Climate Zones, Frost Depth, and Design Loads

How loads travel

Every load on a house has to get to the ground. The roof’s snow pushes on rafters, rafters on walls, walls on floors and girders, those on the foundation, the foundation on the footing, and the footing on the soil, whose bearing capacity is measured in pounds per square foot ¹. Wisconsin’s code says the same thing as a rule: every dwelling needs a complete load path from where each load starts to the foundation ². A beam with no post under its end, or a post with no footing under it, breaks the path.

Frost

Water in the soil expands when it freezes, and freezing soil under a footing lifts it: frost heave. Doors stick, drywall cracks, stoops tilt away from the house. So ²:

• Footings and foundations, including those for landings and stoops, go below the frost penetration level or at least 48 inches below adjacent grade, whichever is deeper.
• Footings may never be placed on frozen material. Pouring in winter means protecting the excavation from freezing.

Exceptions ²:

• Frost-protected shallow foundations, designed to ASCE 32, use insulation around and under the footing to keep the ground below it from freezing, so the footing can be shallower.
• The part of a foundation directly under a window areaway.
• Footings on clean bedrock less than 48 inches down, with the soil cleaned off, clay removed from crevices, and water kept from collecting.

The 48-inch minimum applies statewide; where frost goes deeper than that, the deeper line governs.

Snow and wind

Roofs must carry the snow load from the UDC’s zone map, applied straight down on the roof’s horizontal projection ²:

A 30 by 40 foot house in Zone 1 must be built to carry 1,200 × 40 = 48,000 pounds of snow on its roof.

Wind: walls and roofs must resist a horizontal and uplift pressure of 20 psf over the surface, or the loads from ASCE 7 ². That’s what hurricane ties, anchor bolts, and wall bracing are for.

Floors

Floors are designed for live loads: people, furniture, and stored things ²:

Plus the dead load, the actual weight of the materials. The joist and rafter span tables in SPS 325 Appendix A are built from these numbers, and can be used instead of calculating each member ². Lumber that isn’t grade-stamped must be designed as No. 3 grade for studs, joists, and rafters ².

Energy zones

The energy code has its own map. Zone 2 is 15 northern counties: Ashland, Bayfield, Burnett, Douglas, Florence, Forest, Iron, Langlade, Lincoln, Oneida, Price, Sawyer, Taylor, Vilas, and Washburn. Zone 1 is everything else ³. (The snow map’s numbering runs the other way, so a Vilas County house is snow Zone 1 and energy Zone 2.)

The prescriptive insulation table ³:

R-38 satisfies the ceiling where its full uncompressed depth runs over the wall plates at the eaves, and a cathedral ceiling too shallow for R-49 may use R-30 over up to 500 square feet ³. Instead of the table, a builder can show the whole house’s heat loss (its total UA) is no more than the table’s, usually with REScheck software ³. Gas furnaces and boilers must be at least 90% AFUE unless a tighter envelope table is met ³.

What to take from this

Footings, stoops included, go below frost or 48 inches, whichever is deeper, and never on frozen ground. Roofs carry 40 psf of snow in the northern zone and 30 in the south; floors 40 psf, garages 50; walls and roofs 20 psf of wind. In the energy code, 15 northern counties are Zone 2; both zones need R-49 ceilings and U-0.35 windows, with R-20 (or 13+5) walls in Zone 1 and R-21 in Zone 2. Every load needs a continuous path down to the soil.