Fifteen pounds per square foot is the design ground snow load mapped at Seattle, a number that surprises people who picture the wet Pacific Northwest winter piling up like the Rockies. That ground figure is not what your rafters actually carry, and the wind speed and ice barrier rules built around it matter just as much for a roof here. Get the exact numbers for your address from your local building department before you order trusses, shingles or a re-roof.
What is the ground snow load in Washington?
The design ground snow load at Seattle is 15 psf, under ASCE 7-16, the edition IRC/IBC 2021 makes applicable in most states. That number is what shows up on a permit application, drives a plan review, and is what a truss manufacturer’s drawings are built around when someone in the Puget Sound area applies to build or re-roof. Before you get anywhere near material choices for roofing, this is the figure a builder pulls first.
Ground load versus roof load
Fifteen psf is the load sitting on flat, open ground, not the load a shingle actually bears. The code applies exposure, thermal and slope factors before that number reaches a rafter. For an ordinary heated sloped house roof, that conversion lands near 10 psf of roof snow load, lower on a steep roof that sheds snow before it piles up. Confusing the ground figure with the roof figure is the single most common mistake people make when they read a snow map.
A few jurisdictions are beginning to adopt ASCE 7-22, which maps the same point at Seattle at 46 psf. That is not three times the snow. The 7-22 number is a strength-level figure built on a 1.0 load factor, while the 7-16 figure here uses a 1.6 factor. The gap between them is bookkeeping between code editions, not evidence that existing roofs are undersized or that snowfall has gotten worse.
Seattle’s figure is mapped near sea level, at roughly 141 feet of elevation. Snow load climbs fast with elevation in Washington, and a property in the Cascades or in the state’s northeastern counties works from a very different number than a lot in the city. Confirm the mapped value for your specific address with your county or city building department rather than assuming Seattle’s number travels with you.
How much snow can a roof hold in Washington?
There is no single answer to how much snow a Washington roof can carry. It depends on the roof’s slope, its exposure to wind, and what the snow sitting on it has turned into. Working from Seattle’s 15 psf ground figure, the code lands an ordinary heated sloped roof near 10 psf of roof snow load, and a steeper roof sits below that because snow slides off before it builds up. That is a design floor, not a hard ceiling on every part of the roof.
Drift is where roofs fail
Drift is where roofs actually fail, not the open field of a roof the design number describes. Wind blows snow off a taller section of roof and piles it against a lower roof, a parapet, or the wall below a dormer, and that drifted pocket can carry several times the flat design load. A lower roof tucked beside a taller wing of the same house is exactly the spot to watch, because it collects drift from above with nowhere for the extra snow to go.
The depth-to-weight math matters as much as the design number. Fresh, light snow runs roughly 5 to 7 pounds per square foot for every foot of depth. Once it settles, or gets rained on and refreezes, that same foot of snow can weigh two to three times as much, and a layer of ice on top adds still more. A foot of dry January powder and a foot of March snow that sat through a thaw and refroze are two very different loads on the same roof.
Watch for the physical signs that a roof is carrying more than it should:
- Interior doors that suddenly stick or won’t latch
- New cracks appearing in ceiling drywall, especially near the center of a span
- A ridge line that looks like it’s sagging when viewed from the ground
- Popping or cracking sounds from the attic during or after a heavy snowfall
If you see any of that, rake snow from the ground with a roof rake, and don’t climb onto a loaded roof to check it yourself. A roof that’s already carrying a marginal load is the last place to add a person’s weight, and a building department or a licensed engineer is who can tell you whether the structure needs relief. Roofs in far heavier snow country face the same drift math on a larger scale, as the guide to snow and wind loads in Newfoundland and Labrador lays out for a much colder climate.
What wind speed must a roof withstand in Washington?
The basic design wind speed at Seattle is 98 mph under ASCE 7-16, for Risk Category II buildings, which covers ordinary houses. That number is a 3-second gust measured at 33 feet in open, Exposure C terrain, not a sustained wind and not the same thing a weather forecast means when it calls out a wind speed. Compare it to a hurricane’s headline number and it looks low, but it is describing a different quantity measured a different way, not a weaker standard.
What the number actually governs
ASCE 7-22 maps the same point at Seattle at 98 mph too, so unlike the snow figures, this number hasn’t shifted between editions here. That consistency is useful to know if your project spans a code transition.
This design wind speed governs a shingle’s wind rating, the nailing pattern a roofer follows, how the sheathing is fastened down to the rafters, and the metal connectors that tie the roof framing to the walls beneath it, the uplift connections that keep a roof from lifting in a gust. Edges, rakes and ridges fail first in a wind event, because that’s where uplift pressure concentrates, which is why code nailing patterns tighten in those zones compared to the flat field of the roof.
