strip and eccentric foundations

What is a foundation?

A foundation refers to the lowest part of a building, although you cannot see them they are one of the most crucial elements of a building. Foundations are part of the building that make contact with the soil to help transfer the load of the building. Foundations are made by digging trenches into the soil until a harder more stable level is found. Concrete is then poured into the trenches; generally, the concrete is accompanied by reinforcement cages to prevent cracking.  Foundations help avoid lateral movement in the building, provide a stronger base therefore a more stable structure, provide a proper level and help distribute the load.

What is a Strip foundation?

A strip foundation is generally a type of shallow foundation, they consist of a continuous strip of concrete formed under load-bearing walls. The strip is used to spread the weight of the load-bearing walls. The width of the strip is determined by the weight of the load-bearing walls and the bearing capacity of the soil. The heavier the load-bearing walls the wider the strip will be equally if the soil is of poor-bearing capacity the strip will be wider. 

Careful evaluation of the surrounding ground conditions is crucial to establish the bearing capacity of the soil. The load capacity of the subsoil is generally determined by the material of the soil, rock, chalk, gavel and sand, clay, peat etc. Other factors that may be taken into account when deciding what foundations to use are where utility pipes lay. 

Strip foundations are most suitable where the soil is of good bearing capacity. They are suited to light structures such as domestic buildings. Strip foundations provide some advantages such as;

  • They are relatively inexpensive, Strip foundations require less excavation and construction material compared to other foundation types. 
  • They are quick and easy to install, once the ground has been prepared the excavation and masonry work can be done quickly. 
  • Strip foundations are versatile and can be tailored to suit different soil conditions and load-bearing requirements. 
  • Strip foundations are compatible with existing foundations, they are often used if the extension is adjacent to or connected to the existing structure. 
  • They are easier on topsoil as they do not require deep excavations, so plants and wildlife are less likely to be disturbed.

While strip foundations are versatile, it is important to consider their drawbacks to determine if they are suitable for your construction project:

  • Strip foundations are not suitable for all soil types, they perform best in stable soil with good load-bearing capacity. 
  • Strip foundations require excavation along the length of the structure. This may have implications for neighbouring foundations, invoking the party wall act. The excavation may also involve relocating utilities and vegetation. 
  • They are limited on the maximum load they can support so are not suitable for larger projects. 

As strip foundations are less intrusive, they are a great option for conservatories. and home extensions.  However, excavation for the new foundation mustn't undermine neighbouring buildings. It is good building practice to excavate to the same depth as neighbouring properties. 

What is an Eccentric foundation?

An eccentric foundation is essentially when the wall is built off the centre of the foundation. Sometimes this cannot be avoided, for example, if you are building an extension and plan to build up to the boundary. Eccentric foundations allow the proposed structure to remain within the boundaries of the building owner’s land. Primarily eccentric foundations are used up to the boundary to prevent the tilting of floors. It must connect with another footing, often a strap beam or combine foundation is used. A strap beam is used to connect the independent footings of two columns. 

Eccentric foundations ensure that the outer edge of the foundation lines up with the outer edge of the proposed wall. This allows adjoining owners to undertake similar works in the future without being hindered by subterranean trespass. Eccentric foundations allow the building owner to maintain 100% of the ownership of the wall. It is not uncommon for the foundations of historic walls to project beyond the boundary line, which can often result in the adjoining owner having to cut back their foundation at their cost. 

Eccentric foundations are typically preferred when;

  • Eccentric foundations are usually preferred when building up to the boundary and your neighbour refuses to put foundations on their land.  
  • If your neighbour’s foundation falls into your land a traditional foundation cannot be built so an eccentric foundation is usually preferred. 
  • Eccentric foundations are used in situations where there is limited space, they allow for the construction of load-bearing points without interfering with surrounding structures. 
  • Eccentric foundations are also used if your neighbour already has an extension, therefore a traditional foundation cannot be used. 

Eccentric foundations are also a great option for extensions, as they are simple to design and easy for the contractor to implement on-site. They are very attractive as they allow you to maintain 100% ownership of the wall, which allows the adjoining owner to undertake similar works minimising the likelihood of a future Party wall dispute. 

If you are planning a rear extension, this will require excavations most likely invoking the Party Wall Act. If the project involves excavation of 3 or 6 meters you must inform your neighbours of your plans of excavating by serving them what is known as a Section 6 Notice.

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We are a team of party wall surveying experts based throughout London and the surrounding areas. Here, we share informative property survey blogs created by industry experts.



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