The main things to know
- Find out what ground is below the house before choosing a foundation.
- Building size, slopes, water and neighbouring buildings all matter.
- Compare complete prices for suitable options.
- Have the excavated ground checked before concrete covers it.
Start with the plot and the building
A neighbour may recommend a raft or a deep trench because that is what their builder used. It is useful background, but it is not enough to decide what your house needs.
The engineer considers the soil, suitable ground below it, the building’s weight, slopes, water and nearby structures. If you might add another floor later, say so now. The foundation and the rest of the structure need to be designed for the agreed plans.
Strip foundations: support beneath walls
A strip foundation is a continuous base beneath a wall that carries part of the building’s weight. Where the ground and building suit it, this can be a straightforward solution.
Its width and depth come from the design. If excavation reveals soft soil or different ground from what was expected, the team needs the engineer’s advice before continuing.
Trench fill: more concrete in the trench
With trench fill, much of the excavation is filled with concrete. This can reduce the amount of wall-building work below ground, but it uses more concrete.
Ask for the comparison to include concrete, excavation and labour. A lower price for one part of the work does not necessarily mean the finished foundation costs less.
Pads and beams: support at particular points
Pads are bases that support columns. Beams can connect these supports and carry walls between them. Depending on the design, the floor may also be held clear of the ground.
Where the design leaves space beneath a floor or beam for soil movement, that space must be kept as designed. It should not be filled with leftover soil during site clean-up.
Rafts: a large base under the building
A raft spreads the building’s weight over a large reinforced concrete base. It can be suitable for some ground conditions, but it is not an automatic cure for every difficult plot.
The thickness, steel and any stiffening are part of the engineer’s design. Ask why a raft is suitable here and how its full cost compares with other appropriate options.
Piles: taking support deeper into the ground
Piles are deep supports used where the building or ground calls for them. They can carry loads through unsuitable upper layers to support farther below or along their length.
This work may need specialist equipment and testing. Access for that equipment and the cost of bringing it to site need to be considered early.
Details that can change the answer
- A sloping plot: the design must account for the changing ground levels.
- Water in excavations: the team needs a safe plan for working in those conditions.
- A neighbouring building: excavation must not undermine its support.
- Previously filled land: the quality and depth of the fill need to be investigated.
Tell the engineer what you know about the plot’s history. Old drains, demolished buildings or land that has been levelled can matter even when the surface looks tidy.
Before the foundation is covered up
Agree who will inspect the excavations and steel, and what records you will receive. Ask the project team to explain the drawings and approvals needed for your building.
Keep the foundation drawings, investigation report and inspection records with your house documents. If conditions change on site, make sure the agreed response is recorded.
Questions we get asked
- What foundation types are used for houses?
- Options include strip, trench fill, pads and beams, rafts and piles. The ground and building determine which is suitable.
- Which is the strongest?
- The useful question is which is suitable for your building and ground. Spending more on the wrong type does not make it the right choice.
- How much will my foundation cost?
- You need a quotation based on the investigation and drawings. Check whether excavation, disposal, fill, concrete, steel and drainage are included.
- Is there a standard foundation depth?
- No. The depth must suit the ground and the design.
- Do I need a soil investigation?
- Arrange for your engineer to assess what investigation is needed before designing the foundation.
Written by
Eng. Samson Gitonga
Civil & Structural Engineer
Samson Gitonga is TimberStone’s civil and structural engineer. Based in Nanyuki, he helps clients understand how the ground, the building plans and the materials come together to create a safe, practical structure.
- Registered Civil & Structural Engineer, Engineers Board of Kenya
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