A wet Pacific snowfall in the Willamette Valley weighs nothing like the dry powder that piles up in the Cascades, and that difference is exactly why a roof that handled last December’s storm without a groan can crack under this one. Oregon’s building code does not treat snow as one number for the whole state. It maps a design load at a reference point, then converts that ground figure into what a rafter actually has to hold. Before you assume your roof is covered, find out what design snow load and wind speed apply at your own address, not at the reference city this page uses to illustrate the rule.
What is the ground snow load in Oregon?
At Portland, the design ground snow load is 10 psf under ASCE 7-16, the edition that IRC/IBC 2021 makes applicable in most states and the number a plan reviewer pulls up the moment your permit application lands on the counter. That figure is not what ends up sitting on your rafters. It is the load mapped for open ground, and the code runs it through exposure, thermal and slope factors before it becomes a roof snow load. For an ordinary heated house with a sloped roof, that conversion lands the working number at roughly 7 psf, the value a truss designer actually builds to.
Why the newer ASCE 7-22 number looks so different
This 7 psf figure is what shows up on a truss drawing, in a plan review comment, and in the span tables a builder orders lumber against. A newer edition, ASCE 7-22, maps the same Portland point at 43 psf, but that number is a strength-level value built on a 1.0 load factor, not the 1.6 factor behind the ASCE 7-16 figure. It is not more snow falling on Portland and it is not a sign that a roof built to the current code is undersized. A few jurisdictions are starting to adopt ASCE 7-22, so it is worth asking your building department which edition governs your specific permit, but for now the 10 psf ground figure, and its roughly 7 psf roof equivalent, is what applies.
Portland sits near sea level in the Willamette Valley. Snow load climbs fast with elevation, and a house in the Cascade foothills or near a ski area works from a design figure that has nothing to do with the valley number. Roofing decisions that hinge on snow load should start with the figure your own building department has on file for your address, not with a reference-city number pulled off a map.
How much snow can a roof hold in Oregon?
There is no single number that answers this, because what a roof is actually asked to carry depends on where the snow sits, not just how much falls. Portland’s roughly 7 psf roof snow load, converted from the ASCE 7-16 ground figure of 10 psf, assumes a fairly ordinary heated sloped roof. A steeper roof carries less, because more of the load slides or is shed before it can pile up. A shallow roof, a roof against a taller wall, or a valley where two roof planes meet carries more, sometimes far more, because wind and sliding snow both deposit their load in the same low spots. Drift is where roofs actually fail, not the open field of a plain gable.
Why depth alone does not tell you the load
A foot of fresh, light snow runs somewhere around 5 to 7 pounds per square foot for every foot of depth. Once that snow settles, or falls wet the way Pacific-influenced snow often does, the same depth can weigh two to three times as much. Add a layer of ice from a freeze-thaw cycle and the weight climbs again. That is why a foot of snow in January is not the same load as a foot of snow in March: the March snow has usually already gone through a melt-and-refreeze cycle or fallen wetter to begin with.
Watch for the practical signs that a roof is being asked for more than it was built to hold:
- Interior doors that suddenly stick or stop closing
- New cracks running across ceiling drywall
- A ridge line that looks like it is sagging where it used to run straight
Any of those is a reason to call a building department or a licensed engineer, not a reason to guess. Raking snow off from the ground with a roof rake is the safe response to a heavy accumulation. Climbing onto a loaded roof to shovel it is not, both because the roof is already carrying more than usual and because a snow-covered roof is exactly the surface a person is most likely to slide off.
What wind speed must a roof withstand in Oregon?
At Portland, the basic design wind speed is 97 mph under ASCE 7-16, Risk Category II. That is a 3-second gust measured at 33 feet in open Exposure C terrain, not a sustained wind and not the number a weather forecast reports. A forecaster’s sustained wind of a given speed and the code’s 3-second gust are measuring different things, and comparing them directly makes the code look weaker or stronger than it actually is. ASCE 7-22 maps the same Portland point at the same 97 mph, so this is one of the rare places where the newer edition does not move the working number at all.
What the wind figure controls on a roof
What that 97 mph number buys is concrete: it sets the wind rating a shingle product has to carry, the pattern and spacing of the nails holding those shingles down, how the roof sheathing gets fastened to the rafters or trusses, and the uplift connectors, hurricane clips and similar hardware, that tie the roof structure back down to the walls. Edges, rakes and ridges fail first in a wind event, because that is where uplift pressure concentrates hardest, which is why fastening and flashing requirements get noticeably tighter right along those lines rather than staying uniform across the whole roof plane.
