Boise’s roof snow load works out to roughly 7 psf, converted from a ground figure of 10 psf that applies to the low valley floor around the city. Check with your local building department for the exact figure used on your permit, because elevation changes both numbers fast as you move away from Boise toward the foothills or the mountains.
What is the ground snow load in Idaho?
At Boise, the design ground snow load is 10 psf under ASCE 7-16, the edition that IRC/IBC 2021 makes applicable in most states. That is the number a permit application, a plan reviewer and a truss manufacturer actually use when they size a roof in the Boise area. It is not the number stamped on a bundle of shingles, and it is not what a homeowner should quote as “how much my roof holds,” because a ground figure and a roof figure are two different things measured two different ways.
Ground snow load is measured on open, level ground with no building underneath it. The code does not hand that number straight to a rafter. It runs it through factors for exposure, for whether the roof is heated, and for how steep the roof pitches, and only the result of that conversion becomes the design roof snow load a truss drawing shows. For an ordinary heated house roof with a normal slope, that conversion lands at roughly 7 psf at the Boise reference point, well below the ground figure, and lower still as the roof gets steeper.
A newer edition, ASCE 7-22, maps the same point in Boise at 32 psf. That is not four times more snow falling on Boise. ASCE 7-22 states its figures on a different basis, a strength-level number used with a 1.0 load factor, than ASCE 7-16, a nominal number used with a 1.6 load factor, so the two are not comparable side by side. A few jurisdictions are beginning to adopt the newer edition, but the number governing a Boise-area permit today is the ASCE 7-16 figure, and nothing about the 7-22 map means an existing roof, built to the older figure, is suddenly undersized.
Elevation moves this number fast. Boise sits at roughly 2,740 feet, and ground snow load in the surrounding foothills or the mountains to the north and east runs higher, sometimes dramatically so, simply because elevation is the main driver of the map. A figure calculated for the Boise valley floor tells you nothing about a lot a few thousand feet higher. Anyone building outside the immediate Boise area should ask their local building department for the ground and roof snow load figures that apply to their specific site, since neither number is uniform across the state.
How much snow can a roof hold in Idaho?
There’s no single number that answers that, and a page that gives you one is guessing. What answers it is the same 7 psf design roof snow load at the Boise reference point, converted from the 10 psf ground figure, applied to the specific roof in front of you, plus a close look at where snow is drifting rather than where it simply fell.
Drift is where roofs actually fail, and it has little to do with the average depth on the main slope. Snow blown off a taller adjacent roof, a mechanical unit, or a parapet piles up on the leeward side, against a wall, below a dormer, or in the valley between two roof planes, and that pile can carry several times the load of open, undrifted snow nearby. A lower roof beside a taller one is a classic drift location, and the code treats it as its own design condition rather than folding it into the flat, uniform figure most people picture.
Why depth alone doesn’t tell you the weight
The other trap is judging load by how deep the snow looks. Fresh, light powder runs roughly 5 to 7 pounds per square foot for every foot of depth. Once that snow settles, gets rained on, or partly melts and refreezes, the same depth can weigh two to three times as much, and a layer of ice on top adds more still. A foot of dry January snow and a foot of wet, settled March snow are not the same load, even though They Look Identical from the ground.
Watch for practical warning signs instead of trying to estimate pounds per square foot yourself:
- Interior doors that start sticking or won’t latch
- New cracks appearing in ceiling drywall
- A ridge line that looks like it’s sagging compared with the roofline next door
- Unusual creaking or popping under a heavy snow load
Any of those is a reason to get snow off the roof and call a professional, not a reason to climb up and look. Raking snow from the ground with a roof rake is the safe way to reduce load; walking on a roof already carrying a heavy, possibly ice-laden snowpack adds your body weight to a structure that may already be near its limit, on a surface you cannot see is solid. Homeowners further into the mountains or higher parts of the Snake River Plain carry meaningfully higher snow loads than the Boise reference figure and should treat these signs even more seriously.
What wind speed must a roof withstand in Idaho?
At Boise, the basic design wind speed is 102 mph under ASCE 7-16, for Risk Category II buildings, which covers ordinary houses. That figure is a 3-second gust measured at 33 feet above the ground in open, Exposure C terrain, not a sustained wind and not the kind of number a weather forecast reports. Compare it to a hurricane’s headline speed and it looks unremarkable. Compare it to what it actually measures, a brief peak gust in open exposure, and it is the figure that governs how a Boise-area roof gets fastened together. ASCE 7-22 maps the same point at the same 102 mph, so this is one of the rare figures where the newer edition doesn’t move the number at all.
