hands a builder the raw 1.1 kPa figure and calls it done.
This is the number that shows up on a permit application, on the plan reviewer’s checklist, and on the truss manufacturer’s drawings when they size the lumber for a new build or an addition. Alberta’s own building code, the National Building Code – 2023 Alberta Edition, derived from the 2020 edition of the National Building Code of Canada, has been in force provincewide since May 1, 2024, and it is administered by Alberta Municipal Affairs under the Safety Codes Act. That is the document your local building official is actually checking your drawings against, even though the snow and wind values themselves trace back to the national model’s Table C-2.
Table C-2 lists dozens of reference localities precisely because a province this size has no single value. Calgary sits at roughly 1045 metres of elevation, and its figure applies there. Foothill communities, mountain parks, and parts of northern Alberta sit at different elevations and see different weather patterns entirely, so their design figures are not the same as Calgary’s. Anyone building outside the Calgary area needs the figure for their own reference locality, available from their municipal building department.
How much snow can a roof hold in Alberta?
There is no single number that answers this, because the same snowfall behaves completely differently depending on the roof it lands on and what the snow itself has become. What the code gives is a starting point, not a verdict on any particular house.
Where drift changes everything
For an ordinary heated, sloped house roof, the code’s basic factor typically brings the design load down to somewhere around 0.7 of the Calgary ground value once slope is taken into account, and a steep roof reduces it further still. But that reduction only applies to the open field of the roof. Wherever snow can pile up rather than sit evenly, the shape factor pushes the number back up, sometimes well past the ground figure. That happens against a parapet or taller wall, below a dormer, in a roof valley, and on a lower roof sitting beside a taller section of the same house. Drift is where roofs actually fail, not on the open slope, because the code’s uniform load assumption stops applying exactly where the wind piles snow into a wedge three or four times deeper than the field around it.
The other variable is what the snow itself weighs, and it is not constant. Freshly fallen, dry snow is far lighter per inch of depth than snow that has sat for weeks, partly melted, and refrozen, or that carries a crust of ice on top. A given depth of dense, wet spring snow can weigh two to three times what the same depth weighed as fresh powder back in January. That is why a foot of snow sitting on a roof in March is a different load than a foot that fell in December, even though the tape measure reads the same.
Signs the roof is asking for too much
- Interior doors that suddenly stick or stop latching properly
- New cracks appearing across ceiling drywall or at the wall-ceiling joint
- A ridge line that looks lower, wavier or less straight than the rest of the roof
If any of those show up, the safe response is raking accumulated snow off from the ground with a roof rake, not climbing onto a loaded roof to shovel it. A roof already carrying more than it should is not a stable place to add a person’s weight. The same drift problem shows up just across the provincial line, with its own reference-city figures, on our page covering Saskatchewan’s roof snow load and wind rules.
What wind speed must a roof withstand in Alberta?
Calgary’s reference figure is an hourly wind pressure of 0.48 kPa, tied to a 1-in-50-year return period, published in Table C-2 of the National Building Code of Canada 2020. This is what NBC Part 4 and Part 9 use for structural design at Calgary. It is worth being clear about what kind of number this is: the Canadian code specifies a pressure, in kilopascals, sustained over an hour, not a gust speed in miles per hour. A reader used to seeing hurricane wind speeds quoted in the news has no direct way to compare that figure to this one, because the two measure different things on different bases, and there is no honest way to convert between them.
What the pressure governs on the roof
The same table also carries a 1-in-10-year hourly wind pressure of 0.38 kPa for Calgary, and that lower figure is used for serviceability checks, things like cladding deflection and comfort, never for the structural strength calculations that govern the roof itself. Mixing the two produces an undersized design, so anyone reading a wind figure off a table needs to check which return period it is attached to before using it.
On an actual roof, this pressure feeds into the shingle wind-resistance rating a builder selects, the nailing pattern the roofer follows, the sheathing fastening schedule, and the uplift connections tying the roof structure down to the walls. Edges, rakes and ridges fail first in a wind event, because that is where suction concentrates as wind pulls up and around a roof shape rather than pushing straight down on it. A shingle with a strong wind rating installed with the wrong nailing pattern performs like the weaker product, not the one printed on the wrapper.
