Twenty-five pounds per square foot is the ground snow load a Baltimore-area building permit works from, and that flat number already contradicts the idea that Maryland’s mild, humid summers mean the roof code skips snow engineering entirely. It doesn’t skip it, it just converts it: exposure, thermal and slope factors turn that ground figure into a smaller roof number before a truss ever gets built. If you’re pulling a permit or reviewing truss drawings, ask your county building department which snow and wind figures they used, since elevation and distance from the Chesapeake shift both away from the Baltimore reference point.
What is the ground snow load in Maryland?
The design ground snow load at Baltimore is 25 psf under ASCE 7-16, the edition that IRC/IBC 2021 makes applicable in most states. That figure is what a Baltimore-area permit application, a plan review and the truss drawings a builder orders all reference, and it is why the number matters to someone who never opens a structural code book.
Ground load vs roof load
25 psf is a ground measurement, not the load that actually presses on a rafter. The code takes that ground figure and runs it through exposure, thermal and slope factors before it reaches the roof. For an ordinary heated house with a sloped roof, that conversion lands at roughly 18 psf of roof snow load, and a steeper roof pulls the number down further because snow sheds off a steep pitch instead of sitting on it. Confusing the ground figure with the roof figure is the most common mistake anyone makes reading a snow map, so keep the two separate from here on.
A newer edition, ASCE 7-22, maps the same Baltimore point at 59 psf. That is not more snow and not a sign that a roof built to the older number is now undersized. ASCE 7-22 reports a strength-level value, calculated with a 1.0 load factor, while ASCE 7-16 reports a nominal value used with a 1.6 load factor. The two numbers sit on different scales measuring the same underlying condition, and a handful of jurisdictions are beginning to adopt the newer edition, but the 25 psf figure is what governs a Maryland permit today.
Both figures are tied to Baltimore specifically. Elevation and distance from the Chesapeake shift the mapped value elsewhere in the state, particularly in the higher terrain of western Maryland, so a homeowner outside the Baltimore area should ask their county building department which figure applies to their address rather than assume the reference-city number travels unchanged. If you are scoping any part of a roofing project that touches structural framing, this is the number your engineer or truss supplier will ask for first.
How much snow can a roof hold in Maryland?
There is no single weight a Maryland roof “can take,” because the answer depends on the roof’s design load, its pitch, and what the snow sitting on it has turned into. Start from the Baltimore ground figure of 25 psf and the roughly 18 psf that an ordinary heated sloped roof converts to under ASCE 7-16, then adjust for the fact that snow does not sit evenly.
Where snow piles up: drift loads
A flat stretch of roof rarely carries the highest load. Snow blown off a taller section lands against a parapet or a wall, piles below a dormer, or collects in a valley between two roof planes, and in every one of those spots the local load can run well above the uniform design figure. A lower roof next to a taller one is a classic drift location, because wind strips snow from the upper roof and dumps it exactly where the lower one meets the wall. This is also where structural failures actually start, far more often than on an open, unobstructed field of roof.
Why depth alone tells you nothing
Fresh, light snow runs roughly 5 to 7 pounds per square foot for every foot of depth. Once that snow settles, or falls wet and heavy, the same foot of depth can weigh two to three times as much. An ice layer packed into the base of the snowpack adds more weight still, on top of already being denser than either. That is the whole reason a foot of powder in January and a foot of slush in March are not the same load, even though a tape measure reads the same number both times.
| Snow condition | Approx. weight per foot of depth |
|---|---|
| Fresh, light snow | 5 to 7 psf |
| Settled or wet, packed snow | Roughly 2 to 3 times the fresh-snow rate |
| Ice layer within the pack | Denser again, adds on top of the snow weight |
Signs a roof is overloaded
- Interior doors that suddenly stick or won’t latch
- New cracks in ceiling drywall, especially near a ridge or valley
- A ridge line that looks like it’s sagging when viewed from the street
- Cracking or popping sounds from the attic framing under load
If you see any of that, the safe response is to remove snow from the ground level with a roof rake, working from the eave, rather than climbing onto a loaded roof with a shovel. A roof rake pulls snow off the lower few feet without adding your body weight to a structure that may already be near its limit. Homes further west, in a state like Missouri, deal with a different balance of these same forces, covered in our Missouri snow and wind load guide, and the drift mechanics are worth comparing if you’re weighing roof geometry on a new build.
What wind speed must a roof withstand in Maryland?
The basic design wind speed at Baltimore is 113 mph under ASCE 7-16, and the same point maps at 113 mph under ASCE 7-22 as well, so unlike the snow figures, the two editions agree here. That number is a 3-second gust at 33 feet, measured in open Exposure C terrain, for Risk Category II (an ordinary house). It is not a sustained wind speed, and it is not comparable to the headline number a hurricane forecast reports, which is usually a sustained or 1-minute value. Comparing the two directly makes the building code look weaker than it is.
