The vocabulary on a foundation quote

Underpinning

The general name for extending a foundation downward or sideways so it bears on stronger material than it currently does. Piering is the most common residential form of it. The word describes the goal rather than a particular product, which is why two quotes can both say underpinning and mean quite different systems.

Helical pier

Steel shafting with welded screw plates, wound down through the soil by a torque head until it meets competent bearing. Torque during installation correlates with capacity, so each pier reports what it can carry as it goes in. Suits lighter structures and anywhere a pier has to be set without leaning on the weight of the building.

Push pier

Also called a resistance pier. Sections of steel pipe are driven into the ground hydraulically, using the weight of the structure itself as the reaction force. Suited to heavier buildings, since a light structure cannot supply enough resistance to drive the pier to depth.

Bearing layer

The stratum a foundation or a pier ultimately transfers load onto. Across the Central Basin that is frequently limestone at modest depth, which is why piers here are often shorter than national cost figures assume. The critical point is that a pier is only as good as what it lands on.

Differential settlement

Settlement that varies across a building rather than affecting all of it equally. Uniform settlement causes very little damage; differential settlement causes almost all of it, because the structure has to bend to accommodate one part moving further than another. This is the mechanism behind most stepped cracking.

Shrink-swell soil

Soil, usually with a high content of expansive clay minerals, that increases in volume as it takes on water and contracts as it dries. The seasonal cycle of a wet Tennessee winter followed by a dry late summer means such soils exert a rising and falling pressure on foundations for the life of a house.

Plasticity index

A soils laboratory number describing the moisture range over which a soil behaves plastically. Higher values indicate more clay activity and therefore more shrink-swell potential. It appears in geotechnical reports and is the technical basis for statements about how expansive a particular soil is.

Karst

Terrain formed where soluble rock, typically limestone, has been dissolved by groundwater over geological time, producing sinkholes, solution cavities, and irregular rock surfaces. Its practical effect on foundations is unpredictability: depth to competent rock can change substantially across a single lot.

Depth to bedrock

How far below the surface competent rock begins. It governs pier length, and therefore pier cost, more than any other single factor. Where it is shallow and consistent, underpinning is comparatively cheap; where it is shallow but irregular, one side of a house can behave completely differently from the other.

Footing

The widened base of a foundation wall or column that spreads structural load over a larger area of soil. Almost all settlement problems are footing problems: either the soil under it moved, or the footing was never wide enough or deep enough for what it was asked to carry.

Frost depth

The depth to which soil can be expected to freeze in a given locality. Footings are placed below it so that freezing water in the soil cannot lift the foundation. Tennessee frost depths are shallow compared with northern states, which is one reason footings here are not especially deep.

Slab-on-grade

A foundation where the floor is a concrete slab poured directly on prepared ground, with no crawl space or basement beneath. Common in newer subdivision construction. Its defining characteristic for repair purposes is that there is no access underneath, so diagnosis relies on elevation measurement and leak testing.

Crawl space

The accessible void between the ground and the floor structure of a house raised on a perimeter foundation wall. The dominant residential foundation type across Middle Tennessee, and the place where a great deal of this region’s foundation trouble begins.

Encapsulation

Sealing a crawl space with a heavy reinforced vapour barrier across the floor and up the walls, closing the vents, and controlling humidity mechanically. Distinct from simply laying plastic on the ground, which is a partial measure that leaves the wall area and the vents doing what they were doing before.

Vapour barrier

The sheet material laid over crawl space soil and up the walls to stop ground moisture evaporating into the space. Thickness and how carefully the seams and pier penetrations are sealed matter more to the outcome than the brand printed on the roll.

Girder

The main horizontal beam under a floor, carrying joists and transferring their load to posts and to the foundation. A sagging girder is the usual cause of a floor that dips toward the middle of a crawl-space house, and it is a framing repair rather than a foundation repair.

Sistering

Fastening a new structural member alongside a damaged one so the pair carries the load together. Standard repair for a joist weakened by rot or long-term deflection, and considerably cheaper than removing and replacing the original member.

Sill plate

The horizontal timber bearing directly on top of a masonry foundation wall, to which the floor framing is fastened. Because it is the lowest wood in the building and sits on masonry that can wick moisture, it is where rot in a damp crawl space usually begins.

Lateral pressure

Horizontal force exerted on a foundation wall by the soil against it. It rises sharply when the soil is saturated, because water adds weight, and rises further where clay swells. It is the cause of bowing walls and horizontal cracks, and it has nothing to do with settlement.

Bowing wall

A foundation wall deflecting inward under lateral pressure. The classic presentation is a horizontal crack near mid-height with the wall leaning inward above it. Repair means both restraining the wall and reducing the pressure, since reinforcement alone leaves the load unchanged.

Carbon fibre strap

A high-tensile fabric strap bonded to the inside face of a foundation wall to resist further inward bending. Very strong in tension, which is what a bowing wall lacks. It prevents further movement rather than reversing what has already happened.

Wall anchor

A system tying a foundation wall to a plate buried in stable soil away from the house, tensioned through a rod. Unlike bonded reinforcement, it can sometimes be tightened progressively over months to recover some of the wall’s original position.

Stepped crack

Cracking that follows the mortar joints of a block or brick wall in a staircase pattern, usually rising diagonally. Because it traces the weakest path through a jointed assembly, it is the signature of differential movement in masonry rather than of anything wrong with the blocks themselves.

Epoxy injection

Filling a crack in poured concrete with a structural epoxy under pressure, restoring continuity across the crack. Appropriate for stable, essentially dry cracks. It creates a rigid repair, so it is the wrong choice where slight ongoing movement is expected.

Polyurethane injection

Filling a crack with an expanding polyurethane resin. It tolerates damp and slight movement and is chosen when the priority is stopping water rather than restoring structural continuity. Where water is the problem, it usually outperforms epoxy.

Polyurethane slab lifting

Raising a settled concrete slab by injecting expanding structural foam through small ports drilled in it. Much lighter than the cement slurry used in traditional mudjacking, so it does not add load to soil that has already settled, and it cures within minutes.

Void

An empty space beneath a slab where soil has washed out or consolidated away from the concrete. A slab spanning a void has lost its support and will eventually crack under load even if it has not yet dropped. Filling voids is often done before any lifting is attempted.

Footing drain

A perforated pipe laid in washed stone beside the footing, wrapped or covered with filter fabric, that collects groundwater and carries it to daylight or a sump. Also called a French drain or perimeter drain. It intercepts water before it can press against the wall.

Positive drainage

Ground that falls away from a building so surface water runs off rather than pooling against it. The most commonly recommended and most commonly ignored piece of foundation maintenance, largely because it looks like landscaping rather than structural work.

Floor-level survey

A measurement of relative floor elevation taken across a whole floor plate with a laser or water level, producing a contour of the floor. It converts an argument about whether a house has moved into a set of numbers, and repeating it later shows whether movement is ongoing.

Tell-tale

A simple monitoring device fixed across a crack, or a scribed reference mark, used to record whether the crack is widening. Cheap, low-tech, and frequently the most useful thing that can be done first, because it answers the question that decides everything else: is this still moving.

Surcharge

Additional load applied to soil behind a retaining wall or beside a foundation from something above it, such as a driveway, a parked vehicle, or another structure. It raises the pressure the wall has to resist and is a common reason a wall that stood for years suddenly begins to lean after a change nearby.

Terms that come up on a quote you have already received are worth asking about directly. The main guide puts most of these in context, and the FAQ answers the questions that follow them.