Published
Where the 170 PSF figure comes from — and what to do when a supplier quotes you less
Two documents, one number
The before-you-build page quotes the county ordinance that sets the ground snow load here at 170 PSF, and the Island Park area page covers what that load does to a roof, a load path and a set of footings. This page answers the question that comes after both: where does the number come from, and how do you know it is right?
It matters because 170 is a large figure, larger than most suppliers deal with routinely, and an owner who is quoted something smaller has no obvious way to tell whether they are looking at an error, a different quantity, or a better price.
The reassuring part is that the figure is not a single county official's judgement. It appears in two separately published places.
| Source | What it is | Island Park |
|---|---|---|
| Fremont County ordinance | Adopted county law — the figure your permit is assessed against | 170 PSF ground snow load for the Island Park District |
| University of Idaho, "Ground Snow Loads for Idaho — 2015 Edition" | Civil engineering research, derived from National Weather Service records | 170 psf at the Island Park station, elevation 6,294 ft |
The University's study was compiled at its Department of Civil Engineering and is published openly by the University of Idaho Library. It works from recorded snow depths at weather stations, converted to loads, rather than from anybody's ordinance.
One honest caveat, because this page is about checking numbers rather than reassuring people. The county's figure was adopted before the 2015 edition was published, and earlier editions of the same research existed. So the correct claim is that two separately published sources state the same figure — not that two wholly independent efforts happened to converge. That is still a great deal more than most jurisdictions can show for their snow load, and it is checkable in an afternoon.
The study is emphatic about its own standing, and the point is worth reading in the authors' words:
We urge our readers to use the information with care. The uncertainties associated with snow should suggest that the scatter in the data could be large in spite of our many efforts. … Finally, we remind the user that the design snow loads are the ultimate responsibility of the person in charge of the project. This document is not legally binding. The user is urged to verify ground snow load values with the local authority having jurisdiction.
— Al Hatailah, Godfrey, Nielsen and Sack, "Ground Snow Loads for Idaho — 2015 Edition", University of Idaho Department of Civil Engineering, November 2015 (lib.uidaho.edu, retrieved 2026-08-17)
So the ordinance is the authority and the study is corroboration. That is the right way round, and it is also the answer to anyone who produces a research figure to argue against a permit condition.
The number is a floor, not a target
Ground snow load is an input, not the answer. What a structure is designed to carry is derived from it, and several things can legitimately move the design figure in either direction:
- Roof snow load is calculated from ground snow load, not copied from it. Exposure, thermal condition and roof shape all enter the calculation.
- Drifting and sliding add local concentrations. A valley, a dormer, a lower roof beneath a higher one, or a wall that catches drift can carry far more than the flat-roof figure suggests.
- Unbalanced loading is its own case. Snow rarely leaves symmetrically, and the half-loaded condition governs some framing.
- Elevation varies across the district. The ordinance sets one figure for the district; the study reports one station. Your lot is neither of those points.
This is why the useful question to a supplier is never "what snow load did you use?" on its own. It is which figure, for which condition, derived how.
When a quote arrives with a different number
This happens often enough to be worth a checklist, and there are only a few explanations. Most of them are innocent, and one is not.
| What you see | Likely explanation | What to ask |
|---|---|---|
| A figure well below 170 | A roof snow load, correctly derived from the 170 ground load | "Is that ground or roof snow load, and what ground figure is it derived from?" |
| A figure near but not equal to 170 | A site-specific or engineered value | "Who derived it, and is it on a stamped drawing?" |
| A round number like 100 or 120 | A default from elsewhere in Idaho, or a standard product | "Was this designed for this district, or is it a catalogue item?" |
| No figure named at all | Nobody has been asked yet | Ask before comparing the price to anything |
The one that matters is the third row. A truss package designed for a much lower load is not a cheaper version of the right product — it is a different product, and the difference is not visible in a quotation. Two bids that name different snow loads are not comparable at any price.
How to check the figure yourself
- Get the ordinance figure for the district your lot sits in, from the county rather than from a builder. The districts are county-defined and the boundary is not obvious from the road.
- Ask every supplier to state the ground snow load in writing, on the quote, before you compare prices.
- Ask separately for the roof snow load and how it was derived, so a smaller number can be recognised as correct arithmetic rather than a cut corner.
- Ask who is stamping it. At loads like this the engineered drawings are the document that matters, and the county has its own thresholds for when a stamp is required.
- Keep the numbers with the file. The snow load a structure was designed for is the first thing a future buyer's inspector, insurer or engineer will want, and it is very hard to reconstruct later.
Re-verify before relying on any figure here. The county ordinance figure and the University of Idaho study were both read on 2026-08-17; the study is published at lib.uidaho.edu and the ordinance through the county's document centre. Ordinances are amended, research is re-issued, and a snow load is not something to take from a web page — including this one — when a stamped drawing is what your build actually needs.
Common questions
What is the ground snow load in Island Park?
Fremont County's ordinance sets 170 PSF for the Island Park District. The University of Idaho's Ground Snow Loads for Idaho, 2015 Edition, lists 170 psf for the Island Park station at an elevation of 6,294 feet.
Do those two sources independently confirm each other?
Not quite, and it is worth being precise. The county adopted its figure before the 2015 edition was published, and earlier editions of the same research existed. What is checkable is that two separately published sources state the same number.
Why is my truss quote showing a much smaller number?
Most often because it is a roof snow load rather than a ground snow load. Roof load is calculated from ground load and is usually lower. Ask which quantity is being quoted and what ground figure it was derived from.
Is a lower number always wrong?
No. A correctly derived roof snow load, or an engineered site-specific value on a stamped drawing, can legitimately be lower. What should worry you is a round default figure from elsewhere, or a catalogue product that was never designed for this district.
Can I compare two bids that name different snow loads?
No. They are not the same product. Get every supplier to state the ground snow load in writing on the quote before you look at the prices at all.
Does the district figure apply exactly to my lot?
The ordinance sets one figure for the whole district and the study reports one weather station. Your lot is neither of those points, which is part of why engineered drawings rather than a published figure are what a build is actually based on.
What makes a roof carry more than the published load?
Drifting and sliding create local concentrations — valleys, dormers, a lower roof under a higher one, or a wall that catches drift. Unbalanced loading is also its own design case, because snow rarely leaves a roof symmetrically.
Is the University of Idaho study legally binding?
No, and it says so itself. The study tells readers it is not legally binding and urges them to verify ground snow load values with the local authority having jurisdiction. The county ordinance is the authority; the study is corroboration.
Where can I read the study?
It is published openly by the University of Idaho Library as Ground Snow Loads for Idaho, 2015 Edition, compiled at the Department of Civil Engineering and dated November 2015. This page was written from the text retrieved on 2026-08-17.
Who should I ask about which district my lot is in?
The county, rather than a builder or a neighbour. The snow-load districts are county-defined, and the boundary between them is not something you can read off the road.