Overview of the Boswell House

The Boswell House, a notable example of early 20th-century British architecture, stands as a testament to the ingenuity and craftsmanship of its era. Constructed primarily in the 1920s, these properties were the brainchild of M A Boswell and Co. Ltd, with M A Boswell himself leading the design efforts. Approximately 4,000 of these homes were built, predominantly as two-storey semi-detached and terraced houses. They are easily identifiable by their medium pitch hipped roofs, which are typically covered with plain or concrete tiles, and their fully rendered external walls. Some of these houses also feature bay windows on both the ground and first floors.

Historical Context and Manufacturing

The Era of Construction

The 1920s was a period of significant change and development in the UK, particularly in the housing sector. Post-World War I, there was a pressing need for affordable housing to accommodate the growing population and returning soldiers. The Boswell House emerged during this time as a solution to the housing crisis, offering a blend of affordability, durability, and aesthetic appeal.

The Manufacturer: M A Boswell and Co. Ltd

M A Boswell and Co. Ltd was a prominent construction company known for its innovative approach to building. The company specialized in system-built properties, which were designed to be constructed quickly and efficiently. The Boswell House is a prime example of their work, showcasing their expertise in using pre-cast reinforced concrete (PRC) and clinker aggregate concrete.

Typical Characteristics of the Boswell House

Architectural Features

Boswell Houses are characterized by their medium pitch hipped roofs, which provide a balanced and symmetrical appearance. The roofs are typically covered with plain or concrete tiles, adding to the durability and weather resistance of the structure. The external walls are fully rendered, giving the houses a clean and uniform look. Some properties also feature bay windows on both the ground and first floors, adding to their charm and providing additional interior space.

Structural Elements

The construction of Boswell Houses involves several key structural elements:

• Substructure: The houses are built on clinker aggregate concrete strip footings and underbuilding, with a damp-proof course (DPC) to prevent moisture ingress.
• External Walls: The walls are constructed using L-shaped clinker aggregate PRC corner columns with square cage reinforcement bars. These columns are connected by pre-cast (PC) ring beams at the first floor and eaves levels. The walls themselves are 9-inch clinker aggregate concrete cavity walls with a 2-inch cavity, connected by concrete ribs staggered between 18-inch lifts.
• Separating Wall: The separating wall between properties is also constructed using 9-inch clinker aggregate concrete with a cavity.
• Partitions: Internal partitions are made from 3 1/2-inch or 4 1/2-inch clinker aggregate concrete slabs.
• Floors: The ground floor consists of timber boarding on timber joists, while the first floor features timber boarding on timber joists built into the inner leaf of the external wall.
• Roof: The roof structure comprises timber rafters and purlins, covered with concrete tiles.

Typical Hazards and Risks

Designated Defective

Boswell Houses are designated as defective under the Defective Premises Act. This designation is primarily due to the use of clinker aggregate concrete, which can contain unstable fractions of unburnt coal. This material can lead to several structural issues over time, including:

• Cracking of Render: The external render can crack due to the movement and instability of the underlying concrete.
• Reinforcement Corrosion: The PRC corner columns are susceptible to reinforcement corrosion, which can compromise the structural integrity of the property.
• Friable Concrete Below DPC Level: The concrete below the DPC level can become friable, leading to potential foundation issues.
• Lack of Tying Between Flank and Internal Walls: The lack of adequate tying between the flank and internal walls can result in structural instability.

Common Defects

In addition to the hazards mentioned above, Boswell Houses are prone to several common defects:

• Aggregate Instability: The clinker aggregate used in the construction can contain unstable fractions of unburnt coal, leading to long-term structural issues.
• Render Cracking: The external render is prone to cracking, which can allow moisture ingress and lead to further deterioration of the underlying concrete.
• Reinforcement Corrosion: The PRC corner columns are susceptible to corrosion, which can weaken the structural integrity of the property.
• Friable Concrete: The concrete below the DPC level can become friable, leading to potential foundation issues.
• Lack of Tying: The lack of adequate tying between the flank and internal walls can result in structural instability.

