Foundation types

What it does

Choose how a building transfers load into the ground.

Compare shallow and deep approaches against the illustrated soil, frost and groundwater conditions.

When it fits

Learning the drivers behind a foundation choice.

The trade-offs

A real selection also requires site investigation, settlement assessment and project design.

Context. An educational model. Geometry, material assumptions and the reference building are described in the illustrated study.

A foundation has one job: to hand the weight of the house to ground that can carry it, without moving. The ground decides how. Where firm soil lies near the surface and nothing in it moves, a wide concrete foot is enough. Where the ground freezes, it has to be got under or protected. Where water rises, it has to be kept out or stayed above. Where the top of the ground is too soft to bear anything, the load has to be carried down past it.

Shallow and deep

Foundations come in two families. Shallow foundations bear on soil just below the surface: strip footings under walls, pads under piers, the thickened edge of a slab. They work by spreading the load over enough area that the pressure falls to what the soil can carry, which is why a footing is wider than its wall. Deep foundations carry the load down through weak ground to a firm layer, by bearing on it at the tip, by friction along their length, or both: piles, drilled piers, caissons. A house almost always gets a shallow foundation; it goes deep only when the ground near the surface gives it no choice.

Every shallow foundation has the same few rules. It bears on undisturbed soil or compacted fill, never on topsoil. Its bottom is at least 12 in (305) below grade, and below the frost line unless it is protected from frost. Wood stays 8 in (205) above the soil. Water is led away from it. Within those rules, the rest is a choice.

What decides it

Frost depth: Silty and clayey soils draw water up to the freezing front, where it grows into lenses of ice that lift whatever is above. Footings go below the deepest frost, from nothing on the Gulf coast to 42 in (1065) on this lot and 5 ft (1525) and more in the far north. The deeper the frost, the less extra it costs to keep digging to a basement.
Groundwater: Below the water table, water presses with its depth: 62.4 lb/ft² for every foot. A basement floor 5 ft under a spring water table feels over 300 psf of uplift, while its 4 in slab weighs 50. High water favours foundations that stay at the surface or stand above it.
The soil: How much it bears (2,000 psf for this lot’s stiff clay, more for sand and gravel, nothing reliable for peat or old fill), and how it moves: clay that swells and shrinks with the seasons, soft ground that keeps compressing for years.
Slope: A slab wants level ground, so a sloping lot is cut and filled; a basement on a slope can open out as a walkout; piers simply stand taller on the low side. See Cut and fill.
The space below: A basement is a storey; a crawl space is a place for pipes and ducts; a slab has nothing under it, so its services are cast in or run overhead.
Flood and radon: In a mapped flood zone the floor must stand above the flood, which rules out basements and favours piers and piles. Radon, a soil gas, gathers wherever a floor sits on the ground; a slab or liner is sealed and vented.
Cost: Roughly: a slab least, then a frost-protected slab, crawl space or piers, then piles, and a basement most, though a basement’s cost buys floor area the others do not.

Six ways to stand a wall

Full basement: A 20 × 10 in (510 × 255) footing 8 ft 2 in (2490) down, keyed to an 8 in (205) wall that stands 8 in above grade. Outside, a coat of dampproofing against soil moisture and a perforated drain in stone that leads water away before it can build up against the wall. Inside, a 4 in slab on a vapour retarder, foam and stone, held off the wall by a foam strip, with a radon pipe from the stone. The wall is also a retaining wall: it is braced at the bottom by the slab and at the top by the floor, which is why it is backfilled last.
Crawl space: The same footing and wall, cut short: the footing just below frost, the wall holding the floor frame 1½ to 2 ft (455 to 610) over the ground. Sealed, it has foam on its walls and a heavy liner over the ground taped up them, and is kept dry like the rest of the house. Vented, it has openings in the wall and insulation in the floor, and it is outdoors.
Slab on grade: One pour: a 4 in (100) floor and, around its edge, a thickened beam 12 in (305) wide at its foot that carries the walls. Under it, stone, foam and a vapour retarder; down its face, foam capped by a flashing that the wall’s sheathing laps over. Steel bars run in the edge, welded wire in the slab. The wall’s first plate is a treated sill bolted straight to the concrete.
Frost-protected shallow: A slab whose edge goes only 16 in (405) down even where frost reaches 42 in, because it is wrapped in foam: down its face and, where winters are cold, in a wing 2 to 3 ft (610 to 915) out from it at its foot.
Piers and beam: Square concrete piers 6 ft (1830) apart on pads below frost, steel caps bolting a three-ply beam to their tops, and the floor framed on the beam with its underside open, insulated and boarded.
Piles and grade beam: Steel shafts with helical plates screwed 12½ ft (3810) down through 9 ft of soft fill into sand and gravel, a concrete grade beam cast over their brackets, and a void form under the beam so the ground can move without touching it.

