Philadelphia’s roof is designed to hold a ground snow load of 25 pounds per square foot, and a house forty miles north toward the Poconos needs to hold more than that, because ground snow load climbs with elevation across Pennsylvania. That single figure sets the size of every rafter, the spacing of every truss, and the pitch a builder chooses on a new build. Start by asking your local building department for the ground snow load mapped to your exact address, not the figure for the nearest big city.
What is the ground snow load in Pennsylvania?
Philadelphia’s design ground snow load is 25 pounds per square foot (psf), the value from ASCE 7-16, the edition of the ASCE 7 standard that the 2021 International Residential Code (IRC) makes applicable in most states. That figure is not what actually sits on your rafters. It is the load mapped to the ground at that location, before a code official or a truss engineer ever touches it.
Ground load is not roof load
The code runs that ground number through exposure, thermal and slope factors before it reaches a rafter. For an ordinary heated roof with a normal slope, the result in Philadelphia lands at roughly 18 psf of roof snow load. That lower figure, not the 25 psf ground value, is what actually governs rafter size and truss spacing. Confusing the two is the single most common mistake made when reading a snow load map.
Who actually uses this number
This is the figure a permit application asks for, the one a plan reviewer checks against, and the one a truss manufacturer stamps onto a shop drawing before any lumber gets cut. In Pennsylvania that model code is enforced through the Pennsylvania Uniform Construction Code, which is what your local building department actually applies when it reviews the drawing. If you are pulling a permit for a new roof, an addition, or a truss replacement, this is the number your building department wants referenced, not guessed at.
The newer map, and why it is not more snow
A newer edition of the standard, ASCE 7-22, maps the same Philadelphia location at 34 psf. That is not more snow falling on the region. ASCE 7-22 figures are strength-level values built on a 1.0 load factor, while ASCE 7-16 figures are nominal values built on a 1.6 load factor, so the two are not measured on the same basis and cannot be compared directly. A few jurisdictions are beginning to adopt 7-22, but the 25 psf figure from 7-16 is the one behind the code edition in force across most of the state, and an existing roof built to it is not undersized just because a newer map shows a bigger number.
Where Philadelphia’s number stops applying
Ground snow load rises with elevation, and parts of Pennsylvania, including the Alleghenies and the Poconos, sit well above the Delaware Valley. The 25 psf figure describes Philadelphia, not the state, and a building department in a higher or more exposed county may work from an entirely different mapped value. Pennsylvania’s own elevation swings shift that number the way elevation shifts it far more dramatically in mountain states, such as the roof snow load rules in Colorado. Confirm the design ground snow load for your specific address with your local building department or a licensed engineer before assuming the Philadelphia figure applies to your project.
How much snow can a roof hold in Pennsylvania?
There is no single number for this, because a roof’s real holding capacity depends on the roof’s shape, its framing, and what the snow sitting on it has turned into by the time it matters. The code gives a design value, not a guarantee, and understanding how that value is built shows where a roof actually gets into trouble.
From ground load to roof load, and then to drift
Start from the ASCE 7-16 ground snow load of 25 psf mapped to Philadelphia. For an ordinary heated house with a normal slope, that converts to roughly 18 psf of roof snow load, the figure covered in the section above. A steeper roof carries less, since snow slides off before it can pile up. But snow does not sit evenly across a roof, and that is where the real risk lives. It builds up against a parapet or a taller adjoining wall, below a dormer, inside a valley where two roof planes meet, and on a lower roof section beside a taller one that sheds snow onto it. Drift load, not the flat design load, is where most roof overload failures actually happen, because a roof built to the uniform number was never meant to carry a three- or four-foot drift concentrated in one spot.
Why weight, not depth, is what matters
A foot of snow is not a fixed weight. 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 partially melts and refreezes, the same depth can weigh two to three times as much. An ice layer on top adds weight again, in far less depth. That is why a foot of dry January snow and a foot of wet March snow are entirely different loads on the same roof, even though they look about the same from the ground.
| Snow condition | Approx. weight per sq ft, per foot of depth | Why it differs |
|---|---|---|
| Fresh, dry powder | About 5-7 lb | Loose crystal structure, lots of trapped air |
| Settled or wet snow | Roughly 2-3x fresh powder | Compaction and meltwater fill the air pockets |
| Ice layer | Heavier still, in far less depth | Solid ice has almost no trapped air |
Signs a roof is carrying too much
A handful of warning signs matter more than any depth measurement on the ground:
- Interior doors that suddenly stop closing properly
- New cracks in ceiling drywall, especially near the center of a span
- A ridge line that looks like it is sagging or bowing
- Popping or creaking sounds from the attic under load
If you see any of these, the safe response is to clear snow from the ground with a roof rake, working from below. Climbing onto a snow-loaded roof to shovel it adds your own weight to a structure that may already be near its limit, and it is not a safe substitute for calling a professional or your building department about a specific roof.
What wind speed must a roof withstand in Pennsylvania?
