Twenty pounds per square foot is the number stamped on a roof permit near Indianapolis, not the depth of snow sitting on a roof right now. That ground-level figure gets discounted for slope, adjusted for exposure, and recalculated for drift before a truss company ever draws a rafter. If you’re pulling a permit or reviewing truss drawings, ask your local building department which value they’re actually using on your project, because a reference-city figure is a starting point, not a universal answer for every address in the state.
What is the ground snow load in Indiana?
The design ground snow load at Indianapolis is 20 psf under ASCE 7-16, the edition IRC/IBC 2021 makes applicable in most states. That figure is measured at ground level, not on a roof. Before it reaches a rafter, the code applies exposure, thermal and slope factors that pull the number down.
Ground load versus roof load
For an ordinary heated sloped house roof, that conversion lands near roughly 14 psf of roof snow load, which is the figure a truss designer actually works with. A steeper roof discounts further, since snow sheds before it accumulates as deeply as it would on a low slope. None of this is a number a homeowner calculates themselves. It’s the job of whoever stamps the truss drawings and the plan reviewer who signs off on the permit.
This is the value a permit application, a plan review, and a set of truss drawings all reference for a house near Indianapolis. It’s a structural design input, not a weather forecast. It accounts for a worst-case accumulated condition over the life of a building, not a typical winter’s snowfall total.
A newer edition, ASCE 7-22, maps the same point at 30 psf. That is not more snow, and it is not a sign that a house built to the older figure is undersized. ASCE 7-22 uses a strength-level basis with a 1.0 load factor, while ASCE 7-16 uses a nominal basis with a 1.6 load factor, so the two figures aren’t measured on the same scale even though they describe the same location. A few jurisdictions are beginning to adopt 7-22, but the 20 psf figure is what applies where IRC/IBC 2021 governs.
This number is specific to Indianapolis. Elevation and local topography shift snow load across a state, and a reading near one city says nothing certain about a ridge, a valley, or a county sitting at a different elevation, the same way elevation swings the numbers across the guide to Oregon’s roof snow load and wind rules. For the exact value that applies to a specific address, the local building department or a licensed engineer is the source, not a state-wide map reading.
How much snow can a roof hold in Indiana?
There’s no single number that answers this, because the answer depends on the roof’s shape, its slope, and where the snow has piled up, not just how much fell. What follows is the reasoning the code uses, not a one-size figure for every house.
Start from the ground snow load at Indianapolis, 20 psf under ASCE 7-16. For an ordinary heated roof at a moderate slope, the code discounts that to roughly 0.7 of the ground value, landing near the roughly 14 psf figure described above. But that discount only applies to open, unobstructed roof area, and open area is not where roofs usually run into trouble.
Where drift changes everything
Drift is where roofs actually fail, and the code accounts for it separately from the base figure. Snow blown against a taller adjoining wall, piled below a dormer, or funneled into a valley between two roof planes can carry several times the load of open roof area nearby. A lower roof next to a taller section of the same house is a classic drift location, because wind moves snow off the upper roof and drops it on the lower one. The base ground figure never captures this. It’s a separate calculation a truss designer or engineer runs for the specific roof geometry, not something read off a map.
Snow’s weight also changes as it ages. Fresh, light snow runs roughly 5 to 7 lb per square foot for every foot of depth, but once it settles, gets rained on, or partially melts and refreezes, that same depth can weigh two to three times as much. A layer of ice from a freeze-thaw cycle adds more still, without adding much visible depth. That’s why a foot of snow sitting since January is a different load than a foot that fell yesterday, and why eyeballing depth never tells the whole story.
Watch for signs a roof is carrying more than it should:
- Interior doors that suddenly stick or won’t latch properly
- New cracks spreading across ceiling drywall
- A ridge line that looks like it’s sagging when viewed from the street
- Unusual creaking or popping sounds from the attic or roof framing
Raking snow off from the ground with a roof rake is the safe response. Climbing onto a snow-loaded roof to shovel it is how people get hurt, both from the fall risk and from the added weight of a person standing where snow has already pushed the structure toward its limit.
What wind speed must a roof withstand in Indiana?
The basic design wind speed at Indianapolis is 106 mph under ASCE 7-16, Risk Category II, the wind provisions that IRC/IBC 2021 makes applicable in most states. That’s a 3-second gust measured at 33 feet in open, Exposure C terrain. It is not a sustained wind and not what a weather forecast calls wind speed. Comparing it to a hurricane’s headline gust number invites the wrong conclusion, since the two describe different things measured different ways, not two points on the same scale.
ASCE 7-22 maps the same point at 106 mph, essentially unchanged from the ASCE 7-16 value. Where the newer edition and the older one disagree elsewhere in the country, that gap can change a real design requirement. At Indianapolis it doesn’t move the number, but confirming which edition a plan reviewer is working from still matters, because the edition determines which map and which risk-category table apply to a given project.
