Subgrade vs Foundation: Know the Difference
Many people use "subgrade" and "foundation" interchangeably. The subgrade is the soil or rock beneath the foundation that carries the load — a natural condition of the site itself. The foundation, by contrast, is the buried structural element that spreads the building's load before passing it down to the subgrade. A geotechnical survey report describes the subgrade; the strip foundations and raft foundations on your drawings are the foundation choice. The former decides the latter.
Four Common Foundation Types
Strip foundation: the default for masonry homes
A strip foundation runs continuously beneath the load-bearing walls with an inverted-trapezoid section, usually cast in place with C25 concrete and about 2–3 times the wall thickness wide. It is inexpensive and simple to build, suits sites with decent soil bearing capacity and buildings of modest height (typically 6 storeys or fewer), and is the most common choice for rural self-built homes and low-rise masonry houses.
Pad foundation: the standard partner for frame structures
Each column gets its own square or rectangular pad, tied to its neighbours with grade beams. Pad foundations need less excavation and less material, but they assume even settlement between columns, so they suit sites with good, uniform soil.
Raft foundation: one big raft under the whole house
A raft foundation spreads a single slab of reinforced concrete across the entire footprint — effectively a full base tray under the house. It distributes the load over a much larger area of soil and handles differential settlement well, making it the right call for low bearing capacity, uneven ground or a high water table. The drawback is noticeably more concrete and rebar.
Pile foundation: deep support for soft ground
When the shallow ground is soft — silt or deep fill, for example — the load has to travel through the weak layer to a firm stratum below, and that calls for piles. Homes typically use precast pipe piles or bored cast-in-place piles, tied together at the top by a pile cap connected to the structure above. Piles are the most expensive option and take the longest to build, so the decision should follow the geotechnical survey, not guesswork.
| Foundation type | When to use | Strengths | Drawbacks |
|---|---|---|---|
| Strip foundation | Low-rise masonry homes on decent soil | Low cost, simple construction | Sensitive to uneven ground |
| Pad foundation | Frame structures, uniform soil | Saves material and excavation | Columns need tying to control settlement |
| Raft foundation | Soft soil, high water table, heavy loads | Strong monolithic behavior, resists settlement | Heavy use of concrete and rebar |
| Pile foundation | Deep soft soil, thick fill | Reliable bearing capacity | Expensive, long construction time |
The Full Foundation Construction Sequence
Foundation work is hidden work: once backfilled it is covered forever, so every step must follow the code and be documented with photographs. A typical sequence looks like this:
- 1. Setting out: using the planning boundary and the drawings, the axis lines are laid out with a theodolite or total station and marked with profile boards; errors are held to centimetre level.
- 2. Excavation: machine-dig to 200–300mm above the design level, then finish by hand so the bearing stratum is not disturbed.
- 3. Blinding: cast a 100mm C15 plain-concrete blinding layer to give the rebar a clean, level working surface and to protect the base soil.
- 4. Rebar placement: lay the main reinforcement to the drawings; with a blinding layer the concrete cover should be at least 40mm, held with concrete spacers.
- 5. Formwork and pouring: once the formwork is inspected and approved, pour in a single operation; concrete grade no lower than C25, consolidated thoroughly with a poker vibrator.
- 6. Curing: keep the concrete watered for at least 7 days at normal temperatures, with no foot traffic or stacking loads during that time.
- 7. Striking and backfill: remove the formwork once the concrete reaches its required strength, then backfill and compact in layers no more than 300mm thick.
What Drives the Foundation Design
- Ground conditions: the characteristic bearing capacity of the soil directly sets the foundation type and size. Do a geotechnical survey before work starts — it typically costs just 1–2% of the foundation budget, yet it eliminates the biggest risk of all.
- Water table: a high water table means dewatering during construction and proper foundation waterproofing; a raft foundation with a waterproof membrane is the safer combination.
- Frost depth: in cold regions the foundation must be set below the local frost line, or protected with measures such as sand-bed replacement or perimeter insulation.
- Building load: more storeys and wider spans call for a larger footprint — sometimes enough to step up from strip to raft foundations.
Common Ground Problems and How to Prevent Them
Ground problems often only surface a few years after the house is finished, and the repair costs are punishing:
- Differential settlement: shows up as diagonal wall cracks, often in a herringbone pattern, usually caused by the wrong foundation choice or weak, uneven ground. Prevent it by surveying first and tying the structure together with ring beams and tie columns.
- Rising damp through the foundation: mouldy wall bases and peeling floors, caused by a missing or damaged damp-proof course. Masonry foundations should carry a DPC of waterproof mortar or membrane at the top, and the indoor floor level should sit above the outside paving.
- Impact of neighbouring works: deep excavation next door can disturb your ground. Keep a safe distance from neighbouring foundations and retain settlement observation records.