Des Moines’ roof snow number is 25 psf, and that figure sits on the ground, not on your rafters. The building code takes that ground value and works it down through exposure, thermal and slope factors before it ever reaches a truss drawing, landing around 18 psf for an ordinary heated house roof. That gap between ground and roof is the first thing most snow-load explainers skip, and it is the reason two roofs a few miles apart in Iowa can carry different design figures even under the same map point. Start any project by asking your local building department which numbers your permit actually uses.
What is the ground snow load in Iowa?
The design ground snow load at Des Moines is 25 psf, under ASCE 7-16, the edition IRC/IBC 2021 makes applicable in most states. That number describes snow sitting on open, unobstructed ground near Des Moines. It is not what a rafter has to carry, and it is not what a truss engineer stamps on a drawing without further work.
From ground to roof
The code runs that ground figure through a set of adjustments, exposure factor, thermal factor and roof slope, before it becomes a roof load. For an ordinary heated house roof with a typical slope, that arithmetic brings the 25 psf ground figure down to roughly 18 psf of roof snow load. A steeper roof sheds snow faster and often carries less. A flatter roof, or one that traps drifting snow, can carry more even at the same address.
This is the number a permit application references, the one a plan reviewer checks against the truss package, and the one a builder’s engineer of record uses to size rafters or trusses before ordering lumber. If you are pulling a permit for a new roof structure or an addition, your local building department confirms which load governs your specific site.
A newer analysis, ASCE 7-22, maps the same point in Des Moines at 45 psf. That is not more snow falling on Iowa and it is not a sign that existing roofs built to the older figure are undersized. The 7-22 number is a strength-level value, meant to be used with a 1.0 load factor, while the 7-16 figure is nominal and used with a 1.6 load factor. The two describe the same physical snow with different math underneath them, and only ASCE 7-16 is the edition your permit, plan review and truss drawings are currently built on.
Iowa has real elevation and terrain variation across its 99 counties, and the Des Moines figure is a single reference point, not a statewide constant. A roof in a different river valley or in the state’s northern reaches can sit under a different design value than the capital.
How much snow can a roof hold in Iowa?
There is no single inches-of-snow answer here, because the same depth of snow can weigh three times as much depending on what it has become. The code gives you, at Des Moines, a design roof snow load of roughly 18 psf worked down from the 25 psf ground figure for an ordinary heated sloped roof, and that number assumes snow spread evenly across the whole roof surface. Real snow does not spread evenly.
Where drift changes the math
Drift is where roofs actually get into trouble. Snow blown off an upper roof piles against a wall below it, collects beside a parapet, or banks up in a valley between two roof planes, and in each of those spots the local load can run well above the flat, uniform design figure. A lower roof next to a taller section of the same house, or a roof below a dormer, is exactly the kind of spot engineers check separately, because the map value at Des Moines describes open, unobstructed ground, not the drift pocket next to a chimney.
Depth also hides a lot of variation in weight. Freshly fallen, dry snow runs somewhere around 5 to 7 pounds per square foot for every foot of depth. Once that snow settles, or picks up moisture from a thaw-and-refreeze cycle, the same depth can weigh two to three times as much. A layer of ice on top, from rain falling on snow and refreezing, adds more weight again in a much thinner layer. A foot of powder in January and a foot of wet, settled snow in March are not the same load, even though they look the same on a tape measure.
| Snow condition | Approximate weight | What changes it |
|---|---|---|
| Fresh, dry snow | About 5-7 lb per sq ft per foot of depth | Recently fallen, cold, low moisture |
| Settled or wet snow | Roughly 2-3 times the fresh-snow rate | Time, partial melt, added moisture |
| Ice layer | Heavier still per inch than any snow layer | Rain-on-snow, thaw-refreeze cycles |
A roof under real strain shows signs before it fails: interior doors that suddenly stick or stop closing, new cracks running across ceiling drywall, or a ridge line that looks like it is sagging when you view the roof from the street. None of those are things to diagnose from a ladder mid-storm. Raking snow from the ground, working from the eaves and pulling snow toward you, is the accepted way to lighten a roof without adding a person’s weight to it. Climbing onto a snow-loaded roof to shovel it is where a lot of roof-related injuries happen every winter, and it does nothing the ground-based approach cannot do more safely.
What wind speed must a roof withstand in Iowa?
The basic design wind speed at Des Moines is 110 mph under ASCE 7-16, Risk Category II, the same edition IRC/IBC 2021 makes applicable in most states. That number is not what it sounds like on a weather broadcast. It is a 3-second gust measured at 33 feet above open, flat terrain (Exposure C), the standard the code uses to design an ordinary house, not a sustained wind speed and not a record gust pulled from one storm. Comparing it directly to the headline wind speed a news report gives for a hurricane will make the code figure look low when the two are not measuring the same thing.
What the number governs
This design speed feeds directly into decisions most homeowners never see on the finished roof: the wind rating a shingle product needs to carry, the nailing pattern that attaches each shingle course, how the roof sheathing is fastened to the rafters or trusses, and the uplift connections, hurricane clips, straps, or other engineered hardware, that tie the roof structure down to the walls.
