Concrete foundations
Most Irish houses and extensions stand on strip foundations: a continuous strip of concrete under each load-bearing wall. Trench fill is a strip foundation where the trench is almost filled with concrete. A raft is a reinforced slab that carries the whole building.
Under Technical Guidance Document A, a plain concrete strip foundation is at least 200 mm thick and at least 400 mm wide on good ground, wider as the wall load goes up or the ground gets softer.
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Strip foundations
TGD A's rules for plain concrete strip foundations apply where the ground is known and even: no made ground, no wide variation in the subsoil under the building, and no weaker soil underneath that could affect it. The foundation sits centrally under the wall.
| Subsoil (TGD A Table 5) | 30 kN/m | 40 kN/m | 50 kN/m | 60 kN/m | 70 kN/m |
|---|---|---|---|---|---|
| Compact gravel or sand; stiff clay | 400 mm | 400 mm | 500 mm | 600 mm | 650 mm |
| Firm clay | 400 mm | 450 mm | 500 mm | 600 mm | 650 mm |
| Rock | Wall width + 150 mm | ||||
| Loose sand, soft or very soft clay and silt | Site investigation needed |
- Thickness: at least 200 mm, or the projection P from Table 5 if that's greater.
- Width: never less than the wall plus 75 mm each side.
- Steps: overlap by twice the step height, the foundation's thickness or 300 mm, whichever is greatest. No step higher than twice the foundation's thickness.
- Concrete for plain foundations in non-aggressive ground: C12/15, at least 200 kg/m³ of cement, water/cement ratio no more than 0.85. A 1:7 mix by volume is allowed on small jobs.
Trench fill foundations
In a trench fill foundation the concrete is poured to within about 150 mm of ground level, so no blocks are laid below ground and nobody has to work in the trench. It suits loose soil and a high water table, but in loose ground the amount of concrete is hard to predict and can be large, especially where trenches meet.
In heavy clay, or near trees, trench fill can go deeper, to where the soil's moisture doesn't change. Mesh reinforcement is often needed then, and the trench sides may be lined with a compressible board.
Raft foundations
A raft is a reinforced concrete slab, typically 150 to 300 mm thick, covering the whole footprint. It spreads the walls' load over the ground. It can suit one and two-storey houses on poor ground, such as soft clay or fill, or where settlement is likely. As a rough rule, if strip foundations would cover half the floor area or more, a raft may make more sense.
A raft has to be designed by an engineer: the steel, the thickened edges and the ground under it are all calculated. It is less suited to heavy loads on a few points, and its edges need protecting from erosion.
Choosing between them
| Strip | Trench fill | Raft | |
|---|---|---|---|
| What it is | A strip of concrete under each wall, blocks up to ground level | The trench almost filled with concrete | One reinforced slab under the whole building |
| Suits | Known, even, firm ground | Loose soil, high water table, getting out of the ground fast | Poor or variable ground, settlement risk |
| Concrete | Least | More, hard to predict in loose ground | A reinforced slab, with more steel |
| Designed by | TGD A tables on known ground; an engineer otherwise | Engineer or TGD A widths | Engineer |
The ground decides. TGD A says that where there's doubt about the subsoil, and always on loose sand, silt or soft clay, a site investigation is essential before the foundation is designed.
What has to be ready
- The engineer's foundation drawing and the building control commencement in place
- Trenches dug to the depth and width on the drawing, bottoms clean and firm
- Ground checked by the engineer where it's soft or varies
- Steel fixed, on spacers, if the foundation is reinforced
- Truck access, or a pump booked
- Concrete never placed on frozen ground
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Concrete foundations questions
How wide should a strip foundation be?
On good ground, at least 400 mm for walls up to 40 kN/m, rising to 650 mm at 70 kN/m (TGD A Table 5), and never less than the wall plus 75 mm each side. Soft ground needs a site investigation.
How thick should a strip foundation be?
At least 200 mm of concrete under TGD A, or more where the projection from Table 5 is greater.
What is a trench fill foundation?
A strip foundation where the trench is filled with concrete to within about 150 mm of ground level, so no blocks are laid below ground.
When is a raft foundation used?
On poor or variable ground, where settlement is likely, or where strip foundations would cover half the floor or more. A raft is a reinforced slab, typically 150 to 300 mm thick, designed by an engineer.
What concrete is used for foundations?
For plain strip foundations in non-aggressive ground, TGD A gives C12/15 with at least 200 kg/m³ of cement. Under I.S. EN 206, many foundations are exposure class XC2, C28/35 minimum, for reinforced work.
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