Design snow load is the weight a building code tells a designer to plan a roof for. Moscow and Pullman publish their own criteria, and they are not the same, so the number for a given address comes from the building official who covers that address. Drifting and sliding snow is a design question too, not only a weight question.
Key takeaways
- A design snow load is the figure a building code sets for the weight a roof must carry, and it is set for a jurisdiction, not a region.
- The City of Moscow and the City of Pullman publish their own criteria, and the two sets are not interchangeable.
- Pullman's current criteria, effective March 15, 2024, are built on the 2021 IRC with Washington amendments.
- Snow drifts and slides concentrate load where roof planes meet, at valleys, parapets and roof offsets.
- Eave ice and heavy snow load are separate problems, one about water and the other about weight.
- Outside city limits, the county building department covers the address and can confirm the criteria that apply there.
What is a design snow load?
A design snow load is the weight per unit of roof area that a building code tells a designer to plan for. It is not a measured snowfall, and it is not the depth of snow on your roof on a given day.
Building codes set design loads so that a roof is framed to carry the snow expected at its location, with a margin built in. The figure is expressed as a pressure, in pounds per square foot, and it is set for a jurisdiction rather than for a whole region. Two towns a short drive apart can carry different figures for that reason, even when the weather they share looks the same from the road.
Ground snow load and roof snow load are not the same thing. The ground figure is converted to a roof figure by a procedure in the national standard, and that conversion depends on the roof's shape, its exposure and its slope. A homeowner does not do that calculation. The building official reviews it, and the engineer who stamps a structural change carries it through the design.
How do Moscow and Pullman differ?
Moscow and Pullman are two cities in two states, and each publishes its own design criteria under its own code adoption. The published numbers are not interchangeable and should not be compared as though they were.
Pullman's criteria, effective March 15, 2024, rest on the 2021 IRC with Washington amendments. They set out a ground snow load, a design wind speed, a frost depth and a soil bearing value, and they state that ice barrier underlayment is required. Washington derives its ground snow loads through its own published method, so the figures are produced differently from Idaho's. A reader who sees two different figures for the two cities should first check the year and the code edition each one refers to.
Moscow's criteria come from the city's own handouts and its adopted code amendments, and the University of Idaho publishes ground snow load research for the state. Those sources are not interchangeable with one another or with Pullman's. The Building Safety Division is the right place to confirm which one governs an address today. A reader comparing the two towns should compare the rules that apply, not a pair of numbers pulled from different documents.
What does the building official confirm for my address?
The official for your address confirms the design snow load, the wind and frost criteria, and whether ice barrier underlayment is required. Inside city limits that is the city building department. Outside them, it is the county.
Inside Moscow, the Building Safety Division in Community Development handles plan review, inspections and code administration. Inside Pullman, the protective inspections office handles permits, and its applications can run through the city's online portal. Outside city limits, the county building department covers the address, and it can confirm the design criteria that apply there. The distinction helps when a neighbor or a salesperson quotes a number from memory.
Ask the question in writing if you can. Give the address, the planned covering and the scope of the work, and ask for the design snow load, the ice barrier requirement and any local rule on snow retention or design review. A written answer is what the contractor and the inspector will both work from later, and it prevents a disagreement after the roof is on.
Why do snow drifts and slides matter?
Snow does not sit evenly on a roof. It drifts against walls and collects where roof planes meet, and it slides off steep sections onto whatever lies below.
Moscow's amendment to the building code requires designers to consider snow buildup at valleys, parapets and roof offsets. Drifting and sliding concentrate load in those places, and they are where additions, dormers and lower roofs meet a taller wall. A homeowner who adds a porch, a dormer or a second story should expect the valley and wall details to be checked, not only the main slope. A taller wall beside a lower roof is the classic place to look first when a drift is the concern.
Sliding is the other side of the same problem. Snow that comes off a steep roof can strike people, vehicles and walkways, which is why some owners install snow retention over doors and paths. The structural question and the safety question are separate. A building official answers the first, and a technician can advise on the second when the roof is being looked at. The snow guards service covers the retention side.
How is eave ice different from snow load?
Eave ice is a water problem, and snow load is a weight problem. A roof can pass a structural check and still leak at the eaves, and a roof can carry its load and still need ice protection.
