New Hampshire’s roof snow load isn’t a single number pulled off a national map, the way it is in most states. The state building code sends you to a study built specifically for New Hampshire, one that sets the design load by elevation instead of a flat statewide figure, and at low elevations like Manchester’s roughly 260 feet, that number lands well below what a mountain town further north would need. Start any roofing project by asking your town’s building department which figure from that study applies to your address, not by guessing off a map.
What is the ground snow load in New Hampshire?
New Hampshire does not appear on the ASCE 7-16 national ground snow load map at all. Instead, the state building code points to a dedicated study, Tobiasson et al., CRREL TR-02-6, which sets the design ground snow load as a function of elevation across the state. Land above 2,500 feet is treated as a case study region, meaning the map gives no value there and a site-specific study is required before a permit can move forward. Manchester, at roughly 260 feet, sits well below that threshold, so the state study yields a straightforward look-up value for a project at that elevation.
Why the ground figure isn’t the roof figure
Ground snow load and roof snow load are not the same figure. The ground value describes what accumulates on open, flat ground. Before that number reaches a rafter, the code applies exposure, thermal and slope factors that adjust it for how a building actually sits and how steep its roof is. For an ordinary heated house with a sloped roof, the result generally lands close to 0.7 of the ground value, and lower still on a steep pitch. That converted figure, not the raw ground number, is what a truss engineer actually designs to.
This is the figure a building department reads off the state study when reviewing a permit application, and it’s what a truss manufacturer uses to size rafters and connectors before lumber gets cut. Unlike Wisconsin, where a roof snow load figure comes straight off the ASCE map for the reference city (see Wisconsin’s roof snow load and wind rules), New Hampshire’s reliance on an elevation-based study means that same kind of map simply doesn’t apply here. Skipping the state study and guessing a number off a national map is the most common way a project runs into trouble at plan review.
What the newer map shows, and why it isn’t the answer yet
For reference, ASCE 7-22, the newer edition and not yet the basis for New Hampshire’s adopted code, maps Manchester’s exact coordinates at 80 psf on a strength-level basis, or about 56 psf converted to the allowable-stress basis the older edition uses. That number is not directly comparable to a figure elsewhere in the state under the current edition: the two editions use different load factors, and a gap between them is not evidence of more snow or an underbuilt roof, only of a different way of counting the same weather. The figure a town’s building department actually uses today comes from the state elevation study, not from this map point.
How much snow can a roof hold in New Hampshire?
There’s no single depth of snow a roof in New Hampshire can safely carry, because the answer depends on what the snow has become, not just how deep it sits. Whatever figure a building department pulls from the state elevation study for a given address, the code doesn’t apply it directly to the roof surface. For an ordinary heated house with a sloped roof, the converted design load lands at roughly 0.7 of the ground figure, and lower still on a steep pitch that sheds snow readily. That’s the load a truss was designed to carry when the house was built, assuming it was built to code in the first place.
Where drift changes everything
The place a roof actually fails is rarely the open field of a plain gable. It’s the spot where wind piles snow against a taller wall, below a dormer, in a valley between two roof planes, or on a lower roof section next to a taller one. Drift loads can run well above the flat design figure at exactly those spots, because wind moves snow off the exposed parts of a roof and dumps it where the geometry traps it. A roof engineered for a uniform load can still be locally overloaded at a drift point that was underestimated, or created later by an addition that changed the roofline.
Depth is not weight
A foot of fresh, light snow in January is not the same load as a foot of wet snow in March. Freshly fallen, dry snow runs roughly 5 to 7 pounds per square foot for each foot of depth. Once that snow settles, or falls wet and heavy to begin with, the same depth can weigh two to three times as much. An ice layer, formed when a mid-winter thaw refreezes, adds concentrated weight again in a much thinner layer.
- Fresh, dry powder: roughly 5 to 7 lb per square foot per foot of depth
- Settled or wet, packed snow: roughly two to three times that
- An ice layer from a mid-season thaw: concentrated added weight in a thin layer
That’s why two feet of accumulated snow late in the season, built up over several storms with a thaw in between, can be a heavier load than three feet of dry powder sitting on the roof in early January.
Signs a roof is carrying too much
A few warning signs are worth taking seriously: interior doors that suddenly stop closing, new cracks appearing in ceiling drywall, a ridge line that looks like it’s sagging when viewed from the ground, or popping and cracking sounds from the attic. Raking accumulated snow off a roof from the ground, with a roof rake, is the safe response. Climbing onto a snow-loaded roof to shovel it is not: it adds a person’s weight to a structure that may already be near its limit, standing on a surface that is by definition unstable footing.
What wind speed must a roof withstand in New Hampshire?
The basis: a gust, not a sustained wind
The basic design wind speed for an ordinary house in Manchester is 113 mph under ASCE 7-16, the edition behind the wind provisions in New Hampshire’s current code. That figure is a 3-second gust measured at 33 feet in open, level terrain, Exposure C, for Risk Category II construction, the category that covers most single-family homes. It is not a sustained wind speed, and it’s not the number a weather forecast reports as wind speed. A forecast’s sustained wind and a 3-second gust from the same storm can differ substantially, so comparing this figure to a hurricane headline number will make the code look weaker than it actually is.
