Montana doesn’t run one building code that applies the same way from Yaak to Miles City. What holds in a state with a single statewide amendment doesn’t hold here: whether a snow-load figure or a wind-speed number even shows up on your permit depends on whether your county or city has adopted the Montana Residential Code at all. Check with your local building department before you assume any number on this page is the one your inspector will use.
What is the ground snow load in Montana?
The mapped ground snow load at Billings is 20 psf, under ASCE 7-16, the edition IRC/IBC 2021 makes applicable in most states. That figure is not what a rafter carries. It’s the load on open, level ground before the code applies exposure, thermal and slope factors to get a roof snow load, and for an ordinary heated sloped house roof that conversion lands around 14 psf. A steeper roof, or one with better exposure to wind that scours snow off, ends up carrying less again. Confusing the ground figure with the roof figure is the single most common mistake people make reading a snow map, so keep the two separate in your head from here on.
Where the number comes from and who uses it
This value comes from the ASCE Hazard Tool’s ASCE 7-16 mapping at Billings, and it’s the number that shows up on a permit application, gets checked in plan review, and drives the truss drawings an engineer or truss manufacturer produces for a new roof. If your jurisdiction has adopted the Montana Residential Code, based on the 2021 International Residential Code, this is the load basis the reviewer works from. ASCE 7-22, which some jurisdictions are beginning to look at, maps the same point in Billings at 40 psf. That is not four times the snow. ASCE 7-22 figures are strength-level values built on a 1.0 load factor, while the 7-16 figure above is a nominal value built on 1.6. Different basis, same physical snow, and it says nothing about whether an existing roof is now undersized.
Billings sits at a mapped elevation of roughly 3,142 feet. Montana’s terrain runs from valley floors to mountain passes thousands of feet higher, and ground snow load climbs fast with elevation. A number that fits Billings tells you nothing about a roof outside Bozeman at 6,000 feet or a cabin near a mountain pass. This is background for the section that follows, not a step you’ve already read in the opening: it’s what the 20 psf figure is measured against before we talk about what it means for an actual roof deck. If you’re starting from scratch on a project, our general guide to roofing covers the steps before you get to load calculations at all.
How much snow can a roof hold in Montana?
There’s no single number for this, and anyone who gives you one inches of snow is guessing. What you can work from is the ratio: the Billings ground figure of 20 psf converts to roughly 14 psf on an ordinary heated sloped roof, and that’s before drift is added anywhere.
Drift is where roofs actually fail
Drift loading pushes the local figure well above the flat-roof average, and it collects in predictable places: against a taller wall, below a dormer, in a roof valley, and on a lower roof section next to a taller one where wind deposits snow blown off the upper roof. A design that only accounts for the uniform 14 psf and ignores drift at those spots is the design that cracks a rafter, not the one that fails everywhere at once.
Depth is not weight
The other trap is treating snow depth as a stand-in for load. Fresh, light snow runs roughly 5 to 7 pounds per square foot per foot of depth. Once it settles, or picks up rain or a melt-refreeze cycle, that same foot can weigh two to three times as much. An ice layer on top adds more still, concentrated rather than spread. That’s why a foot of dry January powder and a foot of wet March snow are not the same load, even though they look identical from the ground.
A maritime state like Washington deals with heavier, wetter snow at comparatively low elevation, while Montana’s continental climate tends toward drier snow that still gains weight fast once it compacts or a thaw cycle hits it. Elevation changes that picture inside the state just as much as climate does.
Watch for the physical signs that a roof is carrying more than it should:
- Interior doors that suddenly stop closing or start sticking
- New cracks or popped nails in ceiling drywall
- A ridge line that looks like it’s sagging when viewed from the street
- Creaking or popping sounds from the attic under load
If you see any of that, rake snow from the ground using a roof rake rather than climbing onto a loaded roof. A loaded roof is unpredictable underfoot, and it’s not a place to find out where the weak point is. For anything beyond raking, or if you’re seeing the signs above, that’s a call to a building department or a licensed engineer, not a guess.
What wind speed must a roof withstand in Montana?
Billings’ basic design wind speed is 109 mph under ASCE 7-16, for Risk Category II, which covers ordinary houses. That number is a 3-second gust measured at 33 feet in open (Exposure C) terrain, not a sustained wind speed and not what a weather forecast means when it reports wind speed. Compare it to a hurricane’s headline number and it looks alarmingly low. Compare it on its own terms and it’s the figure a truss uplift calculation and a shingle’s wind rating are built around. ASCE 7-22 maps the same point at the same 109 mph, so there’s no edition gap to flag here the way there is for snow load.
What this figure does not trigger
Montana’s inland location keeps it outside the hurricane-prone and wind-borne debris regions the code maps along the Gulf and Atlantic coasts. States with a Pacific coastline, like Oregon, still fall outside those specific hurricane-prone zones too, but coastal wind-borne debris requirements elsewhere mean impact-rated glazing or shutters for homes inside the mapped region. Nothing in the Billings figure calls for that here.
