What are house foundations and what do they actually do?
A foundation is the part that turns the weight of a building into a pressure the ground can carry without moving. What that means in practice, and why almost every foundation problem is really a ground problem.
- Difficulty
- beginner
- Time
- 9 min
- Read
- 5 min
- Safety
- caution
Short answer
A foundation spreads the load of the walls over enough ground area that the soil beneath is not overloaded, and puts that load at a depth where the ground does not move with the seasons. It does not hold the house up on its own — the ground does. That is why foundation failures are nearly always caused by something that changed the ground: moisture, a tree, a leaking drain, or an excavation next door.
People picture a foundation as a solid thing the house is bolted to, like a keel. It is closer to a snowshoe. The walls carry a large load down to a narrow line, and the foundation widens that line until the pressure on the soil is low enough that the soil does not squash or slide. Everything else follows from that idea, including why depth matters as much as width.
Safety
Step by step
- Follow the load down.Roof onto walls, walls onto the foundation, foundation onto the ground. Each stage spreads the load over a bigger area than the one above. The ground is the last and weakest link, and it is the only one nobody designed.
- Understand what width is for.Width reduces the pressure on the soil. A heavier wall or a weaker soil needs a wider foundation to bring the pressure back down to something the ground can carry without compressing.
- Understand what depth is for.Depth is not about strength. It gets the underside of the foundation below the layer of ground whose volume changes with the weather — frost near the surface, and seasonal wetting and drying much deeper than that in clay.
- Accept that the ground was never designed.A structural engineer chooses the foundation to suit the ground that is found. Nobody chooses the ground. This is why a site investigation comes before a foundation design and not after it.
- Learn the three ways it goes wrong.The soil under part of the building loses volume and that part drops, which is subsidence. The soil gains volume and that part lifts, which is heave. Or the building itself compresses the ground evenly as it beds in, which is settlement and is normal.
- Notice that most causes are outside the house.A tree drawing water out of clay, a leaking drain washing fines out of the soil, a new excavation next door, a change in the level of the ground against the wall. The foundation usually did nothing wrong; its surroundings changed.
- Know what you can safely observe.Crack widths, crack direction, whether doors and windows have started to bind, whether the ground level against the wall has been raised, and where trees and drains are. All of that is visible from standing up. None of it requires digging.
Tips
- The age of a house tells you a lot about what is likely to be under it. Victorian terraces often sit on very shallow brick footings that would not be permitted today, and have stood for a century because the ground under them has never been disturbed.
- "The foundations have gone" is almost never what has happened. Concrete does not decay in the ground on the timescale of a house. What has changed is the soil around and beneath it.
- If a neighbour starts excavating close to a shared or nearby wall, that is the moment to take dated photographs of every existing crack. It costs nothing and it settles arguments later.
Common mistakes
- Digging a hole beside the wall to see what the foundation looks like — It removes the soil the foundation is bearing against, which is the thing keeping it stable, and an unsupported trench beside a loaded footing can collapse into itself. This is the single most dangerous piece of DIY curiosity in the subject.
- Assuming a crack means the foundation has failed — Most cracks in a house are in the plaster or the render and have nothing to do with the ground. Even structural cracks are far more often thermal or moisture movement in the wall itself than foundation movement.
- Filling cracks to make the problem go away — A crack that is still moving reopens, and filling it destroys the record of how wide it was and when. Monitoring across a full year is what tells a surveyor whether the movement is seasonal or progressive.
If it doesn't work
Diagonal cracks appearing in a hot dry summer, closing in winter
Cause: Seasonal clay shrinkage, most likely influenced by vegetation — Fix: Photograph and measure them, and watch across a full year rather than acting in the same summer. Do not remove a mature tree as a first response — on clay, removing one can cause heave that is worse than the shrinkage was.
Cracks that keep widening month after month regardless of the weather
Cause: Progressive movement, which is a different problem from seasonal movement — Fix: Contact your buildings insurer before commissioning anything. Subsidence claims are handled by the insurer, and appointing your own consultant first can complicate the claim.
New cracking that started when work began next door
Cause: Excavation, vibration or dewatering close to your foundation — Fix: Record it with dated photographs immediately and raise it in writing with the neighbour and their contractor. If the work is notifiable under the party wall legislation, there is a formal route for damage.
Questions people ask
How do I find out what foundations my house has?
Without digging: the age and type of the house is the strongest clue, the original drawings may be with the local authority building control records for anything post-war, and a structural survey can determine it properly with a trial pit dug safely. Guessing from the neighbours is unreliable — houses in the same terrace have been underpinned, extended and rebuilt independently.
Do older houses have foundations at all?
Usually something, but often very little by modern standards — a few courses of brick corbelled out below ground, or a shallow spread of lime concrete. That is not automatically a problem. It becomes one when the ground around it changes.
Does a house need deeper foundations for an extra storey?
Possibly, and it is not a question anyone can answer from a description. Adding load is exactly the situation where the existing foundation and the ground beneath it have to be assessed by a structural engineer, and building control will want that assessment.