Who this guide is for
- Owners planning a new build, extension or garden building who want to follow the foundation discussion
- Self-builders reading an engineer's early options before a scheme is fixed
- Buyers trying to understand what a survey or a set of drawings is describing
- Anyone offered two foundation options who cannot tell what separates them
The choice follows what arrives, not what you would prefer
A shallow foundation is asked to hand load to ground near the surface. What separates the families is the shape in which that load arrives and how much the ground varies beneath it. Preference, cost and familiarity all enter later, and none of them changes the test being applied.
- A continuous wall delivers load along a line
- A column or post delivers it at a point
- Ground that changes across the footprint is a different problem again
- What bears on the element decides which entry describes it
Line of load means strip; point of load means pad
The strip foundation entry offers a test that needs no arithmetic. Take the wall away and a strip has nothing left to do, because spreading a line of load is its only purpose. Take the column away from a pad and a base connection is still cast into it, waiting for something.
That connection is the second half of the difference. A pad has to hold a member in position as well as spread its load, and what the frame designer assumed about the base, free to rotate or held rigidly, changes what the pad has to be. The same column on the same ground does not always produce the same foundation.
Trench fill and traditional strip are one family
Trench fill is not a third option. It is the same load path with more of the excavation occupied by concrete and less rebuilt in masonry up to ground level, which changes buildability and the below-ground wall rather than the reason the element exists at all.
When ground variability pushes towards a raft
A raft answers a different problem. Rather than each wall or column finding its own patch of ground, the loads are collected by a single plate stiff enough to move as one, so local weakness is bridged instead of followed. How much variation a plate can accept is a design determination made from investigation.
That stiffness is also why the middle of a footprint begins to matter. A backfilled trench, a pocket of made ground or an abandoned service run under the centre can put a stiff plate into a shape it was never proportioned for, because bending follows the worst ground beneath it rather than the average.
- Ground that varies across the footprint rather than along one wall
- Working space, groundwater or access that make many separate excavations difficult
- A wish to resolve foundation and ground floor in a single element
- Loads that would otherwise need several individual footings of quite different sizes
Where a floor stops being a floor
A raft and a ground-bearing slab look identical once cast, and the library settles them with one question: what bears on the element. If the load-bearing walls and columns stand on it and it takes them into the ground, it is a raft. If they are carried elsewhere and it takes only floor loading, it is a floor.
The answer is not academic. It changes the reinforcement, the stiffenings underneath, whether wall lines have to sit over downstands, and what cutting an opening through the element means years later. A drawing labelled as a slab that carries walls is describing a raft whatever the title block says.
All of it assumes somebody has looked at the ground
Every test above rests on knowing what lies under the plot. Ground investigation is what turns the subgrade from an unknown into a described body, and the subgrade entry is explicit that a neighbouring site does not indicate what will be found, because ground can vary sharply over short distances.
- What was found beneath each part of the footprint, not just at one hole
- Whether the soil is one that changes volume with moisture
- Where groundwater sits, and how that varies through a year
- Whether any part of the plot has been filled or previously built on
Following the decision without trying to make it
You are not being asked to choose. You are being asked to understand why one option was proposed and what would change it, so that a later alteration, a wider opening, a new drain or a raised patio, is discussed against the foundation that was actually built rather than the one you assumed.
Following the shallow foundation decision
- 1Establish whether load above arrives along wall lines, at columns, or as both.
- 2Ask what ground investigation was carried out on this plot, and where the holes sat.
- 3Ask whether the ground varies across the footprint or only in one area of it.
- 4Confirm whether the element being drawn carries the structure or only the floor.
- 5If a raft is proposed, ask where the stiffenings run and whether wall lines sit over them.
- 6If pads are proposed, ask whether anything ties them together and what that is for.
- 7Where columns are involved, ask what base condition the frame design assumed.
- 8Establish where drains and incoming services cross the foundation line.
- 9Ask what finished external ground level the design assumes against each wall.
- 10Ask which drawing shows the wall damp-proof course meeting the floor membrane.
- 11Keep the investigation report and the foundation drawings together as one record.
Common mistakes to avoid
- Choosing a family from what a neighbouring plot used, when ground can change sharply over short distances.
- Treating a deeper trench as automatically better, when founding level is chosen for the stratum rather than as a margin.
- Reading trench fill as a separate system, when it is the same load path with more concrete in the excavation.
- Assuming a raft removes the need for investigation, when a stiff plate is sensitive to what lies under the middle of it.
- Calling any slab on the ground a foundation, when it becomes one only where the structure bears on it.
- Moving a wall during design development without checking it still sits over the stiffening drawn to receive it.
- Assuming isolated pads act as a foundation on their own, when whether they behave as a set depends on what links them.
When to involve a professional
- Foundation design is site-specific work for a qualified structural engineer using ground investigation carried out on that plot.
- Ground investigation and its interpretation belong with geotechnical professionals rather than with an impression of the soil at the surface.
- Where a frame is involved, the base condition assumed by the frame designer and the foundation provided have to be reconciled by those designers together.
- Nothing here states a founding level, a width, a reinforcement arrangement or a bearing pressure, and none can be inferred from it.
Frequently asked questions
Questions readers ask about this topic
How do I tell a raft from a thick floor slab?
By what stands on it. If the load-bearing walls and columns bear on the element itself and it takes them into the ground, it is a raft. If they are carried elsewhere and the element takes only floor loading, it is a floor slab.
Is trench fill a different foundation from a strip?
No. It is the same continuous line of load being spread, with more of the excavation occupied by concrete and less rebuilt in masonry up to ground level. What changes is buildability and the below-ground wall, not the load path or the purpose.
Do isolated pads need to be joined together?
Whether they do is an engineering decision. Left independent, each pad settles according to the ground immediately beneath it and the frame above has to absorb the difference. Ground beams or ties share that difference out and give walls something continuous to bear on.
Can a report from the plot next door be used?
It can inform expectations and nothing more. Ground varies over short distances, which is why investigation is site-specific, and a foundation designed against somebody else's boreholes has been designed against ground that was never tested where you are building.
Why will nobody give me a founding depth?
Because founding level is chosen for the stratum the foundation has to reach and the influences acting around it, not as a safety margin. Two excavations to the same depth can arrive at very different formations, so depth follows the ground rather than preceding it.
Keep reading