Ice forms at the eave when snow sits on a warm upper roof while the lower edge stays below freezing. Meltwater runs down, refreezes at the edge and backs up under the covering. The University of Minnesota Extension describes that sequence for ice dams, and it treats the problem as heat loss first. Warm air leaking into the attic keeps the upper roof above freezing, which is what feeds the dam.
That is why the fix is insulation, air sealing and ventilation, with the covering as the last layer rather than the first. The shingles can be sound and an ice dam can still form if the attic is leaking heat. Ice barrier underlayment is a backstop at the eave. It does not stop the heat that causes the dam. The ice dams guide goes into the mechanism in more detail.
What should a homeowner check before a reroof?
Check the permit requirement, the design criteria the official confirms, the ice barrier rule and whether snow retention is wanted. Those four items shape the scope before the old roof comes off.
Moscow treats a reroof as its own small permit category, which does not need the site plan a larger building job needs. Pullman lists reroofing as a permit category and requires the general contractor and their state registration number on the application. Neither permit is the same as the other, and neither answer carries across the state line.
If you can, look at the roof structure from the attic side before the scope is set. Ask the technician to check the framing, the sheathing and any earlier repairs to the rafters, and to note anything that changes the load picture. A roof that has been reframed or re-decked may need the building official's review even when the covering is the only thing being replaced. Note any sag, stain or repair you already know about, because the technician will want that history before the scope is set.
When should I call a technician?
Call when you see water stains, ice at the eaves that does not clear, or a roof that looks heavily loaded after a storm. The line is answered around the clock, and a technician visits when one is free.
Snow on the roof is ordinary in a Palouse winter. What matters is what it does: sitting for a long time, thawing and refreezing at the eaves, or drifting hard against a wall. If the attic shows wet insulation or the ceiling shows a new stain, the roof needs a look, and the inside of the house is often where the problem first shows.
Do not climb onto a heavily loaded roof to clear it. Clearing snow is a separate matter from working the roof itself, and it is safest to keep the eaves clear from the ground where you can do so. A technician can advise on removal, guards and the details, and the phone line can take the first description of the problem. (833) 751-2102
Does a new covering change the roof snow capacity?
Usually the covering is a repair question, and the framing is the structural question. A change to the framing, such as a new valley, a dormer or a reframed section, goes to the building official for review.
A reroof replaces the covering over the existing structure. The question for the building official is whether the framing and the sheathing are sound, and whether the change alters the load path anywhere. The covering itself is rarely the piece that decides the structure capacity, though the official makes that call for your address and under the rules that apply there.
Additions are different. A new dormer, a porch roof tied into a main slope or a raised section changes where snow collects and how it drifts. Those are the changes the code asks designers to consider at valleys, parapets and roof offsets, so they need drawings and a review. Keep the drawings with the permit so the next owner can see what was built and under which criteria.
Frequently asked questions
Can you tell me the snow load for my street?
Not from this page. The design snow load depends on the jurisdiction and sometimes on the roof's shape and exposure, so the building official for your address is the one who confirms it. Ask in writing and keep the answer with the permit and the scope of work.
Is Pullman's snow load higher than Moscow's?
This page does not compare them, because the two cities publish their criteria differently and a comparison would not be like for like. Use each city's own published criteria, and ask the building official for your specific address before drawing any conclusion about your roof.
Does a metal roof need a higher snow rating?
The design snow load is a structural question for the building official and the engineer. Whether a metal covering sheds snow is a separate question, and it depends on where the snow would go once it starts to move. The metal roofing service and its guide cover that side of the decision.
What is ice barrier underlayment?
It is a membrane laid beneath the covering along the eaves, and Pullman's published criteria require it. The Idaho side depends on the adopted code for the address, so ask the Moscow Building Safety Division what applies to your roof before you assume either answer.
Should I shovel the roof myself?
Clearing snow from a roof is risky work, and it is best left to someone who is equipped for it. Keep the eaves clear from the ground when that is safe. Call a technician if ice is building up at the edge, the attic is wet or a ceiling shows a stain.
Will a bigger roof need a bigger snow allowance?
The allowance depends on the structure and the criteria for the address, not on the size of the covering. A larger roof does collect more snow in total, which is why the designer and the building official look at the whole structure. Ask the official how the criteria apply to the roof you are planning.
Sources
This guide is general information for Latah County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.