Historical Context and Origins

The Duo-Slab house, also known as the Airey house, is a significant part of the UK’s architectural history. Designed by William Airey and manufactured by William Airey & Sons Ltd, these properties were primarily constructed in the 1920s. With a total of 4,650 units built, these homes were a response to the post-World War I housing shortage. They are predominantly found in semi-detached and terraced forms, often located in suburban areas across the UK.

Identification Characteristics

Duo-Slab houses are easily identifiable by their distinct architectural features. These properties are typically two-storey semi-detached or terraced houses with a medium pitch hipped or gable roof covered with tiles. The external walls are rendered throughout, giving them a uniform appearance. The system was also adapted for flats, although these are less common.

Key Features

• Roof: Medium pitch hipped or gable roof covered with tiles.
• External Walls: Rendered clinker concrete columns infilled with clinker aggregate PC blocks.
• Floors: Ground floor of timber boarding on timber battens on concrete; first floor of timber boarding on timber joists.
• Main Frame: Clinker aggregate concrete columns.

Construction Details

Understanding the construction details of Duo-Slab houses is crucial for both buyers and investors. These homes were built using a specific set of materials and techniques that were innovative for their time but may present certain challenges today.

Substructure

The substructure consists of concrete strip footings and concrete under-building, with a damp-proof course (DPC) to prevent moisture ingress.

External Walls

The external walls are a key feature of Duo-Slab houses. They are constructed using rendered clinker concrete columns infilled with 3-inch clinker aggregate PC blocks, followed by a 2-inch cavity and another layer of 3-inch clinker aggregate blocks. Timber formers are also used in the construction.

Separating Wall

The separating wall between properties is made of clinker aggregate concrete columns infilled with clinker aggregate PC block cavity wall. This design provides both structural integrity and sound insulation.

Partitions

The ground floor partitions are constructed using clinker aggregate concrete columns infilled with clinker aggregate PC blocks. The first-floor partitions are made of timber stud lined with plasterboard.

Floors

• Ground Floor: Timber boarding on timber battens on concrete, with concrete flooring in the kitchen.
• First Floor: Timber boarding on timber joists.

Roof

The roof is constructed using timber rafters and tiles, providing a durable and weather-resistant covering.

Typical Hazards and Risks

While Duo-Slab houses have stood the test of time, they are not without their issues. Understanding these potential hazards and risks is essential for anyone considering purchasing or investing in one of these properties.

High Carbonation in Clinker Concrete Columns

One of the primary concerns with Duo-Slab houses is the high carbonation in clinker concrete columns. Carbonation can weaken the concrete, making it more susceptible to cracking and structural failure.

High Sulfate Content

The clinker concrete used in these houses often has a high sulfate content. Sulfates can react with the concrete, causing it to expand and crack over time. This can lead to significant structural issues if not addressed promptly.

Horizontal and Vertical Cracking

Cracking of the render at joints between concrete blocks is a common issue in Duo-Slab houses. These cracks can allow moisture to penetrate the structure, leading to further deterioration and potential mould growth.

Bowing of Flank External Wall

Bowing of the flank external wall is another typical defect. This can be caused by a variety of factors, including settlement, thermal expansion, and moisture ingress. Bowing can compromise the structural integrity of the property and should be addressed by a professional surveyor.

Nominal Bearing of Lintels

The nominal bearing of lintels in Duo-Slab houses can be a concern. Lintels with insufficient bearing can lead to cracking and potential collapse of the masonry above.

Defective Premises Act

Duo-Slab houses are not classified as defective under the Defective Premises Act. However, this does not mean they are free from issues. It is essential to conduct a thorough survey to identify any potential problems before purchasing or investing in one of these properties.

Inspection Guide for Surveyors

When inspecting a Duo-Slab house, surveyors should pay close attention to several key areas. This guide provides a step-by-step approach to ensure a comprehensive assessment.

External Inspection

1. Roof: Check for missing or damaged tiles and inspect the condition of the timber rafters.
2. Walls: Look for horizontal and vertical cracking in the render, particularly at joints between concrete blocks.
3. Flank Wall: Inspect for any signs of bowing or movement.
4. Lintels: Assess the bearing of lintels and look for any signs of cracking or displacement.

Internal Inspection

1. Floors: Check the condition of the timber boarding and joists, particularly in areas prone to moisture, such as the kitchen.
2. Partitions: Inspect the timber stud partitions for any signs of damage or deterioration.
3. Carbonation: Test the clinker concrete columns for signs of carbonation and assess their structural integrity.
4. Sulfate Content: Conduct tests to determine the sulfate content of the concrete and assess the potential for expansion and cracking.

Additional Considerations

1. Dampness: Check for any signs of dampness or moisture ingress, particularly around the DPC and in areas with cracking render.
2. Structural Movement: Look for any signs of structural movement, such as cracks in walls or floors, and assess the potential causes.
3. Mould: Inspect for any signs of mould growth, particularly in areas with high moisture levels or poor ventilation.

Typical Defects and How to Address Them

Understanding the typical defects in Duo-Slab houses and how to address them is crucial for maintaining the property’s value and ensuring its longevity.

High Carbonation

To address high carbonation in clinker concrete columns, it is essential to conduct regular inspections and testing. If carbonation is detected, remedial measures such as applying a carbonation-resistant coating or replacing affected columns may be necessary.

High Sulfate Content

High sulfate content can be managed by conducting regular tests and monitoring the condition of the concrete. If sulfate attack is detected, it may be necessary to replace affected concrete elements or apply protective coatings to prevent further deterioration.

Cracking Render

Horizontal and vertical cracking in the render can be addressed by repairing the cracks and applying a flexible, breathable render system. This will help to prevent moisture ingress and further cracking.

Bowing Flank Wall

Bowing of the flank wall can be addressed by identifying and rectifying the underlying cause, such as settlement or moisture ingress. In some cases, it may be necessary to install additional structural supports or tie rods to stabilize the wall.

Nominal Bearing of Lintels

To address issues with the nominal bearing of lintels, it may be necessary to replace or reinforce affected lintels. This will help to prevent cracking and potential collapse of the masonry above.

Conclusion

Duo-Slab houses, or Airey houses, are a unique and historically significant part of the UK’s housing stock. While they offer many benefits, including their distinctive architectural features and historical value, they are not without their challenges. Understanding the typical hazards and risks associated with these properties, as well as how to address common defects, is essential for anyone considering purchasing or investing in a Duo-Slab house.

At Flettons, we’re committed to safeguarding your investment. When considering a property purchase, trust our seasoned expertise to reveal any hidden threats. For a thorough building survey, get your instant quote through our quote calculator or reach out directly at 0203 691 0451. Your home’s safety is our top priority.

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