Which works where

The figure’s own verdicts, for each foundation against this lot and against one change at a time. “With measures” means it can be made to work at a cost: waterproofing and a sump, a stiffened slab, deeper piers. “More than needed” means it works, but the ground does not call for it.

	This lot	No frost	60 in frost	High water	Expansive clay	Soft fill
Full basement	Works	Works	Works	Poor	With measures	Poor
Crawl space	Works	Works	Works	With measures	With measures	Poor
Slab on grade	Poor	Works	Poor	Poor	Poor	Poor
Frost-protected slab	Works	More than needed	Works	Works	With measures	Poor
Piers and beam	With measures	Works	With measures	With measures	With measures	With measures
Piles and grade beam	More than needed	More than needed	More than needed	More than needed	Works	Works

Beating frost without digging

Frost heave needs three things at once: soil fine enough to draw water up, water to draw, and freezing. Clean sand and gravel drain too freely to heave, which is why a slab on gravel in the north rarely moves; codes still ask for the frost depth, because the soil under a house is seldom that clean all the way down. A plain slab edge, which can only reasonably go 18 in (455) deep, sitting in clay where frost reaches 42 in is the classic failure: lenses grow under it, lift it in winter and drop it unevenly in the thaw.

The frost-protected foundation takes away the freezing instead of the soil. A heated house loses heat into the ground under it; foam down the edge and out in a wing at its foot keeps that heat near the footing, and keeps the cold above the wing out. The 32 °F line, deep far from the house, bends up and passes over the wing, so the ground under a 16 in (405) footing stays unfrozen. The figure draws that bent line. It works only under heated space, and the foam must stay intact: an unheated garage beside it is insulated under its whole floor. Long used in Norway and Sweden, it is in the US code under ASCE 32.

When the ground must be passed

Peat, organic silt and uncontrolled fill have no safe bearing value: under a footing they keep compressing for years, and unevenly. If they are shallow they are dug out and replaced with compacted fill. If they run deep, the house is carried through them. Helical piles suit houses: a machine screws each steel shaft in like a wood screw, and the torque it takes is a running test of the soil, since the capacity of a 2⅞ in (73) shaft is about nine times the installing torque in ft-lb. Each pile here carries a 6 ft share of the wall, about 9,000 lb (40 kN), on two helices bearing in the sand and gravel. A grade beam spans between them, and because it bears on nothing, it sits on a void form the ground can heave or settle into.

Expansive clay asks for the same move for a different reason: its top 8 to 10 ft swell and shrink with the seasons. Piers or piles anchored below that active zone, with beams held off the ground, ride it out; a slab on it has to be stiffened or post-tensioned to bridge the movement.

The choices

What does the house stand on?

