Where are strip foundations used

Strip foundation: Frost-free foundation for your own home

A strip foundation is a common type of foundation and takes the weight of entire residential buildings and transfers it to the ground. This is hard work and requires very special properties from the strip foundation.

Strip foundation: Frost-free foundation for your own home
Strip foundation: Frost-free foundation for your own home

Foundations not only carry the weight of a building built on them, they also protect the building's sub-floor from frost and often from moisture from the ground. First of all, the two most important requirements for a foundation - it must in any case be frost-free and safely load-bearing. Whether point or strip foundations or a continuous floor slab made of concrete - the statics of the building on top and the nature of the subsurface largely determine the foundation construction. If the building is predominantly supported by columns, point or single foundations are built, for which a solid foundation is necessary, as the load is concentrated at certain points. A strip foundation also needs a dense and therefore stable soil. If this is not the case, a solid concrete slab distributes the load of the house much better and above all nicely evenly over a large area on a soft surface. The building does not sink in this way.

What is a strip foundation?

The strip foundation is a common type of foundation and a stable, robust base for buildings or walls. It consists of solid, cast concrete strips that follow the floor plan of the house and form the base for the building's load-bearing walls. In large buildings, however, a thin, non-statically load-bearing floor slab can still be placed on the strip foundation - for better protection against soil moisture or as a basement floor slab.
In contrast to the floor slab, such a foundation saves concrete, as you neither have to concretize the entire base area nor completely cover it - that saves work.

Good to know: Strip foundations must not run too close to each other, as otherwise the bottom pressure, i.e. the pressure on the ground, of both concrete strips would overlap and the ground could settle.

Requirements for a strip foundation

Strip foundations must be carefully planned and built. What you save in the process will later pay off tremendously and endanger the entire construction project. Therefore, a structural engineer should determine the necessary depth, width and the type and position of the reinforcement that your foundation needs for the respective soil type and the planned building.

Frost-free foundation for foundations

When water freezes, it expands. If this happens with ground water under a foundation, it can literally lift it up, which can lead to cracks in the concrete and thus to building damage. All foundations must therefore reach a depth where frost can no longer penetrate. With us, this is the case from a depth of 80 centimeters - the foundation should also extend into the ground that deep, even better a meter deep.

A strip foundation can also protect a floor slab from the effects of frost from the ground - as a circumferential frost protection. With this so-called perimeter insulation, insulation elements made of XPS panels are attached to the outside of a 30 centimeter wide strip foundation so that the ground at the edge of the foundation cannot freeze. XPS stands for polystyrene extruder foam and is a weatherproof, pressure-resistant rigid foam that does not mind direct contact with the ground. The thermal insulation can be used as permanent formwork before the strip foundation is poured.
Such a frost protection apron can also take on a load-bearing function and is then taken into account in the static calculation.

Suitable concrete for strip foundations

So that a strip foundation can support a house or very long, heavy garden walls, you need stable concrete and additional reinforcement. Either finished reinforcement cages or individual, vertical and horizontal rods are used for this purpose, which are linked together by wire (tied up) and finally poured completely into the concrete. Foundations for garages or larger garden houses are poured from concrete of compressive strength class C 20/25, for residential houses C 25/30. With smaller construction projects, you can do without reinforcement and the associated work, the strip foundations hold up that way.

If you want to build a foundation yourself, you buy the dry concrete in sacks, have it delivered ready-made as ready-mixed concrete or mix the concrete yourself. Delivery is only worthwhile for large projects and only if the concrete mixer has direct access to the construction site and the concrete can be processed immediately can. Storage is not possible! The drum of a standard mixer holds six to seven cubic meters of concrete. The required amount of concrete corresponds to the volume of the trench - plus around 15 percent, because the concrete is still being compacted.

Dry concrete is the most convenient way to get foundation concrete - add water, stir, done. However, the convenient way is expensive if you need a lot of concrete. And that is the case with strip foundations.

Do-it-yourself mixing is ideal for larger construction projects: you need water, cement and gravel with a grain size of 0/16 for unreinforced and 0/8 for reinforced concrete. This has to be finer so that it completely fills the space around the iron bars.

How to build a strip foundation

1. Planning

In the case of self-constructed garden sheds, you are usually on the safe side and dimension the substructure rather too big than too small - it fits.
If you buy construction kits for garden houses or prefabricated garages, the assembly instructions usually include a foundation plan. Exact planning is necessary for residential buildings; nothing works without an architect or structural engineer. These determine the concrete required for the building and also the type and position of the reinforcement.

If you want to build the foundation for a garden wall on a slope, you must first compensate for the slope so that the foundation is horizontal. It is then slightly sunk into the ground on the mountain side. Under no circumstances should it simply be built parallel to the surface of the ground. To compensate, dig from the lowest point into the rising surface and remove the earth.

2. Mark the history

The foundation strips follow the course of the load-bearing walls. First remove vegetation and debris from the soil surface and mark the course and outline of the foundation trench to be excavated with wooden pegs and mason's cord. Remember that a strip foundation should be quite a bit wider than the wall, and add another four inches if you want to completely shutter the foundation. In sandy soil this is necessary because the earth is sliding down, with clay it is sufficient to board the top 15 centimeters - but in any case the piece that should be above ground level. A foundation can be aligned and smoothed out much better with formwork, and rainwater is definitely diverted away from the wall in this way.

Spray the outline of the trench on the ground with marking spray and remove the wooden pegs and the cords - a tangled batter board is extremely annoying when digging the trench. The shuttering boards are measured exactly again afterwards anyway, these ensure that the strip foundation is built with centimeter precision.

3. Digging

Now it’s getting tangible - and real hard work. With a spade and shovel you can still dig the trench for the strip foundation of a garden shed, but for a garage it is better to rent a mini excavator, which can be operated with a short instruction.

4. Fit the formwork for the strip foundation

The formwork brings the concrete into shape and ensures that a foundation or a slab foundation comes in exactly the right place and is built at the right height. To do this, stretch wall cords on both sides of the dug trench, to which you align the upper edge of the shuttering boards. Support the boards on the outside with pegs, battens or wedges - the main thing is that the concrete cannot push the boards apart. The boards are screwed together at the ends and corners. Towards the inside, loosely clamped battens hold the shuttering boards in position. Until the first concrete is poured in, then the slats come off again.

5. Concrete the strip foundation

First, fill the trench halfway with concrete and compact it. To do this, poke into the liquid concrete with a shovel or square timber, which also allows trapped air to escape. For larger quantities of concrete, there are special machines for rapid compaction. If reinforcement is provided, this comes on the first concrete layer and is then completely enclosed by the concrete. In the case of residential buildings, there are also the grounding straps for the water and power lines. Then fill in the rest, compact it as well and smooth the surface.