Risk Category II covers ordinary houses and most residential construction. Higher categories, used for buildings like fire stations or schools, get designed to a higher wind speed at the same location, because the consequence of failure is judged more severe. A homeowner’s roof almost always falls under Category II, which is the value given above.
What the wind speed governs
On an actual roof, that number governs the wind rating printed on a shingle product, the nailing pattern used to install it, how the sheathing is fastened down to the trusses, and the uplift connections tying the roof framing to the walls below. Edges, rakes and ridges fail first in a wind event, because that’s where suction from wind moving over a roof concentrates hardest. A shingle rated for high wind speeds still underperforms if it’s nailed to a lower pattern than the rating assumes, or if the connection between roof framing and wall wasn’t detailed for uplift in the first place.
This figure describes Indianapolis specifically, not the state as a whole. Elevation, exposure and local topography shift the design wind speed from one part of a state to another, sometimes considerably, the way coastal exposure changes the figures covered in Delaware’s roof snow load and wind rules. For the value that applies to a specific address, the local building department is the source to confirm, not a single reference-city number applied statewide.
Does Indiana require an ice barrier under the shingles?
An ice barrier is a self-adhering membrane installed under the shingles, run from the eave edge up past the interior wall line of the house. It isn’t there to stop snow from sitting on the roof. It’s there to stop meltwater that backs up behind an ice dam from finding a seam and reaching the roof deck underneath.
The 2020 Indiana Residential Code (675 IAC 14-4.4), based on the 2018 International Residential Code, is the uniform code governing residential construction across Indiana, administered by the Indiana Fire Prevention and Building Safety Commission. Commission rules prevail over local ordinances, so this isn’t a requirement that shifts town to town the way some building rules do elsewhere. The 2018 IRC base includes a provision requiring an ice barrier in any area with a history of ice forming along the eaves and causing water to back up under shingles.
Whether that history applies to a given roof is exactly the kind of judgment a local building official makes, but the climate figures point toward yes across most of the state. NOAA’s 1991-2020 climate normals put the coldest month’s average daily minimum at Indianapolis at 20.9°F, well below freezing for weeks at a stretch, and the same design snow load discussed above, 20 psf on the ground at Indianapolis, means there’s routinely enough snow sitting on a roof for a dam to form. Cold nights and standing snow are the two ingredients an ice dam needs.
What actually causes an ice dam
The dam doesn’t start at the eave. It starts in the attic. Heat escaping upward from the living space warms the underside of the roof deck over the attic, melting the bottom layer of snow even while the air outside stays well below freezing. That meltwater runs down the roof until it reaches the eave, which sits over unheated space, usually a soffit or an overhang, and refreezes there. Ice builds up, water backs up behind it, and eventually finds a shingle seam or a nail hole.
The membrane protects the deck once that water is already there. It doesn’t stop the dam from forming. Air sealing the attic floor and adding enough insulation to keep the roof deck cold and uniform, the same temperature as the outside air, is what actually prevents the melt-refreeze cycle. A roof with an ice barrier and a leaky, under-insulated attic still forms dams every winter. It just doesn’t leak into the ceiling while it does.
What roofing material suits Indiana best?
The honest answer starts from what the roof has to survive here, not from a preference for one material over another: a ground snow load of 20 psf at Indianapolis, a basic design wind speed of 106 mph, and a state that splits between IECC climate zone 5A across 47 counties and zone 4A across 45 counties. Those figures change what each material has to do well, not which one wins outright. Anyone starting from scratch should also look at the broader roofing basics before narrowing down to a material.
Architectural asphalt shingles
Asphalt shingles carry a published wind rating, and at Indianapolis’s 106 mph design wind speed, that rating and the nailing pattern used to install it are what determine whether the shingles stay attached in a wind event, more than the brand or price tier. A shingle rated for high wind speeds, installed with a standard nailing pattern, performs to the standard, not to the rating printed on the wrapper.
Standing-seam metal
Standing-seam metal sheds snow well, which sounds like a clear advantage under a 20 psf ground snow load, and often is. But shedding well means the snow has to go somewhere. A metal roof over a doorway, a walkway or a driveway can dump a slab of snow and ice onto exactly the spot people walk, which is a layout decision to plan for at the design stage, snow guards or a redirected roofline, not something to discover the first winter after installation.
Slate and concrete tile
Slate and concrete tile add their own dead weight on top of whatever snow accumulates, and that dead weight is part of the load the structure carries every day of the year, not just in winter. A roof structure has to be designed for tile from the start. Retrofitting a lighter structure originally built for asphalt shingles isn’t a simple material swap.
Where wind governs, the fastening and the underlayment matter more than the material choice itself. A roofing product tested to a high wind rating is only as good as the pattern it’s actually installed with, and the connections between roof, wall and foundation matter as much as anything printed on the shingle wrapper. None of this tells an individual homeowner what their specific roof needs. That answer comes from the local building department, or a licensed engineer working from the actual structure.