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Point foundations: worth knowing and tips
Point foundations: worth knowing and tips

Video: Point foundations: worth knowing and tips

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Point foundations have the lowest workload of all foundations and can often be built on your own. However, point foundations are only suitable for certain building types.

Table of contents Table of contents Point foundations: Effective foundation for load-bearing parts of buildings

  • What are point foundations?
  • Possible uses
  • The right concrete
  • Formwork
  • Advantages and disadvantages
  • Start frost-free
  • DIY: Point foundation for carport

Table of contents Table of contents Point foundations: Effective foundation for load-bearing parts of buildings

  • What are point foundations?
  • Possible uses
  • The right concrete
  • Formwork
  • Advantages and disadvantages
  • Start frost-free
  • DIY: Point foundation for carport

What are point foundations?

A foundation is the basis of every building. A point foundation is a single, point-by-point foundation or consists of a whole collection of such points that transfer the weight of the structure into the ground. The point foundation is also called a single foundation and is usually cast from concrete - the narrow, but deep concrete pillars are distributed over the base of the building, under each supporting pile there is a separate point foundation. In contrast to the strip foundation or a floor slab, this has to withstand mainly compressive and no or hardly any tensile forces. Another difference is that no walls are built on a point foundation, but iron or wooden posts transfer the weight of the building to the floor. Exceptions are brick corner posts of garden walls or chimneys, which can also stand on a point foundation.

When do you build point foundations and what are they made of?

From garden fences to residential buildings: a point foundation is always required when load-bearing post or column structures such as carports, terrace roofs, fence posts, garden houses or wooden decks are to be securely grounded. Even industrial halls and multi-storey residential buildings can be built on point foundations. These stand on stilts, so to speak.

Point foundations can also support individual, column-shaped building parts and are then built in combination with other types of foundations. For larger buildings, a combination of point and strip foundations is usually built for static, but also for cost reasons, or point foundations are built as a supplement. Even the tallest building in the world with almost 830 meters stands with its massive concrete slab on hundreds of concrete columns reaching up to 50 meters deep into the ground.

But also a garden house with its large footprint can be placed on a point foundation by simply laying its load-bearing planks over numerous individual foundations that have been poured at the appropriate distance.

Gartenhaus
Gartenhaus

Every garden shed needs a firm stand on level ground. A point foundation is particularly suitable for this, because the many individual foundations form an overall foundation.

Photo: living4media / Möller, Cecilia

Where which foundations run is shown in the foundation plan, which you usually get from the manufacturer for garden houses or carports. For larger buildings, this is determined by a structural engineer - also from which concrete they have to be built and whether reinforcement is necessary or not. For this reason, point foundations should only be built for simple and light buildings such as carports or garden houses. Because mistakes in construction can no longer be ironed out later - a disaster for residential buildings.

In the hobby area, point foundations mainly support wooden posts. However, wood rots when it comes into contact with earth as well as in concrete, if not so quickly. Therefore, place the wooden beams on support shoes or H-post anchors so that damp wood can dry quickly in the air after rain.

What concrete do you use for point foundations?

If you want to build a foundation yourself, you can either buy the necessary concrete as so-called dry concrete in bags, mix the concrete yourself or have it delivered as so-called ready-mixed concrete. However, this is only an option for very large construction projects. If you want to mix the concrete for your point foundation yourself, use water, cement and gravel with a grain size of 0/16 for unreinforced and gravel with a grain size of 0/8 for reinforcement, so that the space around the reinforcing bars is completely filled.

For small projects like carports, take ready-mixed screed concrete out of sacks, which you only have to mix with the necessary amount of water. In this way, you are on the safe side in any case and the quantities of concrete required for the DIY sector can also be achieved at an acceptable price-performance ratio. How much water you need is on the package.

Concrete is generally divided into compressive strength classes. The concrete grade C 25/30 is suitable for point foundations. With private construction projects such as garden houses or carports, you can do without reinforcement and the associated effort, the point foundation is also sufficiently stable.