Portland’s figure is a reference-city value, not a statewide one. Higher terrain and more exposed sites elsewhere in Oregon can carry a different design wind speed, and the same national wind maps that produce very different numbers between an inland city and a more exposed one, the way they do for a state like Indiana, covered in our Indiana snow and wind load guide, apply the same logic here: the map value is tied to a location, and your building department can tell you which value governs your address.
Does Oregon require an ice barrier under the shingles?
An ice barrier is a self-adhering membrane installed under the shingles, running from the eave edge up past the point where the interior wall line sits, so that any water working its way backward under the shingles hits a waterproof layer instead of bare roof deck. It is not there to stop snow from accumulating. It is there for the specific failure that happens after snow has already accumulated: water finds a way back up the roof slope and under the shingle layer, and without the membrane that water reaches the plywood or OSB deck underneath.
What actually causes an ice dam
The mechanism matters because it decides whether a membrane is a real fix or just a bandage. Heat escaping from the living space into the 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 warm attic underneath to keep it liquid. That refrozen ridge is the dam, and it backs up more meltwater behind it until that water finds a way under the shingles. The membrane protects the deck from that backed-up water. It does nothing to stop the dam from forming in the first place. Only air sealing the attic floor and adding enough insulation to keep the underside of the roof deck close to outdoor temperature actually prevents the melt-refreeze cycle that builds the dam.
Portland’s coldest-month mean daily minimum, from NOAA’s 1991-2020 climate normals, is 36.2°F, a figure that sits right at the edge of freezing rather than well below it. That does not by itself tell you whether ice damming is a real risk at your address. Higher elevation sites in the Cascades or the Coast Range run colder and see this problem far more often than the Willamette Valley floor does.
The 2023 Oregon Residential Specialty Code, based on the 2021 International Residential Code and administered by the Building Codes Division of Oregon’s Department of Consumer and Business Services, has been mandatory statewide since April 1, 2024. It is a uniform code: no municipality may weaken it or enact a different requirement without authorization from the Division’s Director. That uniformity covers what a building department enforces on your permit, but whether a given jurisdiction’s plan review calls for an ice barrier membrane on a particular roof is still a question for that local building department, not something a state-level map can answer on its own. Ask before you assume either way.
Snow that sits on a roof for a while behaves differently from the fast melt-and-refreeze cycle that drives genuinely severe ice damming. In Quebec, where an ice barrier membrane is close to a given on every re-roof, that cycle runs harder and longer than it typically does across most of Oregon’s lowland cities, which is part of why the same assumption cannot travel unchanged between the two places.
What roofing material suits Oregon best?
The honest answer depends on which load is doing the work at your address: Portland’s roughly 7 psf roof snow load and 97 mph design wind gust, both drawn from ASCE 7-16, plus a climate zone split that puts 18 of Oregon’s 36 counties in IECC zone 5B and the other 18 in zone 4C. That combination, not a single best-material list, is what should drive the decision.
Comparing categories, not products
Architectural asphalt shingles carry a published wind rating, and in a 97 mph gust zone that rating, plus the nailing pattern actually used on the roof, matters more than the shingle line itself. A shingle rated for high wind performs only as well as the fastening schedule it was installed to, so an under-nailed high-rated shingle is not actually a high-wind roof. Standing-seam metal sheds accumulated snow efficiently once it starts sliding, which lightens the standing load on the structure, but it turns snow shedding into a design question rather than a non-issue: where that sliding sheet lands, over a walkway, a doorway or a parked car, has to be planned for. Slate and concrete tile add their own dead weight on top of whatever snow load the roof already carries, so a structure specified for those materials has to be designed for the combined weight from the start.
| Material | What governs its performance here | What to plan for |
|---|---|---|
| Architectural asphalt shingles | Published wind rating combined with actual nailing pattern | Fastening schedule matched to the rated wind speed, not just the shingle itself |
| Standing-seam metal | Sheds accumulated snow quickly once sliding starts | Where the sliding snow sheet lands: doorway, walkway or driveway |
| Slate or concrete tile | Adds its own dead weight on top of the design snow load | Structure sized for the combined weight from the start, not retrofitted afterward |
The same zone split, 18 counties in zone 5B and 18 in zone 4C under the 2021 IECC, also decides how much insulation code requires over the living space, and that insulation level is the real lever behind the ice-dam risk discussed earlier, not the roofing material laid on top of it. A roof assembly built to the zone 5B insulation standard keeps more heat out of the attic than one built to the zone 4C standard, regardless of which material sits on the outside.