Philadelphia’s basic design wind speed is 115 mph under ASCE 7-16, for Risk Category II buildings, which covers ordinary single-family houses. That number is a 3-second gust measured at 33 feet above open, flat terrain (Exposure C), not a sustained wind speed and not the kind of figure a weather forecast reports. A hurricane’s headline wind speed and this design figure are measured on completely different bases, so comparing them directly makes the code look weaker or stronger than it actually is.
ASCE 7-22 maps the same Philadelphia location at 114 mph, essentially the same value on an updated map. That is not a meaningful shift in wind risk, just a refinement in how the map was built. The 115 mph figure from ASCE 7-16 remains the one tied to the code edition in force across most of the state right now.
What the wind speed actually governs
This figure feeds directly into several concrete requirements on a house:
- The wind rating required on asphalt shingles and how they must be fastened
- The nailing pattern and fastener spacing across roof sheathing
- Uplift connectors, such as hurricane ties, between the roof framing and the wall framing
- How exposed edges, rakes and ridges are detailed, since uplift forces concentrate there first
A roof can look identical from the street and still be built to two very different wind standards depending on the fasteners and connectors hidden underneath the shingles. That is why the connection details matter as much as the roofing material itself at a design wind speed this high.
Philadelphia sits well inland from the immediate Atlantic coastline, and the wind-borne debris provisions in the code, which require impact-rated glazing or shutters on windows and glazed doors, apply mainly to mapped coastal wind-borne debris regions. Whether any such requirement reaches a specific project in Pennsylvania is a question for your local building department, since municipalities may adopt stricter ordinances than the statewide minimum. Do not assume it either way without checking.
Does Pennsylvania 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 line of the exterior wall below. It is not there to stop snow from sitting on the roof. It is there to stop meltwater from getting under the shingles and into the house once an ice dam has already formed at the eave.
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 roof until it reaches the cold overhang past the exterior wall, where there is no warm attic below to keep it liquid, and it refreezes. Layer by layer, that refrozen water builds a dam that backs up behind it, forcing water sideways and upward under the shingles instead of off the edge. The membrane protects the deck once that has already started. It does not stop the dam from forming. Only air sealing the attic floor and adding enough insulation to keep the roof deck cold and uniform actually prevents the dam in the first place.
Philadelphia’s mean minimum temperature in the coldest month runs 26.0°F, per NOAA’s 1991-2020 climate normals. Sitting that close to freezing on an average night, in a season that regularly carries measurable roof snow load, is exactly the combination that produces ice dams: enough cold to refreeze meltwater at the eave, enough snow cover to supply the water in the first place. Cold, snow-prone states farther west wrestle with the same eaves-and-attic problem, including the questions covered in our page on the snow load and ice barrier rules in Iowa.
What the code actually says
The code in force across Pennsylvania is the Pennsylvania Uniform Construction Code (34 Pa. Code, chapters 401-405), which adopts the 2021 International Residential Code with the state’s own exceptions. Those 2021-based provisions have been in force statewide since January 1, 2026, and the code is administered by the Pennsylvania Department of Labor and Industry, Bureau of Occupational and Industrial Safety. It sets a statewide minimum, and a municipality may adopt a stricter ordinance on top of it after review. The facts available here do not spell out a specific ice-barrier trigger for this state on their own, and getting that detail wrong on a re-roof is expensive, so this is a case worth a direct call to your local building department before shingles go down. Ask specifically whether an ice barrier membrane is required at the eaves for your project and how far up the roof it must extend.
What roofing material suits Pennsylvania best?
The honest answer depends on which of Philadelphia’s design numbers you are building around: a 25 psf ground snow load, a 115 mph basic wind speed, and, under the 2021 International Energy Conservation Code, a climate that splits between zone 5A across 53 of the state’s 67 counties and zone 4A across the remaining 14. None of the common roofing categories is disqualified by these numbers, but each responds to them differently.
Asphalt shingles
Architectural asphalt shingles carry a manufacturer wind rating, and that rating only means what it says if the shingles are installed with the nailing pattern the design wind speed calls for. At 115 mph, a shingle rated for high wind performs to whatever standard it was actually nailed to, not to the number printed on the wrapper. Underlayment and fastening quality do more work here than the shingle brand.
Standing-seam metal
Metal roofing sheds snow far more readily than shingles or tile, which sounds like an advantage under a 25 psf ground snow load, and often is. But shedding snow means the snow has to land somewhere. A metal roof over a front door, a walkway or a driveway can dump a slab of snow or ice directly onto whatever sits below it, so where that snow is aimed becomes a design decision, not an afterthought, at the point the roof is planned.
Slate and concrete tile
Slate and concrete tile carry substantial dead weight of their own, on top of whatever snow load the structure already has to hold. That combined load has to be accounted for in the framing from the start. Retrofitting a roof originally framed for asphalt shingles over to tile or slate is a structural question, not a cosmetic one, precisely because of that added weight sitting on top of the snow load already discussed above.
Across all three categories, the connection between the roof deck and the framing below it, and between that framing and the walls, is what a 115 mph design wind speed actually tests. For general guidance on inspecting and maintaining any of these systems once installed, our broader roofing guide covers the basics that apply regardless of material.