Wind speed swings sharply with terrain across the region. Montana’s open plains push design speeds well past what a sheltered Puget Sound lot sees, a difference the guide to Montana’s roof snow load and wind speed rules breaks down county by county. Seattle’s 98 mph figure is a Puget Sound lowland number, and exposed coastal bluffs or high ridgelines elsewhere in Washington can map differently. Confirm the design value for your address with your local building department before ordering wind-rated shingles or planning uplift connections.
Does Washington 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 to a point past the interior wall line. Its job isn’t to stop snow. It’s to stop meltwater that backs up behind an ice dam from soaking through the roof deck and into the house below.
What actually builds the dam
Ice dams form when heat escaping from the living space into the attic warms the underside of the roof deck and melts the snow sitting on it. That meltwater runs down the roof until it reaches the cold overhang past the exterior wall line, where it refreezes into a ridge of ice. Water then backs up behind that ridge and, without a membrane in place, works its way under the shingles and into the deck.
Seattle’s coldest-month mean daily minimum, by NOAA’s 1991-2020 climate normals, is 37.1°F, above freezing on an average night even at the depth of winter. That doesn’t rule out an ice dam after a heavy snow or a hard cold snap, but it does mean the freeze-thaw cycle that builds one is less relentless at sea level around Puget Sound than it is at higher elevation or east of the Cascades, where winters run colder and more consistently below freezing.
The Washington State Residential Code, based on the 2021 International Residential Code and administered by the Washington State Building Code Council, has been in force statewide since March 15, 2024, with the next edition not taking effect until May 3, 2027. It sets a statewide minimum that a city or county may exceed but not undercut. Exactly how an ice-barrier requirement is applied can vary by local amendment and by elevation even within one county, so confirm the ice-barrier requirement for your jurisdiction with your local building department before a re-roof rather than assuming a single statewide rule covers every address.
Whatever the local rule turns out to be, the membrane only treats the symptom. It keeps water out at the eave, but it does nothing to stop the ice dam from forming in the first place. The actual fix is upstream: air sealing and attic insulation that keep the roof deck close to outdoor temperature so there’s no warm patch to melt snow in the first place. A roof with a membrane sitting over a leaky, under-insulated attic will still grow ice dams. It just won’t leak from them right away.
What roofing material suits Washington best?
Washington’s own numbers, a 15 psf ground snow load and a 98 mph design wind speed at Seattle, plus a climate spread from IECC zone 4C to zone 6B, point toward different trade-offs depending on which load actually governs a given roof.
Comparing the categories
| Material | Wind performance | Snow behavior | Added dead load |
|---|---|---|---|
| Architectural asphalt shingles | Carries a manufacturer wind rating tied to a tested gust speed | Holds snow rather than shedding it quickly | Low |
| Standing-seam metal | Performs well when properly fastened at panel clips and edges | Sheds snow in sheets once it warms, sometimes suddenly | Low |
| Slate or concrete tile | Heavy and stable in wind when properly attached | Holds snow, sheds slowly | Substantial, adds to the snow load the structure carries |
A standing-seam metal roof sheds snow with far less friction than shingles or tile, which is an advantage on the roof itself but a liability wherever that snow lands. Where the slide comes down over a doorway, a walkway or a driveway is a design decision that has to be made before installation, not something discovered the first time a sheet of snow lets go. Slate and concrete tile add their own dead weight on top of whatever the design snow load already asks the structure to carry, so a structure built for asphalt shingles isn’t automatically ready for tile.
The fastening and underlayment matter more than the material wherever wind governs the roof. A shingle rated for a high wind speed underperforms in the field if it’s nailed to a lower standard than the rating assumes. The nailing pattern and underlayment a roofer actually uses on the day of installation do more to determine whether a roof reaches its rated wind speed than which product line went on the shelf.
Washington’s 39 counties split across four IECC zones under the 2021 IECC: 18 counties in zone 5B, 14 in zone 4C, 4 in zone 5C and 3 in zone 6B. The letter matters as much as the number, since it marks a dry or marine moisture regime, and the insulation tables group zones differently depending on that letter. Where standing-seam metal sheds snow directly over a walkway or an outdoor unit, roofers sometimes add snow guards near the eave to break one large slide into smaller, more frequent drops, a detail that applies to smooth metal roofing and rarely comes up with asphalt shingles or tile, which shed more gradually to begin with.