Inspection Guide for Surveyors

Identifying a Boswell House

When inspecting a property, surveyors can identify a Boswell House by its distinctive architectural features, including:

• Medium Pitch Hipped Roof: Look for a roof with a medium pitch, typically covered with plain or concrete tiles.
• Rendered External Walls: The external walls are fully rendered, giving the property a clean and uniform appearance.
• Bay Windows: Some properties feature bay windows on both the ground and first floors.

Key Areas to Inspect

Surveyors should pay close attention to the following areas when inspecting a Boswell House:

• External Render: Check for signs of cracking or deterioration in the external render, which can indicate underlying structural issues.
• PRC Corner Columns: Inspect the PRC corner columns for signs of reinforcement corrosion, which can compromise the structural integrity of the property.
• Concrete Below DPC Level: Examine the concrete below the DPC level for signs of friability, which can indicate potential foundation issues.
• Tying Between Walls: Assess the adequacy of tying between the flank and internal walls, as a lack of tying can result in structural instability.

Tools and Techniques

Surveyors should use a combination of visual inspection and specialized tools to assess the condition of a Boswell House. Key tools and techniques include:

• Moisture Meters: Use moisture meters to detect signs of moisture ingress, particularly in areas where the external render is cracked.
• Endoscopes: Employ endoscopes to inspect the condition of the PRC corner columns and reinforcement bars.
• Concrete Testing: Conduct concrete testing to assess the stability and integrity of the clinker aggregate concrete, particularly below the DPC level.
• Structural Analysis: Perform a structural analysis to evaluate the adequacy of tying between the flank and internal walls.

Construction Details

Roof

The roof of a Boswell House is constructed using timber rafters and purlins, covered with concrete tiles. The medium pitch hipped design provides a balanced and symmetrical appearance, while the concrete tiles offer durability and weather resistance.

Walls

The external walls are constructed using L-shaped clinker aggregate PRC corner columns with square cage reinforcement bars. These columns are connected by pre-cast (PC) ring beams at the first floor and eaves levels. The walls themselves are 9-inch clinker aggregate concrete cavity walls with a 2-inch cavity, connected by concrete ribs staggered between 18-inch lifts. The separating wall between properties is also constructed using 9-inch clinker aggregate concrete with a cavity.

Floors

The ground floor consists of timber boarding on timber joists, supported by a timber sole plate. The first floor features timber boarding on timber joists built into the inner leaf of the external wall. In some variants, the first floor may be constructed using reinforced concrete slabs.

Main Frame

The main frame of a Boswell House is constructed using clinker aggregate concrete, with PRC corner columns and PC ring beams providing structural support. The use of clinker aggregate concrete offers a balance of strength and affordability, although it can be prone to certain defects over time.

Variants and Modifications

Tie Bars

Some variants of the Boswell House feature tie bars running from front-to-back and from side-to-side at the first floor level. These tie bars are designed to provide additional structural stability and prevent movement between the walls.

Embedded Bars

In some properties, 3/4-inch or 1-inch bars are embedded in the concrete at mid-wall height. These bars have threaded ends to accept a nut and a 4-inch or 6-inch square end-plate, which ties the internal walls and separating wall to the external walls.

Roof Modifications

In certain variants, the roof may be constructed using a steel frame instead of timber rafters and purlins. These roofs are typically covered with plain tiles, offering a different aesthetic and structural profile compared to the standard timber and concrete tile design.

Conclusion

The Boswell House is a unique and historically significant property type, offering a blend of architectural charm and structural innovation. However, these properties are also prone to several common defects and hazards, primarily due to the use of clinker aggregate concrete and PRC corner columns. As a designated defective property type under the Defective Premises Act, it is essential for home buyers and investors to conduct thorough inspections and assessments before making a purchase.

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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