Structural Foundations: Simple Guide to Types, Construction and Why They Matter
Foundations are the part of a building you never see, but they carry everything above. If the foundations are poor, the structure will move, crack or even fail. This guide explains what structural foundations are, the main types used in UK construction, and how they are built for houses and small projects.
What Structural Foundations Do
Simple definition
A structural foundation transfers the weight of a building into the ground safely. It spreads loads so the soil can carry them without sinking too much.
How foundations work with the structure
Loads travel from the roof to the walls, then into beams and columns, and finally down into the foundations. Good design keeps movements small and prevents cracking.
Why Foundations Are So Important
Place your video here, near the top of the article:
Preventing settlement and movement
Without proper foundations, parts of the building can sink at different rates. This causes sloping floors, sticking doors and cracks.
Dealing with weak or variable ground
Foundations are chosen to suit the ground conditions. Clay, sand, rock and filled ground all behave differently. The right foundation type manages those risks.
Main Types of Structural Foundations
Strip foundations
Strip foundations are continuous strips of concrete under load-bearing walls. They are common for traditional brick and block houses on reasonably strong ground.
Trench fill foundations
Trench fill uses deeper concrete trenches with minimal blockwork. It is often used when the ground near the surface is weak or when you want to speed up construction.
Pad foundations
Pad foundations are individual pads of concrete under columns or posts. Loads are carried at discrete points rather than along a full wall.
Raft foundations
A raft foundation is a thick slab that sits under most or all of the building footprint. It spreads the load over a large area, useful on softer or variable ground.
Piled foundations
Piles are deep elements driven or bored into the ground to reach stronger soil or rock. A concrete pile cap or ground beam then links the piles together.
Choosing the Right Foundation Type
Ground conditions and site investigation
Engineers look at soil type, groundwater, existing trees, nearby buildings and drains. A simple trial pit or full site investigation shows what the ground can safely carry.
Building loads and layout
Heavy structures or wide spans need stronger foundations. A small extension on firm ground may use simple strips. A large new-build with poor soil may need piles.
Existing structures and underpinning
If foundations are already in place but not deep enough, underpinning may be used to extend them. This must be carefully designed and sequenced.
How Structural Foundations Are Built in UK Housing
Setting out and excavation
The builder sets out the footprint, then excavates trenches or pads to the required depth. Depth is controlled to meet the engineer's drawings and building regulations.
Concrete placement and curing
Reinforcement is placed if needed, then concrete is poured, compacted and left to cure. The timing of brickwork or blockwork above is planned so the concrete has gained enough strength.
Linking foundations to walls and beams
Starter bars, dowels or straps connect the superstructure to the foundations. This stops sliding and improves stability under wind and lateral loads.
Common Foundation Problems in UK Homes
Settlement and subsidence
Uneven settlement can come from tree roots drying out clay, leaking drains washing away soil, old fill or poor compaction. Visible signs include stepped cracks in brickwork and gaps at skirting boards.
Heave in shrinkable clay
If trees are removed or the ground becomes wetter, clay can swell and push foundations upwards. This is called heave.
Poor or missing foundations
Some older buildings and DIY extensions have shallow or makeshift foundations. These often lead to movement when extra loads are added.
When You Need a Structural Engineer for Foundations
New extensions or major alterations
Any extension, loft conversion with extra loads, or removal of key walls should involve a structural engineer. They will design pad sizes, strip widths and any required reinforcement.
Visible cracking or movement
If you see significant cracking, sloping floors or sticking doors, an engineer can investigate the foundation performance and advise whether repair or underpinning is needed.
Building near trees, slopes or water
These situations increase the risk of movement. A proper foundation design reduces that risk and gives Building Control confidence.
FAQs about Structural Foundations
What is the main purpose of foundations?
Foundations spread the load of the building into the ground so it does not sink or move excessively.
Which foundation type is used most for UK houses?
Strip or trench fill foundations are most common for typical brick and block houses on moderate ground.
When are piles used?
Piles are used when the near-surface soils are weak, highly variable or on sites with deep fill or high groundwater.
Do small domestic extensions need piles?
Most small extensions do not, but they may if the ground is very poor, if there are large trees nearby, or if Building Control / the engineer recommend them.
Who decides the correct foundation type?
A structural engineer designs the foundations, often guided by a site investigation and local knowledge.
Need Help with Structural Foundations?
If you're planning a new build, extension or dealing with cracking and movement, SECalcs can design structural foundations and provide full calculation packs for Building Control.