What Is Underpinning and How Could It Affect Your Home Purchase

Underpinning a house is a crucial construction technique used to strengthen and stabilise the foundations of a building. It involves reinforcing or extending existing foundations to address issues like subsidence or structural instability.

Underpinning is typically necessary when a property’s foundation has been compromised, often due to ground movement or changes in soil conditions.

This method can involve various approaches, such as adding new concrete footings or installing steel screw-in piers to support the structure on more stable soil.

The process aims to prevent further movement or subsidence, ensuring the long-term stability and safety of the property.

Homeowners may encounter underpinning when purchasing a property or addressing foundation issues in their current home.

While it can be a significant undertaking, underpinning is essential for maintaining the structural integrity of a building and can impact factors such as insurance costs and property value.

Understanding Underpinning

Underpinning is a crucial construction technique used to strengthen and stabilise existing foundations. This process involves extending or reinforcing the foundation to improve structural integrity and address issues like subsidence.

Definition and Purpose

Underpinning refers to the method of strengthening and stabilising the foundations of an existing building. It is typically employed when the original foundation is no longer capable of supporting the structure adequately.

The primary purpose of underpinning is to prevent further structural damage and ensure the long-term stability of the building.

Common reasons for underpinning include:

• Subsidence or ground movement
• Increased loads due to building modifications
• Poor soil conditions
• Deterioration of existing foundations

By reinforcing the foundation, underpinning helps distribute the building’s weight more effectively and prevents further settlement or movement.

Types of Underpinning

There are several methods of underpinning, each suited to different situations and soil conditions:

1. Mass concrete underpinning: This traditional method involves excavating sections beneath the existing foundation and filling them with concrete.
2. Beam and base underpinning: A reinforced concrete beam is installed beneath the existing foundation, supported by concrete bases.
3. Pile underpinning: Steel or concrete piles are driven or bored into the ground to transfer loads to more stable soil layers.
4. Mini-piled underpinning: Similar to pile underpinning but using smaller diameter piles, ideal for restricted access areas.
5. Resin injection: A modern technique where expanding resin is injected into the ground to stabilise and lift the foundation.

The choice of method depends on factors such as soil conditions, building type, and project budget.

Signs That A House Requires Underpinning

Recognising the need for underpinning is crucial for maintaining a property’s structural integrity. Several telltale signs can indicate foundation issues that may necessitate this intervention.

Identifying Structural Issues

Cracks in walls are often the most visible indicators of foundation problems. Hairline cracks may be harmless, but wider fissures, especially those larger than 5mm, warrant immediate attention.

Look for diagonal cracks extending from the corners of doors and windows, as these can signify significant movement.

Doors and windows that stick or fail to close properly can also point to foundation shifts. This occurs when the frame becomes misaligned due to the building’s movement.

In severe cases, visible tilting of the structure may be apparent, with one side of the house noticeably lower than the other.

Gaps between walls and ceilings or floors can emerge as the foundation settles unevenly. These separations often appear alongside cracks and may worsen over time.

External signs include bulging or bowing walls, which can indicate pressure from unstable soil beneath the foundation.

Extensions pulling away from the main building are another red flag. This separation occurs when different parts of the structure settle at varying rates, potentially due to inadequate initial foundation work for the extension.

The Underpinning Process

Underpinning reinforces a building’s existing foundation to provide greater stability and support. This process involves several key stages, from initial assessment to final verification.

Preliminary Steps

The underpinning process begins with a thorough structural survey. Engineers assess the building’s condition, soil composition, and extent of foundation damage. They then develop a tailored underpinning plan.

Obtaining necessary permits from local authorities is crucial before work commences. The area around the foundation is cleared, and temporary supports may be installed to ensure structural integrity during the process.

Safety measures are put in place, including protective barriers and equipment for workers. Utility lines are marked and protected to prevent accidental damage during excavation.

Executing the Underpinning

Excavation begins in small sections to minimise destabilisation. Workers dig beneath the existing foundation to the required depth, typically reaching more stable soil layers.

New concrete footings are poured into the excavated areas. These footings extend deeper than the original foundation, providing enhanced support. In some cases, steel reinforcements may be added for extra strength.

The process continues in stages, with each section of the foundation underpinned sequentially. This method ensures the building remains stable throughout the procedure.

Finishing and Verification

Once new footings are in place, the gap between the original foundation and new underpinning is filled with concrete or grout. This creates a solid, continuous support structure.

