Snow Load: The Number That Decides Whether Your Barn Survives

Most of the United States can look up ground snow load on a map in the ASCE 7 standard, find a number, and design to it. Erie County can't. Northwestern Pennsylvania is designated a case-study area in ASCE 7-22, meaning local elevation, terrain, and lake-effect microclimate make the mapped values unreliable and the design load has to be established from local data.

What the numbers look like here

The City of Erie's adopted code specifies a ground snow load of 40 psf. Published guidance for northwestern Pennsylvania near Erie puts the range at roughly 40 to 60 psf or higher, driven by lake-effect snow and by elevation.

That variation is not academic. It is the difference between the lakeshore at Girard and the high ground at Edinboro or Wattsburg, and it is the difference between a truss package that holds and one that doesn't. Buildings on the higher inland ground south and east of the city genuinely need more structure than the identical building put up twenty miles north on the plain.

Why wide-span buildings are where this bites

Snow load is carried by the trusses, and the demand on a truss climbs fast as the span widens. A 24-foot span forgives a lot; a 60-foot clear span forgives nothing. Underestimating ground snow load in this region is one of the recognised paths to roof collapse on wide-span metal and post-frame buildings, and it usually happens in a single heavy lake-effect event rather than gradually.

Drifting is the other half of it

The design load isn't the whole story. Snow drifts against anything that interrupts a roof plane — a lean-to added to an existing barn, a lower roof beside a taller one, a valley between two spans. Those junctions carry far more than the flat-roof design load, and they're exactly where additions go wrong. If you're adding a lean-to onto a building that's already up, the drift load at the junction is the engineering question, not the new roof itself.

The question to ask

Ask every builder who quotes you: what ground snow load are these trusses engineered to, and can I see the truss drawings?

Truss manufacturers stamp the design load on the drawings. A builder who works here routinely will produce them without hesitation. A quote that comes in noticeably cheaper than the others, from someone who can't answer that question, has usually found its savings in exactly that place.

And the one below it

While you're asking: the design frost depth used in the Erie area is 48 inches. Columns and footings have to reach below it. A building set shallow will move with the ground every spring, and no amount of good steel on the roof fixes a foundation that heaves.