- Shingle wind rating and the number of nails per shingle
- Sheathing fastener spacing, especially at panel edges
- Uplift connectors between roof framing and wall framing
- Extra attention at edges, rakes and ridges, where wind uplift concentrates first
Edges, rakes and ridge lines are where wind failures start on a house that otherwise performs fine in a straight-line storm, because wind pulls hardest at a roof’s perimeter and highest points. A roofer who nails the field of a roof correctly but shortcuts the edge courses has built the weak point exactly where the load is highest.
The newer ASCE 7-22 map lands on essentially the same figure here, close to 110 mph, so this is one case where the two editions agree rather than diverge. A re-roof designed to either edition’s wind figure for Des Moines is working from the same basic gust speed. Wind-borne debris regions, which require impact-rated glazing or approved shutters, are mapped along hurricane-prone coastlines rather than inland locations such as Des Moines, so that added layer of rules plays out very differently for a coastal state. Rhode Island’s shoreline communities are a useful contrast: the guide to Rhode Island’s roof snow load and wind rules covers what changes once that flag applies.
As with the snow figure, this wind speed is mapped at Des Moines specifically. Terrain, exposure category and local topography can shift the effective design wind pressure on a particular building even within the same county, which is one more reason the local building department, not a statewide rule of thumb, is the final word on what a specific roof needs.
Does Iowa require an ice barrier under the shingles?
An ice barrier is a self-adhering waterproof membrane installed directly on the roof deck at the eaves, run up far enough to pass the interior wall line of the house below. Its job is not to keep snow off the roof. It is there to stop water from an ice dam, water with nowhere else to go, from finding a seam in the shingles and reaching the wood deck underneath.
Why ice dams form here
Ice dams start with heat, not weather. Warm air leaking from a living space into an unsealed attic warms the underside of the roof deck unevenly, melting the bottom layer of snow while the surface stays frozen. That meltwater runs down until it reaches the roof edge, over the unheated eave beyond the wall line, where it refreezes into a ridge of ice. The dam that ridge creates backs water up under the shingles above it, and that backed-up water is what an ice barrier is built to stop.
Des Moines’ coldest-month average low, per NOAA’s 1991-2020 climate normals, is 13.8°F, cold enough that snowpack sits on a roof for extended stretches through winter rather than melting off quickly. That is exactly the condition that lets an attic’s heat loss build an ice dam over time. Combined with the design roof snow load covered above, that makes ice damming a real, recurring risk in a typical Iowa winter, not an edge case.
Iowa’s own building code is the Iowa State Building Code, based on the 2024 edition of the International Residential Code and administered by the Iowa Department of Inspections, Appeals and Licensing. Its published state amendments cover basement escape openings, not eave protection, so on this specific point the code follows the model IRC language rather than a separate state rule. The catch is adoption itself: the state code binds state buildings and larger cities without their own recognized code, but elsewhere, whether a county or city has adopted it, or adopted anything at all, is a local decision. Whether your roof needs an ice barrier under a given jurisdiction’s code comes down to what that city or county has adopted, and your building department is the only place that can confirm it for your address.
Even where a membrane is required and installed correctly, it only protects the deck from water damage. It does nothing to stop the dam from forming in the first place. Air sealing the attic floor and bringing insulation up to the level appropriate for the local climate zone is what actually keeps the roof deck at a uniform, cold temperature so meltwater never happens in the first place. A membrane without that attic work is a fix for the symptom, not the cause. Pennsylvania’s colder counties deal with the same mechanism, and the guide to Pennsylvania’s roof snow load and wind rules walks through how that state’s code handles it.
What roofing material suits Iowa best?
The honest answer starts from the numbers already on this page: a ground snow load near 25 psf at Des Moines, a design wind speed of 110 mph, and a climate split across two IECC zones. Those three together rule some choices in and shift how others should be installed, more than they crown one single winner.
Comparing the categories
Architectural asphalt shingles carry a wind rating as a category, and that rating only holds if the nailing pattern matches what the 110 mph design speed calls for, with extra attention at eaves, rakes and ridges where uplift concentrates. A shingle with a strong wind rating, nailed to a lower standard than its rating assumes, performs to the lower standard, not the one printed on the wrapper.
Standing-seam metal sheds snow well, often in one large slide rather than a slow melt, which is good for the roof structure itself but a real consideration for whatever sits below the eave. A metal roof over a walkway, a driveway or an entry door needs that slide path accounted for at the design stage, not discovered the first time a warm afternoon lets a season’s accumulation cut loose at once.
Slate and concrete tile add their own dead weight to whatever snow load the roof structure is already carrying, since that weight sits on the roof year-round, not just in winter. Adding tile or slate to a roof already designed to the loads discussed above is a structural decision, not just an aesthetic one, and it is one a licensed engineer should check against the specific rafter or truss sizing already in place.
Iowa’s climate zone split, under the 2021 International Energy Conservation Code, puts 84 of its 99 counties in zone 5A and the remaining 15 in zone 6A. That split drives the insulation and attic sealing side of the roof system discussed above, which matters for ice dam prevention regardless of which roofing material sits on top. None of it changes which material is legally required. It changes which details, fastening schedule, underlayment coverage, attic air sealing, actually determine whether a given material performs the way its rating promises. For general guidance on planning a re-roof project from the ground up, the roofing section covers the steps this page assumes are already underway.