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Home / Code Guides / Roof Snow Load

Roof Snow Load Basics — Ground Snow, Design Limits & Engineering

The IRC does not calculate snow load for you — it hands you your jurisdiction’s ground snow load and tells you which framing tables to use. The key threshold is 70 psf: at or below it you can frame prescriptively from the tables; above it, the roof must be engineered.

RequirementModel Code StandardSection
Ground snow loadThe design ground snow load is established by the local jurisdiction and recorded in its adopted climatic and geographic design criteria tableIRC Table R301.2
Engineering thresholdWhere the ground snow load exceeds 70 psf, the prescriptive framing provisions no longer apply and the roof must be designed in accordance with accepted engineering practiceIRC R301.2.3
Rafter sizingRafter span tables include dedicated columns for 30, 50, and 70 psf ground snow loads — you size rafters directly from the snow value, species, grade, and spacingIRC R802.4.1, Tables R802.4.1(3)–(8)
Low-snow areasWhere snow is not the controlling load, rafters are sized from the 20 psf roof live load tables insteadIRC Tables R802.4.1(1)–(2)
TrussesEngineered trusses are designed by the truss manufacturer for the site’s specified snow load — the design drawings are part of your permit documentsIRC R802.10
Decks and additionsSnow load also drives deck and header sizing — deck framing and header span tables are entered with the same ground snow load valueIRC R507, R602.7

Worth knowing

  • Ground snow load and roof snow load are different numbers. The IRC tables build the conversion in, but an engineer designing above 70 psf will apply ASCE 7 factors for roof slope, exposure, and thermal conditions.
  • Mountain jurisdictions often publish snow load by elevation rather than one number — a lot at 7,800 feet can carry a dramatically higher load than one at 6,500 in the same county. Get the number in writing from the building department.
  • Drifting against a taller wall, a second story, or a parapet can multiply the load on part of a roof. Prescriptive tables assume uniform load, so unusual geometry is a reason to involve an engineer even below 70 psf.
  • Adding solar panels or a heavy roof covering later does not change the snow that falls, but it does eat into the same load capacity — check the framing before re-roofing tile over a roof framed for asphalt.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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