Readers south of the border work from an entirely different wind design system. American states measure design wind speed under ASCE 7, and someone comparing notes with a builder in the United States can see how that system is applied on our page covering New York’s roof snow load and wind rules, though the two codes are not directly comparable figure for figure. As with the snow load, Calgary’s wind pressure is a reference-city figure. Other parts of Alberta, particularly more exposed or higher-elevation locations, carry their own values, available through the local building department for that jurisdiction.
Does Alberta require an ice barrier under the shingles?
An ice barrier is a self-adhering waterproof membrane installed at the eaves, run up the roof deck far enough to clear the interior wall line below. Its job is narrow and specific: it is not there to stop snow, it is there to stop water from getting into the house once an ice dam has already formed and meltwater starts backing up under the shingles.
What actually causes an ice dam
The mechanism matters because it determines whether a membrane alone is a real fix or just a bandage. Heat escaping from the living space into an unvented or poorly insulated attic warms the underside of the roof deck. That warmth melts the snow sitting directly above the warm part of the attic, the meltwater runs down the roof slope, and it refreezes the moment it reaches the cold overhang past the exterior wall, where there is no heat loss underneath to keep it liquid. The ice ridge that builds up there dams the water behind it, and that trapped water finds its way under the shingles and into the deck. The membrane protects the deck from that trapped water. It does nothing to stop the dam from forming in the first place. Only air sealing the attic floor and getting the insulation level right stop the melt-and-refreeze cycle at its source.
Calgary’s climate makes that cycle a real possibility rather than a theoretical one. The mean daily minimum temperature in the coldest month at Calgary runs to -13.2 °C according to Environment and Climate Change Canada’s 1981-2010 climate normals, cold enough that attic heat loss meeting a sun-warmed roof surface can set up the freeze-thaw pattern an ice dam needs. Calgary Int’l A also logs close to 4,979 heating degree days annually, which places it in Natural Resources Canada’s climate zone 6, one of the colder zones on that scale, and a long, cold heating season is exactly the condition under which attic heat loss becomes chronic rather than occasional.
What Alberta’s own code requires at the eaves for a given roof assembly is a detail that varies by roof design and location within the province, and it sits with the National Building Code – 2023 Alberta Edition, administered by Alberta Municipal Affairs under the Safety Codes Act. A local building department or a licensed roofing professional familiar with that code is the right place to confirm what applies to a specific roof, rather than assuming a blanket rule.
What roofing material suits Alberta best?
The honest starting point for choosing a roofing material here is the combination this page has already laid out: a Calgary ground snow load of 1.1 kPa plus rain, an hourly wind pressure of 0.48 kPa, and a climate zone cold enough to make ice damming a genuine concern. No single material wins outright against that combination. Each one trades differently against it.
Matching the material to the load
| Material | Added dead load on the structure | Snow shedding behavior | Wind performance factor |
|---|---|---|---|
| Architectural asphalt shingles | Low | Holds snow on the roof until it melts or is raked off | Rated by a wind-resistance class, but only as strong as the nailing pattern beneath it |
| Standing-seam metal | Low to moderate | Sheds accumulated snow in a single slab once the panel warms | Depends on panel-to-clip engagement and clip spacing rather than the panel material alone |
| Slate or concrete tile | High, adds directly to the structural snow load | Retains snow similarly to shingles rather than shedding it | Heavy units resist uplift through their own mass but still need engineered attachment at edges and ridges |
Standing-seam metal’s snow-shedding habit is worth thinking through before it becomes a surprise. A roof that sheds a heavy slab of snow all at once needs somewhere for that slab to go that is not a doorway, a walkway, or a parked vehicle. Positioning entries and walkways away from that slide path is a design decision made at the drawing stage, not something fixed after the fact.
Where wind pressure is the governing concern, the fastening schedule and underlayment matter more than which material sits on top. A high-wind-rated shingle installed to a lower nailing standard performs at that lower standard, and the same is true of a metal roof with clips spaced too far apart for the panel profile. None of this tells an individual reader which material or installation detail is adequate for their own roof. That determination belongs to the local building department reviewing the specific plans, or to a licensed engineer looking at the actual structure.