ASCE 7-22, the newer edition, maps the same Manchester point at 111 mph. The two figures are close enough here that the practical difference is small, but that shift is a mapping revision, not a sign of reduced wind risk or a loosening of the rules a builder actually has to follow today.
What the number governs on the roof
This design speed determines a shingle’s required wind rating, the nailing pattern used to fasten it, how the sheathing beneath it is fastened to the rafters, and the uplift connectors tying the roof framing down to the walls below. Edges, rakes and ridges take the worst of it because wind uplift concentrates at those transitions, which is why the code calls for tighter nailing patterns and additional fasteners in those zones rather than one uniform pattern across the whole roof.
This 113 mph figure is Manchester’s design value specifically. New Hampshire’s short coastline and its higher inland terrain each read differently off the same wind map, so a figure calculated for a coastal or mountain address can differ from Manchester’s. Confirm the design speed for a specific address with the local building department rather than assuming Manchester’s number applies across the whole state.
Does New Hampshire require an ice barrier under the shingles?
An ice barrier is a self-adhering, waterproof membrane installed under the shingles, run from the edge of the eave up the roof slope to a point past the inside face of the exterior wall. Its job isn’t to stop snow from accumulating. It’s to stop meltwater that has backed up behind an ice dam from working its way under the shingles and into the roof deck.
Why this climate produces ice dams
Manchester’s coldest month averages a daily minimum of 17.1°F, according to NOAA’s NCEI 1991-2020 climate normals, paired with a winter that reliably keeps a snow load on every roof in the state for weeks at a time. That combination, a hard freeze most nights against a standing snowpack, is exactly the condition that produces ice dams: a warm spot in the roof melts snow from underneath, the meltwater runs down the roof deck beneath the snow, and it refreezes the moment it reaches the cold, unheated overhang past the exterior wall.
What the membrane does and doesn’t do
The membrane treats the symptom, not the cause. An ice dam forms because heat is escaping into the attic and warming the underside of the roof deck unevenly, typically through gaps around can lights, bath fan ducts, an attic hatch, or insulation that has settled or was never installed to an adequate depth. The barrier keeps water that backs up from soaking into the deck and dripping into the ceiling below, but it does nothing to stop the dam from forming in the first place. Air sealing the attic floor and bringing insulation up to an adequate depth are what actually prevent the ice from building up.
What the code requires
New Hampshire’s own building code, the New Hampshire State Building Code adopted under RSA 155-A and based on the 2021 International Residential Code, is a uniform statewide code: no city or town may weaken its technical requirements, and it’s administered by the New Hampshire State Building Code Review Board through the Division of Fire Safety. Since July 1, 2026, no local amendment to these technical requirements is permitted at all. That code carries the ice barrier requirement that applies along eaves in a climate like this one. Confirm the exact extent and placement required for a specific roof with the local building department before a re-roof, since the requirement is tied to how far the barrier must run past the interior wall line, not just whether one is present.
New Hampshire’s ten counties split across two federal energy code climate zones, IECC zone 6A in five counties and zone 5A in the other five, which is the range that determines how much attic insulation the energy code calls for. That split affects how much insulation a house needs to keep heat out of the attic in the first place. Check the county table or the ENERGY STAR map for the zone that applies to a specific address rather than assuming one figure covers the whole state.
Ice damming isn’t unique to New Hampshire. The same mechanism, heat escaping into a cold attic and refreezing at the eave, drives the rules covered in Yukon’s roof snow load and wind rules, where a colder, longer winter pushes the same risk further still.
What roofing material suits New Hampshire best?
The right roofing material for a New Hampshire house follows from the same three figures covered above: the state’s elevation-based snow load, Manchester’s 113 mph design wind speed, and the state’s split between IECC climate zones 6A and 5A. General roofing maintenance advice matters too, but the loads on this page decide what’s structurally sound before style enters the conversation.
What each material changes
| Material | Snow behavior | Wind consideration | Added dead weight |
|---|---|---|---|
| Architectural asphalt shingles | Holds snow on the roof rather than shedding it suddenly | Comes with a published wind rating, met only when nailed to the pattern that rating assumes | Low |
| Standing-seam metal | Sheds snow in sheets once it warms, sometimes all at once | Panels and clips carry their own uplift rating, tied to fastening spacing | Low |
| Slate or concrete tile | Holds snow similarly to shingles | Heavy enough that wind uplift is rarely the governing concern | Substantial, added directly to the snow load the structure already carries |
Standing-seam metal sheds snow well, which is exactly why it needs a design decision most homeowners never think to ask for: where the sliding snow lands. A slide that clears the roof over a front door, a walkway, or a parked car is a hazard the roof designer has to plan for with snow guards or a redirected slide path, not something that happens to work out on its own.
Slate and concrete tile carry a real weight of their own, and that weight is added directly to whatever snow load the structure already has to hold, on top of the state study figure for the address, not instead of it. A structure sized for asphalt shingles isn’t automatically capable of carrying a heavier roofing material without review.
Where wind is the governing load, the fastening and the underlayment matter more than which material sits on top. A shingle rated for a high wind speed only performs to that rating when it’s nailed with the pattern, nail count and placement the rating assumes. The same is true of metal panel clip spacing and tile fastening. A high-rated product installed to a lower standard performs at the lower standard, not the one printed on the wrapper.