Idaho’s inland location means Boise doesn’t fall inside a wind-borne debris region, so the added requirement for impact-rated glazing or approved shutters, the kind found in hurricane-prone coastal areas, doesn’t apply here. Readers comparing this to a state with a very different wind profile, such as Oklahoma’s roof snow load and wind speed page, will see how much that obligation can change from one part of the country to another.
What the wind number actually governs
The 102 mph figure drives three practical decisions on a re-roof or new build: the wind rating stamped on the shingle package, the nailing pattern the manufacturer requires to achieve that rating, and the fastening schedule for the roof sheathing itself. It also governs the uplift connections, straps and clips, that tie the roof structure down to the walls below. None of that is visible once the roof is finished, which is exactly why it has to be right the first time. Edges, rakes and ridges fail first in a wind event because that’s where uplift pressure concentrates, so a skipped fastener or an undersized nailing pattern in those areas shows up as missing shingles long before the field of the roof does.
As with the snow figures, 102 mph is the value for the Boise reference point. Other parts of Idaho, particularly at different elevations or exposures, can carry a different mapped wind speed, and the local building department is the source for the number that actually applies to a given site.
Does Idaho 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 the roof slope to a point past the interior line of the exterior wall below. Its job isn’t to stop snow from sitting on the roof. It’s to stop meltwater that backs up behind an ice dam from finding a seam in the shingles and soaking into the roof deck underneath.
Ice dams form for a specific reason, and understanding it decides whether a membrane fixes the problem or just protects against its symptom. Heat escaping from the living space into the attic warms the underside of the roof deck unevenly, snow over the warm part melts, and that meltwater runs down until it hits the roof edge, which stays cold because it overhangs the unheated eave. There the water refreezes, builds into a dam, and backs up under the shingles upslope of it. The membrane only protects the deck once that backup happens. It does nothing to stop the dam from forming in the first place. Air sealing the attic floor and adding enough Insulation to keep the whole underside of the roof deck close to outdoor temperature is what actually prevents the dam. A reader who installs the membrane and stops there has bought protection against water damage, not a fix for the ice dam itself.
What Idaho’s code says
Idaho’s statewide code is the Idaho Residential Code, based on the 2018 International Residential Code, in force since January 1, 2021, and administered by the Idaho Building Code Board under the Division of Occupational and Professional Licenses. It sets a statewide minimum that individual cities and counties can exceed but not weaken. Whether a given jurisdiction’s amendments spell out ice barrier requirements, and exactly where on the roof they must start, is a local detail a state-level source can’t settle. Check with the building department that issues the permit for the project’s specific location.
Given Boise’s coldest-month average low of 25.4°F under NOAA’s 1991-2020 climate normals, combined with the design roof snow load built into the code at that location, the conditions for ice dam formation, cold enough to refreeze meltwater and enough snow on the roof to supply it, are real in the Boise area for a good part of the winter. That combination is exactly why ice barrier detailing exists as a code topic in cold-winter states at all, whatever a specific jurisdiction ultimately requires on a given permit.
What roofing material suits Idaho best?
The honest answer depends on which load is doing the governing on a given roof, and in the Boise area that means weighing a 7 psf design roof snow load against a 102 mph basic wind speed and a climate that spans two energy code zones across the state’s 44 counties, rather than picking a material because it’s popular.
Architectural asphalt shingles carry a wind rating, and that rating is only as good as the nailing pattern actually used during installation. A shingle rated for a high wind speed but fastened to a lower standard performs at the lower standard, not the one printed on the wrapper. Fastening and underlayment matter more than the shingle itself where wind is the governing load, which describes most of Idaho outside the highest snow country.
Standing-seam metal sheds snow cleanly off a steep pitch, which is an advantage for the structure since less snow sits on the roof at any one time, but that advantage becomes a hazard at the ground. A slab of snow that slides off a metal roof in one piece can come down over a doorway, a walkway or a driveway with real force. Where the snow goes when it lets go is a design decision, made with snow guards and roof-edge planning, not something to discover the first time it happens.
Slate and concrete tile add real, permanent dead weight to a roof, on top of whatever snow load the structure already carries, so specifying either one means the framing has to be designed for both loads together from the start, not retrofitted onto framing sized for a lighter material. That’s a structural conversation for the project’s engineer, not a swap a homeowner makes on their own during a re-roof.
All of this is Boise-area math. A site closer to the mountains, with a higher mapped ground snow load, shifts the balance further toward materials and fastening details chosen for load first and appearance second, and a licensed engineer or the local building department is the right source for how that balance plays out on a specific structure. Readers weighing the same trade-offs under a heavier, drift-prone snow regime, such as the one covered on South Dakota’s snow and wind load page, will recognize the same logic applied to a different set of numbers. For the ground-level steps this page assumes are already done, the roofing section covers material basics and maintenance.