Pour a foundation afterwards


Especially with old existing buildings, the existing foundation is often not really sufficient or available. Then an extension of the existing foundation or the subsequent casting of a foundation is considered.

Not always foundations were like today

Almost every suburban construction project requires a foundation. The foundation provides stability, but also distributes loads and discharges them into the ground. However, in older buildings in particular there is either no foundation or a foundation that is incomprehensible according to today's standards:

  • old building stands on strip foundations
  • old building stands on natural stone foundation

Reasons to subsequently add a foundation

The floor area of ​​the basement or ground floor often consists of a natural floor. The soil under the foundation is slurried or it is a loamy soil. The outer walls are meanwhile mostly on a natural stone foundation.

Now, if a homeowner wants to modernize the basement or ground floor, there is often the suggestion in the room to pour a concrete floor slab. However, the insufficient or non-existent foundation is a problem.

Ring beam with tension anchors

In fact, there are ways to subsequently cast or expand a foundation. For this purpose, as with ring beams made of concrete for the knee joint of the roof, such are created next to the foundation and connected with tension anchors. The tension anchors are anchored in the existing foundation using a two-component adhesive.

However, this technique can only be used to a limited extent. The static challenges are high. The situation is similar with a basically similar, but somewhat simpler extension: Monier irons are anchored in the existing foundations and then additionally laid in the new foundations to be poured.

The endeavor of an existing foundation

In contrast, the use of buildings or foundations is more frequently used. The new "foundation" is created deeper than the old foundation and also under the old foundation.

Any technical problems with traditional foundations

The most common demand for the subsequent casting of a foundation comes in the absence of a concrete floor or, if a natural soil is present. But then there is usually no foundation, but there is a strip foundation. If a base plate or a slab foundation is to be re-cast here, this can lead to mechanical problems.

Due to the new load distribution, for example, with a natural stone foundation, individual stones can be pushed outwards so that they literally break out of the foundation. The project can therefore lead to major technical problems in most projects around buildings, subsequently to pour a foundation or a floor slab.

Tips & Tricks

If a foundation is to be subsequently poured into a building, a structural engineer, architect or soil surveyor is required. The changed statics may involve more risks than before.

Product Image: brocko / Shutterstock


Video Board: Concrete Slab Foundation - Process & Best Practices