Decided by frost depth · groundwater · what the soil bears and how it moves · slope · use of the space below · flood and radon · cost

Full basementThis house
A storey of concrete walls below grade on footings 8 ft down, with a slab floor inside.
Choose it when frost is deep, so footings go 3 to 5 ft down anyway and the rest of the dig buys a storey; the water table stays well below the floor all year; you want room for storage, the furnace and water heater, or a future finished room; the lot slopes enough for a walkout on its low side.
The catch the most excavation, concrete and time of any; needs dampproofing, a footing drain and often a sump; a wet site floods it, and a high water table can float the slab; radon gathers in it unless the slab is sealed and vented.
Digging, walling and draining it No. 11 →
Crawl space
Short foundation walls on footings below frost, holding the floor frame 1½ to 2 ft over the ground.
Choose it when frost is moderate and a basement's space isn't worth its cost; pipes and ducts should stay reachable under the floor; the ground is uneven or gently sloping; a slab would sit on wet or termite-prone ground.
The catch a cramped, damp space unless sealed and conditioned; vented ones let humid air in to condense in summer and freeze pipes in winter; still needs footings below frost and a drain where the floor is below grade.
A crawl-space footing, sized No. 12 →
Slab on grade
A concrete floor poured on stone at the ground, thickened at its edge to carry the walls.
Choose it when frost is shallow, a foot or less; the site is flat or graded level; the water table is high: nothing is dug below the surface; cost and speed matter most: one form, one pour.
The catch in deep frost its edge heaves unless it gets a frost wall or insulation; pipes cast under it are hard to reach or change; on expansive clay it needs a post-tensioned or ribbed slab; no space below for ducts and services.
Frost-protected shallow
A slab with a 16 in edge whose face, and in colder places a wing beside it, is wrapped in rigid foam, so the house's heat keeps the ground under it from freezing.
Choose it when frost is deep but a basement isn't wanted; the building is heated all winter; excavation is expensive: ledge, a high water table, a tight site.
The catch only works under heated space; an unheated garage needs a different design; the foam must be protected from damage, sun and burrowing insects; less familiar to some builders and inspectors.
Piers and beam
Concrete piers on pads below frost, with beams across them and the floor framed above open ground.
Choose it when the site slopes or the ground must not be disturbed; a flood zone requires the floor to stand above the flood; the house is small, light or in a mild climate; piers can be drilled past expansive clay to firm ground.
The catch the floor is outdoors underneath: insulate it and keep pipes from freezing; less stiff against wind and earthquake: it needs bracing or anchored piers; an open underside invites animals unless skirted.
Piles and grade beam
Steel helical, driven or drilled piles carried down through weak ground to a firm layer, joined at the top by a concrete grade beam.
Choose it when the top of the ground is soft fill, peat or loose sand; expansive clay is deep and active; a steep slope or tight access keeps machines out; loads must reach a firm layer far below.
The catch two to three times the cost of a footing where firm ground is near the surface; needs a geotechnical report and an engineer's design; specialist rigs and crews.

Here The lot's frost line is 42 in down, so the footings had to go 3½ ft deep anyway; going on to 8 ft buys an 864 sq ft storey. The water table, 14 ft down, stays below the slab, and the stiff clay bears 2,000 psf under a 20 in footing.

If a crawl space is the answer, there is a second question. Older codes vented it to the outside; in most climates that brings in summer humidity that condenses on the cool floor, so the current preference is to seal it and treat it as part of the house.

Should a crawl space breathe outside air, or be sealed into the house?

Decided by climate and summer humidity · groundwater · pipes and ducts below the floor · radon · cost of conditioning

Vented
Openings in the walls let outside air through; the floor above is insulated and the ground covered with a thin liner.
Choose it when the climate is dry; no pipes or ducts run below the floor; local code still asks for it and no one will maintain a sealed space.
The catch humid summer air condenses on the cool floor framing and grows mould; pipes and ducts sit outdoors in winter; floor insulation sags and falls out over time.
Sealed and conditioned
No vents: foam on the walls, a heavy liner sealed to them over the ground, and a little conditioned air or a dehumidifier.
Choose it when summers are humid or the site is wet; ducts, pipes or the furnace live under the floor; radon has to be kept out.
The catch costs more to build, and the liner must stay intact; foam needs a fire covering or an ignition barrier; it becomes part of the heated house: it must be kept dry and conditioned.