Where the wind speed actually shows up in construction
The 109 mph figure feeds a shingle’s published wind rating, the nailing pattern a roofer follows, the fastening schedule for roof sheathing, and the uplift connections, hurricane clips or straps, tying the roof framing down to the wall framing. Uplift concentrates at edges, rakes and ridges first, which is why those areas get tighter fastener spacing than the field of the roof. A roof that meets the wind speed on paper but skips the edge detailing is still vulnerable exactly where wind loads are highest. As with the snow figures, this number belongs to Billings specifically, and other parts of the state, especially exposed ridgelines and open plains areas, can see different mapped values.
Does Montana require an ice barrier under the shingles?
An ice barrier is a self-adhering waterproof membrane installed under the shingles, run from the edge of the eave up past the interior line of the exterior wall. Its job isn’t to stop snow. It’s to stop water that has backed up behind an ice dam from working its way under the shingles and into the roof deck.
What actually causes an ice dam
The mechanism matters because it decides whether a membrane alone fixes anything. Heat escaping from the living space into an unsealed attic warms the underside of the roof deck. Snow sitting over that warm section melts, runs down the roof, and refreezes the moment it reaches the cold overhang past the exterior wall, where there’s no heat loss underneath. That ice builds into a dam, and meltwater backs up behind it looking for a way in. NOAA’s 1991-2020 climate normals put Billings’ coldest month mean minimum at 17.9°F, cold enough, combined with a heated attic and a snow load on the roof, for that melt-refreeze cycle to be a real, recurring risk rather than a rare event.
The membrane protects the deck if an ice dam forms, but it does not stop the dam from forming in the first place. Only air sealing the attic floor and adding enough insulation to keep the roof deck close to outdoor temperature actually prevents the melt-refreeze cycle. A homeowner who installs the membrane and stops there has bought insurance against water intrusion, not a fix for the dam itself.
What the code actually says in Montana
Montana publishes its own code, the Montana Residential Code, based on the 2021 International Residential Code, administered by the Montana Department of Labor and Industry, Building Codes Bureau. Whether it applies to your roof depends on your local jurisdiction: the state code excludes residential buildings of fewer than five dwelling units unless the county, city or town has specifically made the code applicable to them, and where a jurisdiction has certified and adopted it, that local body cannot enforce anything stricter than the state code itself. That means an ice-barrier requirement, where one applies, is a matter of which edition and amendments your specific county or city has adopted. Your building department can tell you what applies to your address and your roof pitch. This page can’t answer that for you.
What roofing material suits Montana best?
The honest answer depends on which of the three loads above governs your project, and Montana’s whole-state IECC climate zone 6B (dry, cold, no county exceptions inside the state) adds a fourth factor: insulation and ventilation performance under the roof deck, which ties straight back into the ice-dam mechanism above.
Architectural asphalt shingles
Architectural asphalt shingles carry a published wind rating from the manufacturer, and that rating is only as good as the installation under it. A shingle rated for high wind, nailed with the standard nailing pattern instead of the pattern the rating requires, performs to the lower standard, not the one printed on the wrapper. In a location with a 109 mph design gust, the nailing pattern and the number of fasteners per shingle matter as much as which product line you buy.
Standing-seam metal
Standing-seam metal sheds snow far more readily than shingles do, which is an advantage on the roof and a hazard everywhere the snow lands. A metal roof over an entry door, a walkway or a driveway needs a snow-retention strategy or a redesigned eave detail, because an unmanaged slide off a steep metal roof drops a slab of snow and ice with real force, right where people walk.
Slate and concrete tile
Slate and concrete tile add their own dead weight to whatever snow load the structure is already carrying, on top of the ground and roof snow figures discussed above. A roof structure sized for a lighter covering can’t simply be re-clad in tile without checking that the framing was designed to carry the additional dead load along with the snow load, which is a structural question, not a finish choice.
| Material | Wind sensitivity | Snow behavior | Added dead load |
|---|---|---|---|
| Architectural asphalt shingles | Depends heavily on nailing pattern and edge fastening | Holds snow in place, gradual melt | Low |
| Standing-seam metal | Fastener and clip spacing critical at edges | Sheds snow quickly, can slide as a slab | Low |
| Slate and concrete tile | Heavy units resist uplift but fasteners still matter | Holds snow, minimal sliding | Significant, adds to structural load |
None of these is universally best. Each answers a different part of the load picture: shingles are forgiving and inexpensive to detail correctly for wind, metal solves snow retention on the roof but relocates the problem to the ground below it, and tile solves both wind and snow shedding at the cost of a heavier structure underneath it. Whichever direction you go, the fastening schedule and the underlayment specified for your wind zone matter more to how the roof performs than the brand of material sitting on top of it.