Quick answer: A combined footing is a single reinforced concrete footing that supports two or more columns. It is used when columns stand so close that their separate footings would touch or overlap, when a column sits right on the plot boundary, or when weak soil needs a larger base. The common types are rectangular, trapezoidal, strap and raft.
A foundation spreads the weight of a building over the ground. Shallow foundations, also called spread footings, are used when the soil near the surface is firm enough to carry that weight. Most house columns get their own pad, called an isolated footing. A combined footing is the next step, used where separate pads will not fit or will not behave well.
What is a combined footing?
A combined footing is one concrete base shared by two or more columns. Instead of each column pushing down on its own small pad, the columns sit on a common slab that spreads their loads together onto the soil below. It is usually rectangular or trapezoidal in plan. It is one of the shallow types of footing, so it is dug to roughly the same depth as the isolated footings on the same site.
When is a combined footing used?
The structural engineer chooses a combined footing in these situations:
- Columns are close together. If two isolated footings would touch or overlap, they are merged into one.
- A column is on the plot boundary. A footing cannot extend into the neighbour’s land. A pad placed only on your side would be loaded off-centre and would tend to tilt, so the boundary column is tied to the next column inside the plot.
- The columns carry unequal loads. A shared footing can be shaped so that both columns settle by the same amount, which avoids differential settlement.
- The soil is weak near the surface. When the bearing capacity is low, each column needs a bigger pad. Bigger pads run into each other, and one shared footing becomes the simpler answer.
- The layout forces it. Lift cores, staircases and narrow plots often put columns nearer to each other than usual.
- It works out cheaper. Where columns are close, one combined footing can need less excavation and shuttering than several separate pads.
Types of combined footing
Combined footings are grouped by their shape and by how the columns are connected.
| Type | Shape | Used when |
|---|---|---|
| Rectangular | Flat rectangular slab under both columns | The column loads are close to equal and there is room to extend the slab at both ends |
| Trapezoidal | Wider at one end than the other | One column is much heavier than the other, or the footing cannot project beyond one of the columns |
| Strap | Two separate pads joined by a concrete strap beam | A boundary column has to be balanced by an inner column that is some distance away |
| Raft | One slab under many columns and walls | Loads are heavy or the soil is poor across the whole building |
Rectangular combined footing
A flat, elongated slab that carries two or more columns in a row. It is the simplest type and is used when the columns carry similar loads.
Trapezoidal combined footing
When one column is much heavier, the footing is made wider under that column and narrower under the lighter one. The extra width under the heavy column keeps the pressure on the soil even along the length of the footing.
Strap footing
Two pads are linked by a stiff concrete beam called a strap beam. The beam does not rest on the soil to carry load. Its job is to transfer the turning effect of the off-centre boundary column to the inner column. This is also called a cantilever footing.
Raft footing
A raft, or mat, is the same idea taken to the whole building: one slab under all the columns and walls. Many engineers treat it as a separate foundation type rather than a combined footing.
How does a combined footing work?
The footing is sized and positioned so that its centre lines up with the combined centre of the column loads. When that is done, the soil pushes back with even pressure across the whole base and the footing settles level instead of tilting.
Structurally the footing behaves like an upside-down beam. The soil pushes up along its full length and the columns hold it down at two points. That is why a combined footing has steel at the top between the columns as well as at the bottom, and extra bars across the width under each column. An isolated footing normally has only a bottom mat. This difference matters on site, because a bar fixer who is used to isolated pads may leave the top steel out.
Combined footing vs isolated footing vs raft
| Point | Isolated footing | Combined footing | Raft |
|---|---|---|---|
| Supports | One column | Two or more nearby columns | All columns and walls |
| Suits | Firm soil, well-spaced columns | Close columns, boundary columns, moderate soil | Weak soil or heavy loads |
| Design and steel | Simplest, bottom steel only | More involved, top and bottom steel | Most involved, steel throughout the slab |
| Cost | Lowest | Between the two | Highest |
A typical house does not use one type throughout. Most columns sit on isolated footings and only the boundary columns or closely spaced pairs get a combined footing.
How is a combined footing built?
- Soil test and design. The engineer fixes the size, shape, depth and steel from the column loads and the soil test report.
- Marking and excavation. The pit is marked from the column centre lines and dug to the design depth. The bottom is levelled and compacted.
