Foundation types

Four families

A slab on grade puts the floor of the house directly on the ground, on a prepared base, with thickened edges or a perimeter footing carrying the walls. There is no space underneath it and no access to anything buried in it.

A crawlspace raises the floor structure on a low perimeter wall, leaving a space beneath that is tall enough to reach services and not tall enough to use. A full basement is the same idea taken down far enough to produce usable rooms. And a pier or pile foundation carries the building on discrete points that reach down to competent soil or rock, spanning between them with beams.

The four are not interchangeable. Each answers a different set of ground conditions, and choosing on preference produces the expensive version of somebody else's correct answer.

Frost is a requirement, not a judgement

Water expands when it freezes, and soil containing water lifts as it freezes. A footing placed above the depth to which the ground freezes will be lifted by that expansion, unevenly, every winter, and the building above it will crack.

Codes address this by requiring footings to bear below the local frost depth, or by permitting specific alternative designs where the assembly is detailed to tolerate frost. The depth is local, it is published by the jurisdiction, and it is not a matter of opinion or of what the neighbours did in 1974.

The practical consequence is that in a cold climate the choice between a crawlspace and a basement is often not a choice at all. If the footing has to be at that depth regardless, the marginal cost of continuing down to a usable basement is the excavation and the taller wall rather than the whole foundation. That arithmetic is why basements are common in cold regions and rare in warm ones.

Water is the failure mode

Almost every foundation problem that is not settlement is water. Water in a basement, water under a slab, water standing in a crawlspace, water finding its way through a wall.

The strategies are unglamorous and they are all about keeping water away rather than resisting it once it arrives: grading the ground so surface water runs away from the building, guttering and discharging roof water far enough out, drainage at footing level with somewhere for the water to go, and damp proofing or waterproofing on the outside face of below-grade walls.

Two things make this worse than it needs to be. A high water table changes the problem from drainage to hydrostatic pressure, which is a design question rather than a detailing one. And a crawlspace that is ventilated in a humid climate can introduce moisture rather than remove it, which is why crawlspace strategy is now argued about in envelope terms rather than structural ones.

Soil, slope and the cases that force a decision

Expansive clay shrinks and swells with moisture, which moves foundations bearing on it. The usual answers are a stiffened slab designed for that movement, a deeper foundation reaching past the active zone, or a pier system, and all three are engineered rather than selected from a table.

Made ground, meaning fill placed by somebody rather than deposited by nature, has to be treated as suspect until it is investigated. A neighbouring house standing happily is not evidence about your parcel, because fill is rarely uniform.

A slope pushes hard toward a stepped foundation, a daylight basement or piers, and the choice interacts with drainage: on a slope, water arrives at the uphill wall. Rock at shallow depth cuts the other way, making excavation expensive and often making a slab or a pier system the sensible answer.

Where any of those conditions exists, the sequence set out on The Build Sheet under land and site assessment matters more than the choice itself. The report comes before the decision.

The one chosen for the wrong reason

Basements are frequently specified because the buyer wants one, in markets where the frost arithmetic does not support them and the water table argues against them. That is a legitimate thing to want and an expensive thing to buy.

The honest framing is that a basement in a warm, wet region is additional floor area with a difficult moisture problem attached, purchased at a premium over a slab. If that is worth it to you, it is worth it. What it is not is the default, and treating it as one is how a foundation budget doubles before the house has a shape.

The mirror image happens too. A slab is specified on a sloping site because it is the cheap option on flat ground, and the cut and fill needed to produce flat ground costs more than the foundation it was chosen to avoid. Both mistakes come from picking the type first and looking at the parcel second, which is the wrong way round in every case.