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.
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.
Common questions
What does the code say about ground snow load for Roof Snow Load?
The design ground snow load is established by the local jurisdiction and recorded in its adopted climatic and geographic design criteria table. (IRC Table R301.2)
What does the code say about engineering threshold for Roof Snow Load?
Where 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 practice. (IRC R301.2.3)
What does the code say about rafter sizing for Roof Snow Load?
Rafter 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 spacing. (IRC R802.4.1, Tables R802.4.1(3)–(8))
Adopted edition by state
Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.