Strip footings, sometimes referred to as strip foundations, are shallow foundations commonly utilised in low- to medium-height residential constructions. Although they may be built quickly and cheaply, they are only suitable in situations where the ground is firm and capable of supporting a large weight.
Let’s explore strip foundations and learn why they are important for creating dependable and strong structures.
What is a Strip Foundation?
One kind of shallow foundation used to support a building is called a strip foundation. It consists of a continuous strip of concrete that runs along the length of the building’s walls, evenly distributing the structure’s weight to the ground below. This foundation type is essential in ensuring that the building stays stable and that the load is properly transferred to the soil.
Strip foundations are a form of shallow foundation, often used for buildings with load-bearing walls. They are generally suited for stable soil conditions where the foundation doesn’t need to reach great depths. The primary goal of a strip foundation is to support the building’s weight and prevent structural failure. By distributing the load evenly across the ground, it helps ensure that the building remains levelled, stable, and safe. A structure may sink or collapse if its foundation is not properly planned.
What are the Components of Strip Foundations?
When it comes to strip foundations, there are several key components that play a vital role in their structure and stability. Let’s explore what makes up this essential building block!
Trench
The process of laying a strip foundation begins with a trench that is dug into the ground, typically along the length of the building’s walls. This trench provides the required area for the concrete to be poured and acts as the foundation’s base.
Concrete
Once the trench is prepared, it is filled with concrete. The concrete creates a solid strip that supports the structure above, providing stability and ensuring the building’s long-term endurance. This smooth, continuous layer is key to the foundation’s strength and performance.
Load-bearing Walls
Load-bearing walls are essential structural components constructed on a strong concrete strip foundation. They preserve stability and integrity by distributing and supporting the building’s weight. They avoid settling or structural failure by ensuring proper engineering and placement of weight transmission from the roof and upper levels to the foundation.
Formwork
Strong formwork is built around the trench before the concrete is poured. This mould, made of metal or wood, stops leaks and defines the shape of the concrete. Accurate measurements and strong anchoring are necessary for stability during curing. Until the concrete solidifies, the formwork offers crucial support.
Steel Reinforcement
The judicious placement of steel reinforcing is important before the concrete flows into the trench. This sturdy framework lays the foundation for a strong construction with precise positioning, guaranteeing the strength and longevity of the structure.
The next revolutionary pour will take place with the reinforcing in place. Linear structures like walls or columns are supported by strip foundations. In the UK, they are a common choice for new residential buildings due to their affordability and ease of construction.
What Are the Types of Strip Foundations?
Strip foundations come in various varieties. Apart from a traditional strip base, there are additionally:
Foundations with Deep Strips
A concrete strip that supports walls is occasionally reinforced with steel mesh. This is the least expensive option when the ground conditions are favourable.
Foundations Made of Trench Fill
It is a variation of strip foundations in which concrete is filled into the hole. This method is used for laying strip foundations quickly but costs more than a deep strip foundation due to the amount of concrete to be filled within the width of the trench.
Foundation Made of Wide Strip
In areas where the soil is weak or has insufficient load-bearing ability, broad strip foundations that are reinforced with steel can be employed to distribute the weight over a wider surface and lower the load per square metre.
Foundation Made of Offset Strip
When traditional strip footings are impractical because of their closeness to an existing building or limited access, an offset strip foundation can serve as a budget-friendly alternative to piled or reinforced raft foundations. Usually measuring 750mm in width and 450mm in depth, it incorporates a layer of A193 mesh at the base with a minimum cover of 50mm. This option is generally appropriate for single-storey buildings.
A Comprehensive How-To for Constructing Strip Foundations
- Locate each load-bearing wall and indicate its location.
- Dig trenches, following the central line of each wall as a reference.
- If necessary, dig the trenches deeper than needed and use compacted hard-core for backfill.
- Build formwork to hold up the concrete that has been poured.
- Install rods, mesh panels, or steel reinforcement, and secure with the appropriate ties and supports.
- Fill the pit with concrete, making sure it is uniformly covered.
- Before removing the formwork, let the concrete cure for the specified time.
- The area from where the formwork has been removed can be backfilled with excavated spoil.
Benefits and Drawbacks of Strip Foundations
Like many designs, strip foundations have strengths and weaknesses, which investors must consider before choosing one.
Pros
- Cost-effective construction: They are relatively easy to build from a technical perspective and generally do not involve extensive equipment, making them cheaper.
- Versatile walls: To increase efficiency, the walls of the basement can be the same as those of the foundation walls.
- Adaptable for slopes: Applicable to buildings developed on sites with sloppy grounds.
- Improved floor insulation: Strip foundations extend the range of thermal insulations for the house floors.
Cons
- Limited suitability: It is most suitable for small to medium houses on flat terrains and thus not ideal for high-rise or complicated structures.
- Vulnerability to water damage: Due to their shallow draught, these foundations are particularly susceptible to water damage or flooding.
- High formwork requirements: It takes longer to build than trench-fill foundations due to more formwork.
- Thermal efficiency challenges: Achieving thermal efficiency can generally be challenging, and other strategies may be necessary for implementation.
Key Consideration
The suitability of strip foundations depends on factors like building size, soil conditions, site accessibility, and budget. Always consult a structural engineer to ensure the foundation design is appropriate for the project.
What Should Be the Minimum Depth for Strip Foundation?
A strip foundation should have a minimum depth of around 450mm (1.5 feet) to avoid frost heave unless it is built on rock. Most experts recommend a depth of at least 1 meter (3.3 feet) for optimal stability; the exact depth will depend on soil conditions and local building regulations.
Key points about Strip Foundation Depth
- Frost Protection: The primary factor determining minimum depth is the need to place the foundation below the frost line to prevent damage from freezing soil.
- Soil Conditions: In areas with poor soil quality, a deeper foundation may be required to achieve adequate load-bearing capacity.
- Building Regulations: Always consult local building codes for specific depth requirements for your area.
How Much Is The Cost of Strip Foundation Installation?
Strip foundation installation can cost anywhere from £95 to £130 per linear metre (LM), but it typically depends on multiple factors, such as region and the company from where you get the services. The size of the area, the kind of soil, and other variables can affect the overall cost of a strip foundation:
- Soil type: The site’s soil type can determine the type of foundation and overall cost required.
- Water table: If water is found during excavation, adjustments to the building’s position may be necessary, potentially increasing costs.
- Proximity to Water Sources: Locations near lakes, rivers, or wells may have higher costs due to potential water table effects over time.
- Site Level: If the site requires levelling or bank cuts, expect higher expenses.
- House design: The size and shape of the house will affect how much the foundation costs.
Conclusion
Strip foundations are a practical and cost-effective solution for supporting low—to medium-rise buildings on stable ground. While they offer benefits such as versatility and ease of construction, they also have limitations, including susceptibility to water damage and suitability only for certain conditions.
Proper planning, site evaluation, and professional guidance are essential to ensuring that strip foundations meet your project’s specific requirements. Consult experts like Southern Foundations & Piling for tailored advice and reliable installation.