The surrounding soil is carefully backfilled and compacted to prevent future settling. Drainage systems may be installed or improved to protect the new foundation from water damage.

Engineers conduct final inspections to verify the underpinning’s effectiveness. They may use monitoring equipment to track any structural movement over time, ensuring the building’s long-term stability.

Post-underpinning repairs are carried out on the structure if needed, such as fixing cracks or realigning doors and windows affected by previous foundation issues.

Cost Considerations

Underpinning a house can be a significant expense. The cost varies widely based on factors like property size, damage extent, and chosen methods.

Estimating Underpinning Expenses

Underpinning costs typically range from £6,000 to £50,000. For a semi-detached house, initial excavation trenching may cost between £1,000 and £3,750. The price per linear metre of basic trenching is £75 to £150, depending on depth and width requirements.

Concrete costs contribute substantially, ranging from £1,500 to £2,000 per cubic metre. Structural engineer fees can add £50 to £100 per hour to the total.

Additional expenses may include:

• Party wall agreements (for semi-detached or terraced houses)
• Temporary support and shoring for deep excavations
• Surveyor inspections for boundary walls

Budgeting for the Project

When budgeting for underpinning, homeowners should consider the project’s full scope. It’s crucial to factor in potential additional costs beyond the underpinning itself.

Key budgeting considerations:

• Property size and number of walls requiring underpinning
• Complexity of the job and estimated timeframe (typically 3-5 weeks)
• Method of underpinning chosen
• Possible unforeseen issues that may arise during the process

To ensure accurate budgeting, it’s advisable to obtain multiple quotes from reputable contractors. This helps provide a realistic range of costs for the specific property and circumstances.

Addressing Technical Aspects

Underpinning a house involves crucial technical considerations to ensure structural integrity and safety.

Key factors include selecting appropriate materials and applying sound engineering principles throughout the process.

Material Selection

Concrete is the primary material used in underpinning due to its strength and durability.

A high-strength concrete mix, typically with a compressive strength of 30-40 MPa, is essential for load-bearing capacity. Steel reinforcement bars are incorporated to enhance tensile strength and prevent cracking.

For beam and base underpinning, engineers may specify a concrete grade of C30 or higher. In some cases, specialised grouts or resins might be employed for specific soil conditions or limited access areas.

The choice of materials must account for soil type, moisture levels, and chemical composition to prevent deterioration over time.

Structural Engineering Principles

Underpinning design adheres to strict structural engineering principles to ensure the building’s stability. Load calculations are crucial, considering both dead and live loads, as well as potential future loads.

Engineers must:

• Assess soil bearing capacity
• Determine foundation depth requirements
• Design appropriate reinforcement layouts

Soil mechanics play a vital role in underpinning. Geotechnical surveys inform decisions on foundation depth and width. Settlement analysis helps predict and mitigate potential movement.

Underpinning detail drawings specify precise dimensions, reinforcement placement, and concrete pouring sequences. These details are critical for maintaining structural integrity during and after the underpinning process.

Legal and Safety Considerations

Underpinning a house requires careful attention to legal requirements and safety protocols. Proper planning and execution are essential to ensure compliance and protect workers and residents.

Regulatory Compliance

Obtaining the necessary permits is crucial before beginning any underpinning work. Local building authorities typically require detailed plans and structural engineer reports.

Failure to secure proper permits can result in fines, work stoppages, and difficulties with future property sales or insurance claims.

Building regulations often dictate specific standards for underpinning wall construction, including minimum depths and materials. Homeowners must ensure their chosen contractor is licensed and insured to perform underpinning work. It’s advisable to check references and verify credentials before hiring.

Some areas may have restrictions on underpinning in conservation zones or for listed buildings. In these cases, additional approvals from heritage organisations might be necessary.

Ensuring Safety During Construction

Safety is paramount during underpinning projects. Contractors must follow strict health and safety guidelines to protect workers and occupants. This includes proper shoring of excavations to prevent collapse and ensuring adequate ventilation in confined spaces.

Personal protective equipment (PPE) is mandatory for all workers on site. This typically includes:

• Hard hats
• Steel-toed boots
• High-visibility vests
• Safety glasses

Regular site inspections by qualified safety officers help identify and mitigate potential hazards. Emergency protocols should be established and communicated to all personnel.

For occupied properties, contractors must create safe access routes for residents and erect barriers to keep them away from work areas. Dust control measures and proper waste management are essential to maintain a safe living environment during construction.