A footing is the lowest part of a foundation. It spreads the weight of columns, walls, and the superstructure onto the soil so the ground never carries more pressure than it can safely resist. Every load path in a building ends at a footing, and the footing type shapes settlement behavior, cracking risk, and the usable life of the structure. Three checks govern every footing design: bearing pressure below the allowable soil capacity, settlement within acceptable limits, and resistance to shear and bending. Footings divide into shallow and deep families. Shallow footings sit close to the ground surface and spread load horizontally, while deep foundations carry load down through weak surface soils to stronger material. The main shallow options are isolated footings, continuous strips, combined footings, strap footings, and raft slabs, with piles as the standard deep alternative. This article compares each type, the site conditions that suit it, and the checks that settle the choice. For edge columns and off-center loads, balanced and cantilever footings provide a way to control overturning and keep soil pressure uniform without moving the column.
Isolated Footings: The Standard Spread Footing
An isolated footing supports a single column and is the most widely built concrete footing in residential and light commercial construction. The base widens under the column to distribute the concentrated load over a larger soil area. One pad serves one column, so isolated footings are usually the most economical option when columns sit on a regular grid and the soil carries the load at shallow depth. The pad is sized by dividing the column service load by the allowable bearing capacity, then checking the area against one-way and two-way shear and bending. Depth is then set by shear rather than by bearing.
Design Variations of Isolated Footings
Uniform Thickness Pads
Uniform thickness footings keep the same depth across the whole pad. They are simple to form, reinforce, and pour, and they work well for lightly loaded columns where bending and shear demands stay low. Steel bars sit in a single mat near the bottom of the pad, where tension develops as the soil pushes upward against the column load.
Sloped and Stepped Footings
When the column load grows, the depth at the column face must increase to resist bending and shear. A sloped footing tapers from the column face to the edge, saving concrete compared with a uniform pad of equal strength. A stepped footing achieves the same depth gain with horizontal steps on the top surface. Both shapes cut concrete volume while keeping the critical depth where it is needed. The main types of isolated footings, including reinforcement layouts and cost trade-offs, are covered in a separate breakdown.
- Columns are spaced at least 3 m apart on a roughly regular grid
- The soil has adequate bearing capacity at shallow depth
- Column loads are similar in magnitude across the building
- Groundwater sits well below the footing level
Continuous and Strip Footings for Load-Bearing Walls
Continuous footings support walls instead of individual columns. A continuous footing runs as a long strip beneath a brick, block, or random rubble wall and spreads the wall load along its length, so soil pressure stays low and even. In low-rise and older construction, these strips are often rubble foundations using stone or brick laid in mortar. When the soil is weak and settlement is a risk, the strip is cast as reinforced concrete to keep soil pressure under the foundation within safe limits.
Rubble Strips vs Reinforced Concrete
Rubble continuous footings rely on the strength of masonry laid in a trench, and they perform well where the soil is firm and the wall load is modest. Reinforced concrete strips add a steel mesh or longitudinal bars that tie the footing together and resist bending when soil support is uneven across the width. The choice usually comes down to the soil report and the wall height above. Three types of footings for supporting foundation walls are compared in a project guide that covers the practical side of this decision.
- Excavate the trench to the depth in the geotechnical report, below the frost line where codes require it.
- Level and compact the trench bottom; remove soft or organic pockets.
- Set formwork, or use the trench sides where the soil is stable enough to stand.
- Place reinforcement bars or mesh on chairs at the specified cover.
- Pour the concrete in one continuous operation to avoid cold joints.
- Keep the strip damp and cure it for the specified period before building the wall.
Combined Footings for Close Columns and Weak Soils
A combined footing supports two or more columns on a single foundation pad. Builders turn to this arrangement when columns are so close that individual pads would overlap, when the soil bearing capacity is too low to size separate footings economically, or when an edge column would push a footing past the property line. In the edge column case, the footing connects to the next interior column so the centroid of the pad aligns with the resultant of the two loads, which keeps soil pressure close to uniform.
Rectangular and Trapezoidal Shapes
A rectangular combined footing is easiest to form and suits columns with similar loads. When the exterior column carries a much heavier load, a trapezoidal shape shifts the center of gravity toward the load resultant and evens out pressure distribution. Combined footings need double reinforcement nets, one near the bottom and one near the top, because the cantilever span between columns bends in the opposite direction to a normal pad. A rundown of the top five footing types used in building construction shows where combined footings fit against isolated, strip, and raft options.
