GR-003The ground
Snow Load and Snow Storage on a Rural Lot
A roof is built to a ground snow load set by the adopted code. A driveway needs its own plan for where the plowed snow actually goes. Both decide what a lot can carry.
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A house in the Inland Northwest carries two separate snow problems: the weight sitting on the roof, which the building code answers with a design number, and the pile sitting on the ground, which the lot answers with space and a drainage path. A household moving onto a new lot usually asks about the first and forgets the second, until the first thaw sends the second one toward the foundation.
What is a ground snow load, and who sets the number?
A ground snow load is the design figure a jurisdiction adopts for how much snow weight a roof in that area has to be built to carry, measured in pounds per square foot. It is not a single national number. The International Residential Code, which Washington and Idaho adopt by reference at the state level with their own amendments, sets the baseline structural provisions a building department applies, and the county's own adopted snow load map or table is what a plan reviewer actually checks against a roof design. Elevation changes the figure sharply over a short distance in this region, so a lot a few hundred feet higher than its neighbor can carry a meaningfully higher design load.
A buyer or a builder does not need to calculate this number alone. The local building or planning department publishes the adopted ground snow load for a given address as part of the permit process, and a licensed designer or engineer applies it to the specific roof geometry, pitch, and span. What a household can usefully check before buying is simpler: whether the existing roof was built under a permit at all, since an unpermitted addition or carport is the structure most likely to have skipped this calculation entirely.
Why does roof pitch change how a house handles snow?
A steep pitch sheds snow faster than a shallow one, which sounds like an advantage until the snow sheds onto a walkway, a deck, or a parked car below. A roof designed with the structural load in mind but without a plan for where the shed snow lands creates a second, separate hazard: a sliding mass of wet snow releasing without warning over an entry or a path people actually use. Snow guards, a wider roof overhang set away from walkways, or simply routing foot traffic away from the eave line are the common fixes, and they are a design decision separate from the structural load calculation.
A shallow or low slope roof keeps more snow in place longer, which raises the sustained load the structure carries through a heavy winter but removes the sliding hazard underneath it. Neither pitch is automatically safer. Each trades one risk for the other, and a household inspecting a house before an offer can at least ask which trade the existing roof made.
Where does plowed and shoveled snow actually go?
This is the question a building code does not answer and a site plan should. A rural lot in the Inland Northwest routinely carries a winter's worth of plowed driveway and shoveled walkway snow somewhere on the property, and where that pile sits determines what happens in March.
A pile pushed against the foundation melts directly into the same soil the footing drainage is supposed to keep dry, and a slow seep along a foundation wall through an entire thaw season does more damage over years than a single hard freeze. A pile pushed over or near a septic drainfield packs down the snow cover that insulates the field and can affect how the field freezes and drains. A pile pushed downhill from a well head risks sending contaminated meltwater toward the wellhead seal rather than away from it.
The fix is a storage area chosen before the first storm, not improvised at the first plow pass: a spot with a clear downhill grade away from the foundation, the drainfield, and the well, with enough square footage that a normal winter's accumulated pile does not eventually surround one of those three. On a small or oddly shaped lot, that storage area is worth marking on a site sketch the same way a drainfield reserve area is marked, because both are easy to forget until the day there is nowhere left to push the plow.
What should a household check before the first storm?
Three things. First, confirm the roof over any addition, carport, or outbuilding was actually permitted, since that is the structure most likely to be under-built for the local ground snow load. Second, walk the eave lines and note where a sliding roof load would land, and keep that path clear or add a deflector before the snow does the deciding instead. Third, pick the snow storage spot for the winter, away from the foundation, the drainfield, and the wellhead, and tell everyone who plows or shovels where it is before the first storm, not during it.
A roof that is built to the right number, a slide path that lands somewhere safe, and a snow pile that drains away from the house are three different problems with three different fixes. Treating them as one problem, solved once the permit is approved, is how a lot that passed inspection in July still finds water at the foundation in April.