What the number governs
This design wind speed is what determines the shingle wind rating a roofer specifies, the nailing pattern used to fasten each course, how the sheathing is fastened to the rafters, and the uplift connections tying the roof structure down to the walls below. Roof failures under wind load concentrate at the edges, rakes and ridge, the places where wind accelerates around a corner and lifts rather than pushes. A roof built to the right material rating but fastened with a lighter nailing pattern than the design calls for performs like the lighter pattern, not like the rating on the shingle wrapper.
Wind-borne debris regions
The mapped figure at Baltimore does not carry a flagged wind-borne debris region in the data behind this page. States with exposed hurricane coastline handle this very differently: South Carolina’s coast, for instance, runs to far higher basic wind speeds and triggers mandatory wind-borne debris requirements, impact-rated glazing or approved shutters, detailed in our South Carolina snow and wind load guide. Homeowners along Maryland’s own Atlantic coast and the Chesapeake shoreline should still confirm the wind speed and any debris-region requirement for their specific address with their county building department, since the Baltimore reference point does not necessarily hold along the coast.
Does Maryland require an ice barrier under the shingles?
An ice barrier is a self-adhering waterproof membrane installed under the shingles, running from the edge of the eave up past the interior line of the exterior wall. Its job is not to stop snow from sitting on the roof. It is there to stop meltwater that backs up behind an ice dam from finding a gap in the shingles and reaching the wood deck underneath.
What actually causes an ice dam
An ice dam forms when heat escaping from the living space into the attic warms the underside of the roof deck enough to melt the snow sitting on it. That meltwater runs down the slope until it reaches the eave, which sits over the unheated overhang and stays cold, and there it refreezes into a ridge of ice. Once that ridge forms, more meltwater ponds behind it instead of draining off the edge, and given enough time and enough head pressure, it finds its way under a shingle lap. The membrane is there to catch that water if it happens. It does nothing to stop the dam from forming in the first place. Only sealing attic air leaks and adding enough attic insulation actually keeps the roof deck cold and even, which is what prevents the melt-refreeze cycle from starting.
Does Maryland’s climate make this a real risk?
Baltimore’s coldest month averages a daily minimum of 25.4°F under NOAA’s 1991-2020 climate normals, cold enough to hold a snowpack on a roof for days at a stretch, which is exactly the condition an ice dam needs. Under the 2021 International Energy Conservation Code (IECC), Maryland’s two zone 5A counties sit in a colder design category than the 22 zone 4A counties covering the rest of the state, which means attics in those two counties have that much more heat trying to escape upward through the ceiling in a cold snap. Combined with the design snow load discussed above, the ingredients for ice damming, sustained cold plus real accumulated snow, are present across much of the state.
The code in force is the Maryland Building Performance Standards (COMAR 09.12.51), based on the 2021 International Residential Code with Maryland’s own modifications, administered statewide as a minimum by the Maryland Department of Labor’s Division of Labor and Industry, Building Codes Administration, with permits and inspections still handled locally. Maryland’s listed amendments to that code cover radon control, not the roofing underlayment section, so the base model provision for ice barrier protection in areas with a history of ice-dam formation carries through unless a county has amended it further. Confirm the exact requirement for your address, including how far up the roof the membrane must run, with your county building department before a re-roof, since that is a detail worth getting from the office that will actually inspect the work.
What roofing material suits Maryland best?
The honest answer depends on which load governs on a given roof, and at Baltimore’s figures, that means weighing a 25 psf ground snow load, a 113 mph basic wind speed, and a climate split under the 2021 IECC between zone 4A in 22 of the state’s 24 counties and zone 5A in the other 2, against whatever roof geometry and material you’re considering.
Comparing material categories
Architectural asphalt shingles are rated for wind performance as a system, and that rating only holds if the nailing pattern matches what the rating assumes. At Maryland’s 113 mph design speed, the fastening schedule matters as much as the shingle itself, and a contractor should be specifying both together, not choosing a high-wind shingle and installing it to a standard pattern.
Standing-seam metal sheds snow cleanly off a sloped roof, which is an advantage for the roof structure itself but a liability for whatever sits below the eave. A metal roof that slides its full snow load off in one event can drop a genuinely dangerous weight onto a walkway, a deck, or a parked car, so where that snow will land needs to be part of the design, not an afterthought discovered the first winter.
Slate and concrete tile carry substantial dead weight of their own, and that weight adds directly onto whatever snow load the structure is already carrying, on top of the roof’s own weight rather than instead of it. A structure sized for asphalt shingles is not automatically sized for tile, and switching material categories on an existing roof is a structural question, not just a materials one.
Whichever category you choose, the underlayment and fastening detail at the edges, rakes and ridge matter more than the material label once wind is the governing load, because that is where uplift concentrates regardless of what sits on top. Any roofing decision here should start with the design values your county building department applies, not with a national average, since the Baltimore figures used throughout this page are a reference point and not a guarantee for every parcel in Maryland.