Two design checks dominate combined footing work. The first keeps maximum soil pressure below the allowable bearing capacity, using the combined column loads and pad self-weight. The second checks one-way and two-way shear at the critical sections around each column, since the footing spans between supports like an inverted beam and punching shear governs near heavily loaded columns.
| Footing type | Typical use | Load path | Relative cost |
|---|---|---|---|
| Isolated | Single column on a regular grid | Column to a square pad | Lowest |
| Continuous strip | Load-bearing walls | Wall to a long strip | Low |
| Combined | Two close columns | Two columns to one pad | Moderate |
| Strap | Edge column near a boundary | Column through a beam to a neighbor pad | Moderate |
| Raft | Weak soil, heavy building | Whole structure to one slab | High |
| Pile | Deep soft soil, high loads | Structure to a deep bearing layer | Highest |
Strap Footings and Raft Foundations
Strap footings and raft foundations solve problems that isolated pads handle poorly: the strap footing fixes the eccentric edge column, and the raft spreads heavy or closely spaced loads over the full footprint.
Strap Footings for Edge Columns
A strap footing, also called a cantilever footing, links an exterior column footing to an interior column footing with a rigid beam. The strap does not bear on the soil; it transfers the moment from the eccentric edge column into the interior footing so the pressure under both pads stays close to uniform. It suits sites where a property line or existing wall stops the edge footing from being widened. The beam must be stiff enough to distribute the moment, and it is designed for bending and shear like other beams.
Raft Foundations as One Slab
A raft foundation, or mat foundation, supports the whole structure on a single continuous slab that spreads the total building load across the entire footprint. Rafts suit soft clays, basements where the slab doubles as the floor, and sites where differential settlement between separate pads would crack the frame. Raft thickness commonly lands between 300 and 600 mm for residential and light commercial buildings. Because the slab is large, reinforcement runs in two directions top and bottom, and design checks include punching shear at each column. The five most common footing types in building work are ranked against each other in a separate overview.
Pile Foundations: Deep Support When Shallow Footings Fail
Pile foundations are sometimes listed alongside footings, but they are deep foundations rather than shallow ones. Piles carry the load through weak surface soils to a stronger stratum below, by end bearing on rock or dense soil or by friction along the shaft. They are the answer when the top few meters cannot support spread footings, when the water table is high, or when the structure is so heavy that shallow pads would settle too much.
End Bearing vs Friction Piles
End bearing piles transfer load through their tips into a firm layer, while friction piles develop resistance along the contact between the shaft and the surrounding soil. Most installed piles work as a combination of both, and the designer sets the working capacity from the soil profile. Driven precast and cast-in-place bored piles are the common options, the choice depending on noise limits, access, and depth to the bearing stratum. A pile cap on top then spreads the column load into the group. A wider listing of the different types of footings and their uses places piles in context with the shallow options.
Selection Checks, Pier Footings, and Reinforcement Detailing
The right footing type falls out of two inputs: the geotechnical report supplies the allowable bearing pressure and groundwater level, and the structural drawings supply column loads and spacing.
Selection Checklist
- Compare the allowable bearing capacity with the required footing area for each column.
- Check column spacing; close columns favor combined or strap footings.
- Check the property line; edge columns near boundaries favor strap footings.
- Check soil consistency and groundwater; weak or wet soil favors rafts or piles.
- Check the wall system; load-bearing walls favor continuous footings.
Pier Footings for Decks and Porches
Light structures such as decks and porches use pier footings, small pads or bells at the base of a post that spread a point load into the soil. Sizing depends on the tributary area, the deck load, and the soil bearing value, and pour sequence matters for frost protection in cold climates. Sizing and pouring pier footings for decks is covered in a dedicated guide that walks through the numbers.
Reinforcement Placement and Cover
Whatever the footing type, the steel must sit at the right depth. Bars go near the bottom of a pad because the upward soil pressure puts the bottom fibers in tension, and chairs or spacers hold them at the specified cover, typically 50 to 75 mm for concrete cast against soil. Bar spacing, lap lengths, and support spacing follow the design drawings, and the placement rules for steel reinforcement in concrete footings are worth reviewing before any pour.