Beton anrühren
Beton anrühren

The concrete class plays a role in pouring the point foundation.

Photo: iStock / Imagesines

For larger point foundations, reinforcement made of gate steel may be required. To do this, you twist structural steel mats or reinforcing bars together or you simply simply place a reinforcement cage in the hole before pouring the concrete and pour it into the concrete.

How big is a point foundation? Of course, that depends entirely on the building. For a large rose arch, for example, even 50 centimeter long KG pipes are sufficient, which you use as lost casing. Depending on the size, 30 x 30 centimeters or 40 x 40 centimeters are sufficient for a carport; for large buildings, a single foundation can also measure one meter and more.

Do point foundations need formwork?

Complete formwork is almost only necessary for sandy soils, otherwise earth will constantly slide into the hole. If the foundation should protrude above the ground level, a good ten centimeter high frame made of boards serves as formwork, so that the concrete can be precisely aligned and smoothed out.

If the point foundation consists of several holes, you should connect the shuttering boards for the individual holes. This is not much more complex to build than several individual frames, but it is much easier to align the resulting wooden frame horizontally as a whole than each formwork board by itself. In addition, the formwork becomes more stable and the concrete cannot move or push up the formwork boards.

Advantages and disadvantages of point foundations

Point foundations are generally the easiest and fastest way to give a building a stable concrete base. If you want to create a concrete foundation yourself, the point foundation makes the least amount of work, the earthworks are manageable and you need the least concrete of all foundation variants. Nevertheless, you should be careful when planning and executing, because later problems with the point foundation can no longer be corrected. In a garden shed, this type of foundation enables constant floor ventilation and the wood does not rot. In this case, however, the prerequisite is that the space between the individual foundations is filled with gravel. However, point foundations can only be built on flat terrain or at most areas with a very low slope and the foundations should always be arranged symmetrically.

Establish point foundations frost-free

But all foundations have one thing in common - they must somehow reach a frost-free depth of 80 centimeters. Deeper in heavy residential buildings or in cold winter regions. In heavy buildings, reinforcement made of structural steel is also necessary. After you have dug or drilled the hole for the point foundation, first pour a 10 to 20 centimeter layer of gravel or crushed stone onto the compacted soil and compact the crushed stone as well. This way no water can collect under the concrete. This is important for wet floors and if you cannot fill the concrete to a depth of 80 centimeters.

Build a point foundation for a carport yourself

The work is easy to do on your own, you can expect around two days of work.

  1. First, level the ground and use a string board to pin out the outline of the building with a masonry cord. Exact right angles are important and the two diagonals of the surface must be exactly the same. A little bit of geometry and the Pythagorean theorem are required.
  2. Determine the foundation points and then transfer the outline with the marking paint to the floor, otherwise the cords will always be in your way later.
  3. Define the individual foundation points. All foundation points must be exactly aligned and at one level.
  4. Dig out the holes with a spade, hole spade (hand excavator) or even an earth auger and attach the casing. Check the position of the formwork again and again with a spirit level and a masonry cord or a long stick. Attention: The two arms of H anchors often run slightly towards each other, so they cannot serve as a template for alignment.
  5. Pour in gravel and the concrete on top. This must be viscous and pourable. If you are using a reinforcement cage, fill in some concrete and place the cage on it before continuing to fill the hole. So you can be sure that the structural steel is enclosed on all sides. Don't forget the foundation earth electrode!
  6. If you are not using reinforcement, pour the concrete in two steps and vent it by poking with a broomstick before the next layer comes. If you do not compact, air pockets can make the concrete unstable.
  7. When the point foundation is filled to the top of the formwork, smooth out the concrete and insert the H-anchors into it. If the concrete is too liquid, support the anchor until it stays in place by itself. If you use post supports for screwing on, let the concrete harden first before drilling the dowel holes with a hammer drill.

Thomas Hess

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