- PCC bed. A layer of plain cement concrete is laid to give a clean, level surface for the steel.
- Reinforcement. The bottom mat, the top mat and the column starter bars are tied as per the drawing and lifted on cover blocks and chairs.
- Shuttering. Side forms are fixed and checked for size and line.
- Concreting. The whole footing is poured in one go and vibrated so that no joint is left between the columns.
- Curing and backfilling. The concrete is kept wet, then the pit is backfilled in layers and compacted.
What to check on site before the concrete is poured
- The pit size and depth match the foundation drawing, and the bottom is firm natural soil, not loose fill.
- Both steel mats are in place. The top bars between the columns are the ones most often missed.
- Bar diameter, spacing and the transverse bars under each column match the drawing.
- Cover blocks are fixed under and beside the steel so no bar touches the soil or the shuttering.
- The column starter bars are at the correct centre lines. A column that starts in the wrong place cannot be moved after the pour.
- The pit is dry. Any water is pumped out before concreting.
Advantages of a combined footing
- Even load distribution. Loads from several columns are spread together, which reduces differential settlement.
- Makes use of tight space. It works where columns are close or the plot line leaves no room for a centred pad.
- Can cost less. For closely spaced columns it can be cheaper than several individual footings.
- Flexible in shape. The engineer can pick a rectangular, trapezoidal or strap arrangement to suit the loads.
- Frees up the layout. Columns can be placed where the plan needs them, including on the boundary.
- Ties the structure together. Columns sharing a footing move together, which adds to stability.
Disadvantages of a combined footing
- Harder to design. The footing has to be checked for unequal loads, bending along its length and shear around each column.
- Limited capacity. For very heavy loads or tall buildings, a raft or a pile foundation is usually the better choice.
- Slower to build. More steel fixing and a larger single pour take more time than isolated pads.
- Hard to alter. Once columns share a footing, moving or adding a column later is difficult and costly.
- Depends on the site. Its performance changes with soil type and groundwater, so it must be designed from a soil report.
Common mistakes with combined footings
- Building it like two isolated pads placed side by side, with bottom steel only.
- Pouring it in two parts and leaving a cold joint between the columns.
- Changing the size or shape on site because the pit hit a pipe, an old wall or the neighbour’s footing, without asking the engineer.
- Founding one end on filled soil and the other on natural ground.
- Copying the footing from another house instead of designing it for this plot’s loads and soil.
A combined footing is a design decision, not a site decision. If the foundation drawing shows one, build it exactly as drawn. If the site suggests one is needed where the drawing shows isolated pads, stop and ask the structural engineer. For the wider picture, read how structural design works from soil to roof, how a strip footing carries walls, and how an eccentric footing deals with a column at the edge of its pad. If you want the foundation designed and built by one team, see our construction company page.
Frequently asked questions
What is a combined footing?
A combined footing is a single reinforced concrete footing that supports two or more columns. It spreads their loads together onto the soil and is used when the columns are too close for separate footings or when one column sits on the plot boundary.
When is a combined footing used instead of an isolated footing?
It is used when isolated footings would touch or overlap, when a boundary column cannot have a footing centred under it, when column loads are unequal, or when weak soil needs larger pads than the spacing allows. The structural engineer decides this from the loads and the soil report.
What are the types of combined footing?
The main types are rectangular, trapezoidal and strap footings. A rectangular footing suits columns with similar loads, a trapezoidal one suits unequal loads, and a strap footing joins two pads with a beam. A raft, which covers many columns, is sometimes listed as a fourth type.
What is the difference between a combined footing and a strap footing?
In a combined footing the columns share one continuous slab that bears on the soil along its full length. In a strap footing each column has its own pad and a stiff beam connects them. The beam balances the boundary column but does not itself transfer load to the soil.
Why does a combined footing need steel at the top?
Between the columns the footing bends upward under soil pressure, like an upside-down beam, so the top face is in tension there. Top bars resist that tension. An isolated footing does not bend this way, which is why it usually has only a bottom mat.
Is a combined footing suitable for tall buildings?
Usually not on its own. Combined footings suit low and medium loads. For very heavy loads, tall buildings or poor soil across the whole plot, engineers generally move to a raft or a pile foundation.
