The physics guide

How metal roofs perform in New Hampshire snow country

A roof in this state is a structure with a winter-long load case, then a shedding machine, then a target for wind-driven water, in that order. Metal handles each differently than asphalt, and the differences are physics with primary sources, not marketing. This guide covers the load numbers, the shedding behavior, and the fastener and profile choices that decide whether a metal roof actually delivers in hard-winter service.

The load case comes first

Every New Hampshire town has a design ground snow load in CRREL TR-02-6, the study the state's structural practice leans on under the building code (RSA 155-A): about 50 pounds per square foot around Nashua and Salem, climbing past 120 in the White Mountains, with an elevation adjustment for sites above each town's reference point. Design works from that number regardless of material. What metal changes is how much of the design load the roof actually carries in practice: a surface that sheds spends most of the winter well under its rating, while an asphalt roof carries the blanket until spring. The rating is not a license to underbuild, it is margin, and margin is what March wants.

Shedding is a designed behavior

Snow leaves a standing seam roof in releases: sun or mild air warms the panel, the bond at the surface lets go, and a slope's worth of blanket moves at once. Three design consequences follow. Retention hardware goes over everything a slab could hit, engineered in distributed rows (the snow guards page covers layout). Shed paths need landing zones, planned at design time, not discovered by the propane regulator. And roof geometry matters: dormers, valleys, and roof-to-roof dumps interrupt the slide and build drifts, which is where a snow-country installer earns the difference between a catalog and a roof.

Fasteners: the quiet decider

Hard winters sort metal roofs into two populations by fastener. Concealed-clip standing seam holds the panel while letting it move: thermal cycling from subzero nights to sunlit afternoons expands and contracts the metal, and the clips absorb it for decades. Exposed fastener panels put thousands of gasketed screws through the metal instead; each cycle works them, gaskets age, and the roof develops its signature drip list in 15 to 20 years, the pattern described on the repair page. Both are legitimate products; only one belongs on a 40-year house plan in snow country. The full system anatomy is at standing seam metal roofing.

Gauge and profile for the loaded end of the map

As design values climb toward the Notches, the specification climbs with them: 24 gauge over 26 for panel stiffness, mechanical double-lock seams over snap-lock for their grip under sliding and wind loads, tighter clip spacing straight from the manufacturer's load tables, and taller seams where drifting runs deep. None of this is exotic; all of it should be legible in the written quote, which in a state with no contractor license is your real protection. Mountain-specific practice has its own page at high snow load mountain roofing, and it applies beyond the mountains: hilltop sites in the Monadnock Region and Upper Valley ridgelines quote like the north country.

What this costs and where to go next

Snow-country specification is most of why the standing seam range runs $10 to $18 per square foot installed rather than less; the cost guide breaks down where the money goes. The whole decision, region by region, starts at the New Hampshire Metal Roofing Guide.

Snow performance questions

How much snow can a metal roof hold?

Whatever it was engineered for. The design number comes from your town’s ground snow load in CRREL TR-02-6 (about 50 psf in southern NH to more than 120 in the mountains), converted to a roof load under the state building code. The panel, gauge, and clip spacing are then specified to that number; the metal itself is rarely the limit, the assembly is.

Does snow really slide off by itself?

On a smooth standing seam surface with reasonable pitch, yes, in releases rather than a season-long melt. Sun on the panel warms it enough to break the bond and the blanket goes. That is the feature, and it is why retention hardware over doorways and equipment is part of any honest design.

Is a steeper pitch always better for snow?

Steeper sheds sooner and holds less, but the assembly matters more than the angle: a well-specified low-slope mechanical seam roof outperforms a steep roof with the wrong system. Below each profile’s rated minimum pitch, the question answers itself, because the manufacturer will not warrant it.

What breaks on metal roofs in hard winters?

Mostly the edges and accessories, not the field: gutters and eave trim hit by sliding slabs, undersized snow guards ripped out, and on exposed-fastener systems, the screws and gaskets themselves. Engineered retention and concealed fasteners remove most of that list, which is the pattern behind the repair page.