Foundations and Enclosure for a Small Frame

Frost

Wisconsin’s ground freezes deep in winter, and wet soil heaves as it freezes, lifting anything resting on it unevenly. Foundations deal with frost in one of two ways:

• Go below frost depth. Footings for heated buildings are placed below the local frost line set by code, commonly around 4 feet in much of Wisconsin.
• Float. For small, unheated outbuildings, a foundation designed to move with the ground (a gravel pad with a timber sill, or skids) can rise and fall as a unit without harm, since the light frame doesn’t mind.

Check local requirements; accessory buildings are often treated differently from houses.

Foundation options for small frames

• Concrete piers (poured in tubes) below frost depth under each post or under the sill at intervals. Economical and solid; the sill spans between piers.
• A gravel pad with a timber sill or skids. Common for sheds: compacted gravel for drainage, with a pressure-treated or decay-resistant sill on it. Floats with frost.
• Stone piers or dry-laid stone. Traditional for barns and still beautiful under small frames; the Log and Nordic Building course covers dry-laid foundations.
• A slab-on-grade, insulated at the edges (a frost-protected shallow foundation), for heated buildings.
• A full foundation wall or basement for houses.

Sills

The sill ties the posts together at the bottom and spreads load to the foundation. Keep it dry:

• Raise it well above the ground, with the ground sloping away and gutters or a generous roof overhang to keep splash off.
• Use a decay-resistant species (white oak, tamarack, cedar) or a treated timber, and a moisture break (such as a membrane or a metal flashing) between concrete and wood.
• Anchor it to the foundation against uplift and sliding.

Most decay in old timber frames starts at sills and post feet that stayed wet.

Enclosing the frame

A timber frame is a skeleton; walls and roof wrap around it. For outbuildings:

• Board siding nailed to girts or nailers: vertical board-and-batten or horizontal clapboard. On a frame without many girts, horizontal nailers are added between posts.
• Roof boards over the rafters, then underlayment and roofing (metal, shingles, or wooden shingles).
• Finishes: a penetrating oil, pine tar mixture, Falu red, or linseed oil paint; rough-sawn boards hold penetrating finishes longest ¹. Leave the interior frame unfinished or oiled.

For heated buildings and saunas, the frame is usually enclosed by structural insulated panels (SIPs) or a stick-framed wall built outside the timber frame, keeping the timbers inside the insulated envelope where they stay warm, dry, and visible ².

Moisture

Whatever you build, don’t trap water:

• In a heated building, control air leakage and vapor so warm, moist interior air can’t reach cold sheathing and condense.
• In an unheated outbuilding, ventilate: open eaves and a ridge vent let damp air out.
• Keep wood off the ground and out of splash, and let it dry when it gets wet.

What to take from this

In Wisconsin, foundations either go below frost depth (about 4 feet in much of the state) or float with the ground under light, unheated buildings. Small frames sit well on piers, gravel pads with sills, or stone. Keep sills high, dry, decay-resistant, and anchored. Enclose outbuildings with boards and roofing finished with penetrating coatings, put heated buildings’ insulation outside the frame, and never trap moisture against the wood.